Luminal flagellates can be found in the mouth, vagina, or intestine of almost all vertebrates, and it is common for an animal host to harbor more than one species. Humans may serve as host and reservoir to eight species (Table 53–1), but only two cause disease. Of these, G duodenalis (=lamblia) inhabits the intestinal tract, and T vaginalis inhabits the vagina and genital tract.
TABLE 53–1Luminal Flagellates Infecting Humans ||Download (.pdf) TABLE 53–1 Luminal Flagellates Infecting Humans
|FLAGELLATE ||PATHOGENICITY TO HUMANS ||SITE |
|Giardia lamblia ||+ ||Intestine |
|Dientamoeba fragilis ||? ||Intestine |
|Chilomastix mesnili ||− ||Intestine |
|Enteromonas hominis ||− ||Intestine |
|Retortamonas intestinalis ||− ||Intestine |
|Trichomonas hominis ||− ||Intestine |
|Trichomonas tenax ||− ||Mouth |
|Trichomonas vaginalis ||+ ||Vagina |
Found in flora of vertebrates
These organisms are elongated or oval and typically measure 10 to 20 μm in length. They often possess a rudimentary cytostome (mouth aperture) and organelles, such as ventral discs or axostyles, which help maintain their intraluminal position. They are readily recognized in body fluid or excreta by their rapid motility and some can be specifically identified in unstained preparations. All can be cultivated on artificial media.
Morphology and rapid motility are distinctive
Some luminal flagellates, most notably T vaginalis, possess only a trophozoite stage and are sexually transmitted. Most, including G duodenalis, possess both trophozoite and cyst forms. The latter, which is the infective form, is transmitted via the direct or indirect fecal–oral route. Human-to-human infection is thus found in populations where inadequate sanitation or poor personal hygiene favors spread.
May or may not have the cyst stage
Three members of the genus Trichomonas parasitize humans (Table 53–1), but only T vaginalis is an established pathogen. The three species closely resemble one another morphologically, but confusion in identification is rare because of the specificity of their habitats.
Three Trichomonas species have similar morphology
The T vaginalis trophozoite (Figure 53–1) is oval and typically measures 7 by 15 μm. Organisms up to twice this size are occasionally recovered from asymptomatic patients and from cultures. In stained preparations, a single, elongated nucleus and a small cytostome are observed anteriorly. Five flagella arise nearby. Four immediately exit the cell. The fifth bends back and runs posteriorly along the outer edge of an abbreviated undulating membrane. Lying along the base of this membrane is a cross-striated structure known as the costa. A conspicuous microtubule containing a supporting rod or axostyle bisects the trophozoite longitudinally and protrudes through its posterior end. It is thought that the pointed tip of this structure is useful for attachment. In unstained wet mounts, T vaginalis is identified by its axostyle and jerky, nondirectional movements.
Trichomonas vaginalis. The parasite and its structures are shown in relation to the size of a polymorphonuclear leukocyte (top). The micrograph below illustrates their use for motility. (Reproduced with permission from Nester EW: Microbiology: A Human Perspective, 6th edition. 2009.)
Protruding axostyle may mediate attachment
The organism can be grown on artificial media under anaerobic conditions at pH 5.5 to 6.0. Soluble nutrients are absorbed across the cell membrane. A variety of carbohydrates are degraded to short-chained organic acids. Pyruvate is produced via glycolysis and reduced to lactate, part of which enters structures called hydrogenosomes. Molecular hydrogen and ATP are produced in the hydrogenosomes. These structures are analogous to mitochondria, which T vaginalis lacks. Although T vaginalis lacks a cyst form, the trophozoite can survive outside of the human host for 1 to 2 hours on moist surfaces. In urine, semen, and water, it may be viable for up to 24 hours, making it one of the most resistant of protozoan trophozoites. Attempts to infect laboratory animals have met with limited success.
Cultivable in vitro
Lacks cyst form, but may survive a few hours outside host
Trichomoniasis is a sexually transmitted disease, which produces vaginitis with pain, discharge, and dysuria. The infection fluctuates over weeks to months. Men are usually asymptomatic but may have urethritis or prostatitis.
Trichomoniasis is a cosmopolitan disease usually transmitted by sexual intercourse. An estimated 8 million infections occur in the United States annually. Worldwide this figure reaches 180 million cases. Twenty-five percent of sexually active women become infected at some time during their lives and 30% to 70% of their male sexual partners are also parasitized, at least transiently. As would be expected, the likelihood of acquiring the disease correlates directly with the number of sexual contacts. Infection is rare in adult virgins, whereas rates as high as 70% are seen among prostitutes, sexual partners of infected patients, and individuals with other venereal diseases. In women, the peak incidence of trichomoniasis is between 16 and 35 years of age, but there is still a relatively high prevalence in the 30- to 50-year age group.
Transmission usually sexual
Prevalence linked to sexual activity
Nonvenereal transmission is uncommon
Nonvenereal transmission is uncommon. Transfer of organisms on shared washcloths may explain, in part, the high frequency of infection seen among institutionalized women. Female neonates are occasionally noted to harbor T vaginalis, presumably acquiring it during passage through the birth canal. High levels of maternal estrogen produce a transient decrease in the vaginal pH of the child, rendering it more susceptible to colonization. Within a few weeks, estrogen levels drop, the vagina assumes its premenarcheal state, and the parasite is eliminated.
PATHOGENESIS AND IMMUNITY
Direct contact of T vaginalis with the squamous epithelium of the genitourinary tract results in destruction of the involved epithelial cells and the development of a neutrophilic inflammatory reaction and petechial hemorrhages. Attachment appears to be mediated by adhesins, laminin-binding proteins, and lectin-binding carbohydrates. Trophozoites are capable of secreting a variety of proteinases that undoubtedly help initiate contact-dependent cytolytic events. These proteinases are also capable of degrading immunoglobulin-G (IgG) and IgA. A contact-independent mechanism of cell damage has also been shown to correlate with the presence of a 200 kDa glycoprotein that is heat and acid labile. Changes in the microbial, hormonal, and pH environment of the vagina as well as factors inherent to the infecting parasite are thought to modulate the severity of the pathologic changes.
Infection of the vaginal epithelium triggers innate responses by stimulating Toll-like receptors that trigger secretion of proinflammatory cytokines. This brings about a neutrophil and CD4+ response. Humoral and cellular immune responses follow, although they do not appear to result in clinically significant immunity. Because of the proinflammatory response produced, women with this infection are at greater risk of human immunodeficiency virus (HIV) infection. Trichomonas vaginalis is also capable of phenotypically varying surface antigenic determinants to help it escape immune detection. Parasite damages epithelial cells on contact.
TRICHOMONIASIS: CLINICAL ASPECTS
In women, T vaginalis produces a persistent vaginitis. Although up to 50% are asymptomatic at the time of diagnosis, most develop clinical manifestations within 6 months. Approximately 75% develop a discharge, which is typically accompanied by vulvar itching or burning (50%), dyspareunia (50%), dysuria (50%), and a disagreeable odor (10%). Although fluctuating in intensity, symptoms usually persist for weeks or months. Commonly, manifestations worsen during menses and pregnancy. Eventually, the discharge subsides, even though the patient may continue to harbor the parasite. In symptomatic patients, physical examination reveals reddened vaginal and endocervical mucosa. In severe cases, petechial hemorrhages and extensive erosions are present. A red, granular, friable endocervix (strawberry cervix) is a characteristic but uncommon finding. An abundant discharge is generally seen pooled in the posterior vaginal fornix. Although classically described as thin, yellow, and frothy in character, the discharge more frequently lacks these characteristics. Trichomoniasis may increase the risk of preterm birth and enhance susceptibility to HIV infections.
Chronic vaginitis lasting weeks to months
The urethra and prostate are the usual sites of trichomoniasis in men; the seminal vesicles and epididymis may be involved on occasion. Infections are usually asymptomatic, possibly because of the efficiency with which the organisms are removed from the urogenital tract by voided urine. Symptomatic men complain of recurrent dysuria and scant, nonpurulent discharge. Acute purulent urethritis has been reported rarely. Trichomoniasis should be suspected in men presenting with nongonococcal urethritis, or a history of either prior trichomonal infection or recent exposure to trichomoniasis.
Urethral and prostatic infection in men usually asymptomatic
The diagnosis of trichomoniasis rests on the detection and morphologic identification of the organism in the genital tract. Identification is accomplished most easily by examining a wet mount preparation for the presence of motile organisms. In women, a drop of vaginal discharge is the most appropriate specimen; in men, urethral exudate or urine sediment after prostate massage may be used. Although highly specific when positive, wet mounts have a sensitivity of only 50% to 60%. They are most likely to be negative in asymptomatic or mildly symptomatic patients and in women who have douched in the previous 24 hours. Giemsa- and Papanicolaou-stained smears provide little additional help. The recent introduction of a commercial system that allows direct, rapid microscopic examination without the need for daily sampling may ameliorate this situation. Direct immunofluorescent antibody staining has a sensitivity of 70% to 90%. Parasitic culture, though more sensitive, requires several days to complete and is frequently unavailable. Nucleic acid amplification (NAA) methods have been shown to be the most sensitive for diagnosis.
Wet mount examination for motile trophozoites sufficient in most symptomatic cases
Oral metronidazole is extremely effective in recommended dosage, curing more than 95% of all Trichomonas infections. It may be given as a single dose or over 7 days. Simultaneous treatment of sexual partners may minimize recurrent infections, particularly when single-dose therapy is used for the index case. Because of the disulfiram-like activity of metronidazole, alcohol consumption should be suspended during treatment. The drug should never be used during the first trimester of pregnancy because of its potential teratogenic activity. Use in the last two trimesters is unlikely to be hazardous but should be reserved for patients whose symptoms cannot be adequately controlled with local therapies. High-dose, long-term metronidazole treatment has been shown to be carcinogenic in rodents. No association with human malignancy has been described to date, and in the absence of a suitable alternative drug, metronidazole continues to be used. NAA-based studies have shown that this infection is under diagnosed, and therefore infections are undertreated contributing to the continued high incidence of this parasite.
Metronidazole cures 95% of cases
Giardia lamblia was first described by Anton von Leeuwenhoek 300 years ago when he examined his own diarrheal stool with one of the first primitive microscopes. It was not until the last several decades, however, that this cosmopolitan flagellate became widely regarded in the United States as a pathogen. Of the six other flagellated protozoans known to parasitize the alimentary tract of humans, only one, Dientamoeba fragilis, has been credibly associated with disease. Definitive confirmation or refutation of its pathogenicity will, it is hoped, not require the passage of another three centuries.
Unlike T vaginalis, Giardia possesses both a trophozoite and a cyst form (Figure 53–2). It is a sting-ray–shaped trophozoite 9 to 21 μm in length, 5 to 15 μm in width, and 2 to 4 μm in thickness. When viewed from the top, the organism's two nuclei and central parabasal bodies give it the appearance of a face with two bespectacled eyes and a crooked mouth. It is uncertain why this organism has two nuclei, but both are transcriptionally active. Four pairs of flagella—anterior, lateral, ventral, and posterior—reinforce this image by suggesting the presence of hair and chin whiskers. These distinctive parasites reside in the duodenum and jejunum, where they thrive in the alkaline environment and absorb nutrients from the intestinal tract. They move about the unstirred mucous layer at the base of the microvilli (Figure 53–3) with a peculiar tumbling or “falling leaf” motility or, with the aid of a large ventral disk, attach themselves to the brush border of the intestinal epithelium. The exact molecular mechanism by which the ventral disk mediates attachment has not been resolved but is thought, in part, to involve flagellar motility. Unattached organisms may be carried by the fecal stream to the large intestine.
Giardia lamblia. A. Cyst structures. B. Trophozoite structures. C. Cyst in stool iodine preparation. D. Trophozoite in stool. (C and D, Reproduced with permission from Connor DH, Chandler FW, Schwartz DQ, et al: Pathology of Infectious Diseases. Stamford CT: Appleton & Lange, 1997.)
Giardiasis. Scanning electron micrograph of G lamblia trophozoites in human intestine. (Reproduced with permission from Nester EW: Microbiology: A Human Perspective, 6th edition. 2009.)
Trophozoite and cyst stages
Move about duodenum and jejunum with tumbling motility
In the descending colon, if transit time allows, the flagella are retracted into cytoplasmic sheaths and a smooth, clear cyst wall is secreted. These forms are oval and somewhat smaller than the trophozoites. With maturation, the internal structures divide, producing a quadrinucleate organism harboring two ventral discs, four kinetosomes, and eight axonemes. When fixed and stained, the cytoplasm pulls away from the cyst wall in a characteristic fashion. The mature cysts, which are the infective form of the parasite, may survive in cold water for more than 2 months and are resistant to concentrations of chlorine generally used in municipal water systems. They are transmitted from host to host by direct and indirect fecal–oral routes. In the duodenum of a new host, the cytoplasm divides to produce two binucleate trophozoites.
Cystic forms develop in colon
Resistant cysts transmitted from host to host
Giardia is amitocondriate like Trichomonas. Instead, Giardia possesses mitosomes, which like the hydrogenosomes of Trichomonas are thought to represent mitochondrial adaptations in these aerotolerant anaerobe parasites. Giardia can respire aerobically or anaerobically with glucose as the main substrate for respiration. Axenic cultivation of this organism has been achieved in vitro. Bile salts enhance the parasite's growth. Although Giardia has largely been thought to be an asexual parasite, evidence for genetic recombination, hinting at a form of sexual recombination, has recently been reported.
Organisms of the genus Giardia are among the most widely distributed of intestinal Protozoa; they are found in fish, amphibians, reptiles, birds, and mammals. At first, it was assumed that Giardia strains found in different animals were host specific; on this basis, some 40 different species were described. Since it is now recognized that some strains can infect multiple animal hosts, the practice of assigning species status by the host from which the parasite was recovered is considered invalid. At present, only five species are considered valid and of these, only G duodenalis infects humans. This parasite is also commonly referred to as G lamblia or G intestinalis in much of the current literature.
Giardiasis, an intestinal infection acquired from untreated water sources, is most often symptomatic. When disease occurs, it is in the form of a diarrhea lasting up to 4 weeks with foul-smelling, greasy stools. Abdominal pain, nausea, and vomiting are also present.
Giardiasis has a cosmopolitan distribution; its prevalence is highest in areas with poor sanitation and among populations unable to maintain adequate personal hygiene. In developing countries, infection rates may reach 25% to 30%; in the United States, G lamblia is found in 4% of stools submitted for parasitologic examination, making it, along with Cryptosporidium, this country's most frequently identified intestinal parasite. All ages and economic groups are represented, but young children and young adults are preferentially involved. Children with immunoglobulin deficiencies are more likely to acquire the flagellate, possibly because of a deficiency in intestinal IgA. Giardiasis is also common among attendees of day care centers. Attack rates of over 90% have been seen in the ambulatory non–toilet-trained population (age 1-2 years) of these institutions, suggesting direct person-to-person transmission of the parasite. The frequency with which secondary cases are seen among family contacts reinforces this probability. Undoubtedly, direct fecal spread is also responsible for the high infection rate among male homosexuals. In several recent studies, the prevalence of giardiasis and/or amebiasis in that population has ranged from 11% to 40% and is correlated closely with the number of oral–anal sexual contacts.
Transmission facilitated by poor hygiene and IgA deficiency
High attack rates in day care centers
Giardiasis common among male homosexuals
Waterborne and, less frequently, foodborne transmission of G lamblia has also been documented, and probably accounts for the frequency with which American travelers to Third World nations acquire infection. Unlike the typical bacterial diarrhea syndrome seen in travelers, the diarrhea begins late in the course of travel and may persist for several weeks. More than 20 waterborne outbreaks of giardiasis have also been reported in the United States. The sources have included swimming pools, untreated pond or stream water, sewage-contaminated municipal water supplies, and chlorinated but inadequately filtered water. In a few of these outbreaks, epidemiologic data have suggested that wild mammals, particularly beavers, served as the reservoir hosts. In spite of the evidence for zoonotic transmission, this remains a controversial topic. In some areas of the world, where different animals, including man's closest friend, the dog, and many have been shown to be infected with Giardia, the infecting genotypes differed. In others, the same genotypes were demonstrated in man and animals. In most cases, humans sampled were shown to predominantly harbor human genotypes. Extensive infectivity studies using human genotypes have not been conducted.
Water- or foodborne traveler's diarrhea lasts for weeks
Beavers and other mammals possible sources
Disease manifestations appear related to intestinal malabsorption, particularly of fat and carbohydrates. Disaccharidase deficiency with lactose intolerance, altered levels of intestinal peptidases, and decreased vitamin B12 absorption have been demonstrated. The precise pathogenetic mechanisms responsible for these changes remain poorly understood. Mechanical blockade of the intestinal mucosa by large numbers of Giardia, damage to the brush border of the microvilli by the parasite's ventral disc, organism-induced deconjugation of bile salts, altered intestinal motility, accelerated turnover of mucosal epithelium, and mucosal invasion have all been suggested. None of these correlates well with clinical manifestations. Patients with severe malabsorption have jejunal colonization with enteric bacteria or yeasts, suggesting that these organisms may act synergistically with Giardia. Eradication of the associated microorganism, however, has not uniformly resulted in clinical improvement. Jejunal biopsies sometimes reveal a flattening of the microvilli and an inflammatory infiltrate, the severity of which correlates roughly with that of the clinical disease. Generally, both malabsorption and the jejunal lesions have been reversed with specific treatment. The demonstration of occasional trophozoites in the submucosa raises the possibility that these changes reflect T-lymphocyte–mediated damage.
Basis for malabsorption and jejunal pathology remains uncertain
Susceptibility to giardiasis has been related to several factors, including strain virulence, inoculum size, achlorhydria or hypochlorhydria, and immunologic abnormalities. In one experimental study, humans were challenged with varying doses from as few as 10 cysts. They were uniformly parasitized when 100 or more were ingested. Several workers have noted the frequency with which giardiasis occurs in achlorhydric and hypochlorhydric individuals. Giardia infection produces little or no host inflammation suggesting that local responses may help control the infection. Both innate responses involving nitric oxide, defensins, phagocytic, mast and dendritic cells, and adaptive responses involving IgA and T cells have been identified in mouse models of infections and are thought to operate in human infections as well. Animal studies have demonstrated that Giardia-specific, secretory IgA (sIgA) antibodies inhibit attachment of trophozoites to intestinal epithelium, perhaps by blocking parasite surface lectins. Moreover, antitrophozoite IgM or IgG antibodies, plus complement, are known to be capable of killing Giardia trophozoites. Another indication that antibodies play a role in controlling infections is that humans with immunodeficiencies involving antibody production are more likely to suffer from chronic giardiasis. Giardia trophozoites are also capable of changing their surface coat variant surface proteins (VSPs). VSP switching appears to be transcriptionally controlled. Over 200 VSP genes have been identified for this organism. This process occurs once every 6 to 16 generations. The process of VSP switching undoubtedly helps the organism evade host responses.
Predisposing factors include hypochlorhydria and immunocompromise
GIARDIASIS: CLINICAL ASPECTS
In endemic situations, over two-thirds of persons infected with giardiasis are asymptomatic. In acute outbreaks, this ratio of asymptomatic to symptomatic patients is usually reversed. When they do occur, symptoms begin 1 to 3 weeks after exposure and typically include diarrhea, which is sudden in onset and explosive in character. The stool is foul-smelling, greasy in appearance, and floats. It is devoid of blood or mucus. Upper abdominal cramping is common. Large quantities of intestinal gas produce abdominal distention, sulfuric eructations, and abundant flatus. Nausea, vomiting, and low-grade fever may be present. The acute illness generally resolves in 1 to 4 weeks; in children, however, it may persist for months, leading to significant malabsorption, weight loss, and malnutrition.
Subclinical infections common in endemic areas
Diarrhea, cramping, flatus, and greasy stools
In many adults, the acute phase of giardiasis is often followed by a subacute or chronic phase characterized by intermittent bouts of mushy stools, flatulence, “heartburn,” and weight loss that persist for weeks or months. At times, patients presenting in this fashion deny having experienced the acute syndrome described previously. In the majority, symptoms and organisms eventually disappear spontaneously. It is not uncommon for lactose intolerance to persist after eradication of the organisms. This condition may be confused with an ongoing infection, and the patient may be subjected to unnecessary treatment.
Subacute and chronic infections with weight loss in adults
Lactose intolerance may persist
The diagnosis of giardiasis is made by finding the cyst in formed stool or the trophozoite in diarrheal stools, duodenal secretions, or jejunal biopsy specimens. In acutely symptomatic patients, the parasite can usually be demonstrated by examining one to three stool specimens after appropriate concentration and staining. In chronic cases, excretion of the organism is often intermittent, making parasitologic confirmation more difficult. Many of these patients can be diagnosed by examining specimens taken at weekly intervals over 4 to 5 weeks. Another approach is to perform an enterotest, in which a bead encapsulated in a gelatinous capsule and attached to a thread is swallowed and then retrieved. The recovered bead is washed onto a slide and examined for active trophozoites. Alternatively, duodenal secretions can be collected and examined for trophozoites in trichrome or Giemsa-stained preparations. There are now a number of reliable, commercially available, enzyme immunoassays (EIAs) for the direct detection of parasite antigen in stool. They appear to be as sensitive and specific as microscopic examinations. Immunofluorescent assays for the detection of cysts are also available. The organism can be grown in culture, but the methods are not currently adaptable to routine diagnostic work. NAA assays are highly sensitive and can distinguish infecting genotypes.
Demonstration of trophozoites and cysts in stool or duodenal aspirates diagnostic
EIAs detect Giardia antigen in stool
Five drugs are currently available for the treatment of giardiasis in the United States: quinacrine hydrochloride, metronidazole, tinidazole, furazolidone, and paromomycin. Quinacrine and metronidazole are effective (70%-95%) and are preferred for patients capable of ingesting tablets. Furazolidone is used by pediatricians because of its availability as a liquid suspension, but it has the lowest cure rate. These three agents require 5 to 7 days of therapy. Tinidazole, an oral agent that has been widely used in many countries for more than 25 years outside the United States, is safe and effective as a single-dose treatment. This drug has been shown to be the most effective. It has been available in the United States since 2004. Because of the potential for person-to-person spread, it is important to examine and, if necessary, treat close physical contacts of the infected patient, including playmates at nursery school, household members, and sexual contacts. None of the aforementioned agents should be used in pregnant women because of their potential teratogenicity. Paromomycin, a nonabsorbed but somewhat less effective agent, may be used in this circumstance.
Several drugs available
Close contacts should be examined
Hikers should avoid ingestion of untreated surface water, even in remote areas, because of the possibility of contamination by feces of other people and potentially by feces of infected animals. Adequate disinfection can be accomplished with halogen tablets yielding concentrations higher than that generally achieved in municipal water systems. The safety of the latter results from additional flocculation and filtration procedures. Use of portable filtration units having a nominal pore size of 1 μm is even more effective. Boiling of water, if possible, is even better.
Avoid drinking untreated surface water
BLOOD AND TISSUE FLAGELLATES
Two of the many genera of hemoflagellates, Leishmania and Trypanosoma, are pathogenic to humans. They reside and reproduce within the gut of specific insect hosts. When these vectors feed on a susceptible mammal, the parasite penetrates the feeding site, invades the blood and/or tissue of the new host, and multiplies to produce disease. American trypanosomes differ somewhat in that the infective parasite is passed in the feces of the specific vector during the act of feeding on its host and later rubbed into the feeding site wound. The life cycle is completed when a second insect ingests the infected mammalian blood or tissue fluid. During the course of their passage through insect and vertebrate hosts, flagellates undergo developmental change. Within the gut of the insect (and in culture media), the organism assumes the promastigote (Leishmania) or epimastigote (Trypanosoma) form (Figure 53–4). These protozoa are motile and fusiform and have a blunt posterior end and a pointed anterior end from which a single flagellum projects. They measure 15 to 30 μm in length and 1.5 to 4.0 μm in width. In the promastigote form, the kinetoplast complex is located in the anterior extremity, and the flagellum exits from the cell immediately. The kinetoplast complex of the epimastigote form, in contrast, is located centrally, just in front of the vesicular nucleus. The flagellum runs anteriorly in the free edge of an undulating membrane before passing out of the cell. In the mammalian host, hemoflagellates appear as trypomastigotes (Trypanosoma) or amastigotes (Leishmania, T cruzi). The former circulate in the bloodstream and closely resemble the epimastigote form, except that the kinetoplast complex is in the posterior end of the parasite. The amastigote stage is found intracellularly. It is round or oval, measures 1.5 to 5.0 μm in diameter, and contains a clear nucleus with a central karyosome. Although it has a kinetoplast complex and an axoneme, there is no free flagellum.
Stages in the life cycle of the hemoflagellates (Trypanosomidae).
Life cycle includes insect host stage
Promastigote and epimastigote forms in insects
Trypomastigote and amastigote forms in humans
The flagellated forms move in a spiral fashion, and all reproduce by longitudinal binary fission. The flagellum itself does not divide; rather, a second one is generated by one of the two daughter cells. The organisms use carbohydrate obtained from the body fluids of the host in aerobic respiration. Glycolysis is carried out in structures called glycosomes. In addition, these organisms possess a kinetoplast/mitocondrion complex. Up to 15% of total cellular DNA is found within the kinetoplast. Profound changes occur in this complex as the parasite transits from its vertebrate to invertebrate host since the parasite needs to respire more efficiently under conditions encountered in the latter host.
Leishmania species are obligate intracellular parasites of mammals. Several strains can infect humans; they are all morphologically similar, resulting in some confusion over their proper speciation. Definitive identification of these strains requires isoenzyme analysis, monoclonal antibodies, kinetoplast DNA buoyant densities, DNA hybridization, and DNA restriction endonuclease fragment analysis or chromosomal karyotyping using pulse-field electrophoresis. The many strains can be more simply placed in four major groups based on their serologic, biochemical, cultural, nosologic, and behavioral characteristics. For the sake of clarity, these groups are discussed as individual species. Each, however, contains a variety of strains that have been accorded separate species or subspecies status by some authorities. The organisms can be propagated in hamsters and in a variety of commercially available liquid media.
Species morphologically similar; differ in molecular features
It is estimated that over 20 million people worldwide suffer from leishmaniasis, and 1 to 2 million additional individuals acquire the infection annually. Leishmania tropica in the Old World and L mexicana in the New World produce a localized cutaneous lesion or ulcer, known popularly as oriental sore or chiclero ulcer; L braziliensis is the cause of American mucocutaneous leishmaniasis (espundia); and L donovani and L infantum are the etiologic agents of kala azar, a disseminated visceral disease.
Cutaneous ulcer or visceral infection (kala azar) the primary diseases
All five groups are transmitted by phlebotomine sandflies. These small, delicate, short-lived insects are found in animal burrows and crevices throughout the tropics and subtropics. At night, they feed on a wide range of mammalian hosts. Amastigotes ingested in the course of a meal assume the flagellated promastigote form, multiply within the gut, and eventually migrate to the proboscis. When the fly next feeds on a human or animal host, the promastigotes are injected into the skin of the new host together with salivary peptides capable of inactivating host macrophages. Here, they activate complement by the classic (L donovani) or alternative pathway and are opsonized with C3, which mediates attachment to the CR1 and CR3 complement receptors of macrophages. After phagocytosis, the promastigotes lose their flagella and multiply as the rounded amastigote form within the phagolysosome. In stained smears, the parasites take on a distinctive appearance and have been termed Leishman–Donovan bodies. Intracellular survival is mediated by a surface lipophosphoglycan and an abundance of membrane-bound acid phosphatase, which inhibit the macrophage's oxidative burst and/or inactivate lysosomal enzymes. Continued multiplication leads to the rupture of the phagocyte and release of the daughter cells. Some may be taken up by a feeding sandfly; most invade neighboring mononuclear cells.
All five groups transmitted by nocturnally feeding sandflies
Complement activation mediates attachment to macrophages
Intracellular survival by inhibiting macrophage killing mechanisms
Amastigotes released from macrophages can infect feeding sandfly
Continuation of this cycle results in extensive histiocytic proliferation. The course of the disease at this point is determined by the species of parasite and the response of the host's T cells. CD4+ T cells of the TH1 type secrete interferon (IFN)-γ in response to leishmanial antigens. This, in turn, activates macrophages to kill intracellular amastigotes by the production of toxic nitric oxide. In the localized cutaneous forms of leishmaniasis, this immune response results in the development of a positive delayed skin (leishmanin) reaction, lymphocytic infiltration, reduction in the number of parasites, and, eventually, spontaneous disappearance of the primary skin lesion. In infections with L braziliensis, this sequence may be followed weeks to months later by mucocutaneous metastases. These secondary lesions are highly destructive, presumably as a result of the host's hypersensitivity to parasitic antigens. Scrapings from these lesions show a noticeable absence of lymphocytes indicating that the cell-mediated immune response has been impaired.
In localized cutaneous disease, cellular immune responses produce spontaneous cure
Mucocutaneous metastases in L braziliensis infections
Some strains of L tropica and L mexicana fail to elicit an effective intracellular immune response in certain hosts. Such patients appear to have a selective suppressor T-lymphocyte–mediated anergy to leishmanial antigens. Consequently, there is no infiltration of lymphocytes or decrease in the number of parasites. The skin test remains negative, and the skin lesions disseminate and become chronic (diffuse cutaneous leishmaniasis). In infections with L donovani, there is a more dramatic inhibition of the TH1 response. The leishmanial organisms are able to disseminate through the bloodstream to the visceral organs, possibly because of a relative resistance of L donovani to the natural microbicidal properties of normal serum, and/or their ability to better survive at 37°C than strains of Leishmania, causing cutaneous lesions. Although dissemination is associated with the development of circulating antibodies, they do not appear to serve a protective function and may, via the production of immune complexes, be responsible for the development of glomerulonephritis. A simplified outline of the immune responses in different forms of leishmaniasis is presented in Table 53–2.
TABLE 53–2Immune Response to Leishmaniasis ||Download (.pdf) TABLE 53–2 Immune Response to Leishmaniasis
|HUMAN DISEASE ||PARASITE ||LEISHMANIN SKIN TEST ||NUMBER OF LYMPHOCYTES ||NUMBER OF PARASITES ||PROGNOSIS ||HUMORAL ANTIBODY TITER |
|Localized skin ulcer (oriental sore, chiclero ulcer, uta) || |
|Positive ||Many ||Few ||Good ||Low |
|Mucocutaneous lesions (espundia) ||L braziliensis ||Positive ||Many ||Few ||Poor ||Low |
|Disseminated cutaneous || || || || || || |
|Ethiopian ||L tropicaa ||Negative ||Few ||Many ||Poor ||High |
|American ||L mexicanaa || || || || || |
|Disseminated visceral (kala azar) ||L donovani ||Negative ||Few ||Many ||Poor ||High |
Lack of cellular immune response in disseminated and chronic infections
LOCALIZED CUTANEOUS LEISHMANIASIS
Cutaneous leishmaniasis is a zoonotic infection of tropical and subtropical rodents. It is particularly common in areas of Central Asia, the Indian subcontinent, Middle East, Africa, the Mediterranean littoral, and Central and South America. In the latter area, L mexicana infects several species of arboreal rodents. Humans become involved when they enter forested areas to harvest chicle for chewing gum and are bitten by infected sandflies. In the Eastern Hemisphere, the desert gerbil and other burrowing rodents serve as the reservoir hosts of L tropica. Human infection occurs when rural inhabitants come in close contact with the burrows of these animals. In the Mediterranean area, southern Russia, and India, human disease involves urban dwellers, primarily children. In this setting, the domestic dog serves as the reservoir, although sandflies may also transmit L tropica directly from human to human.
Geographic distribution related to human and rodent reservoirs
Canine reservoir in urban disease
LOCALIZED CUTANEOUS LEISHMANIASIS
Lesions usually appear on the extremities or face (the ear in cases of chiclero ulcer) weeks to months after the bite of the sandfly (Figure 53–5). They first appear as pruritic papules, often accompanied by regional lymphadenopathy. In a few months, the papules ulcerate, producing painless craters with raised erythematous edges, sharp walls, and a granulating base. Satellite lesions may form around the edge of the primary sore and fuse with it. Multiple primary lesions are seen in some patients. Spontaneous healing occurs in 3 to 12 months, leaving a flat, depigmented scar. Occasionally, the lesions fail to heal, particularly on the ears, leading to progressive destruction of the pinna. A permanent strain-specific immunity usually follows healing. Multiple, disseminated nonhealing lesions may be seen in patients with acquired immunodeficiency syndrome (AIDS).
Cutaneous leishmaniasis. A well-developed lesion on the forehead of a 7-year-old girl. (Reproduced with permission from Connor DH, Chandler FW, Schwartz DQ, et al: Pathology of Infectious Diseases. Stamford CT: Appleton & Lange, 1997.)
Chronic, self-limiting skin ulceration
In endemic areas, the diagnosis of localized cutaneous leishmaniasis is made on clinical grounds and confirmed by the demonstration of the organism in the advancing edge of the ulcer. Material collected by biopsy, curettage, or aspiration is smeared and/or sectioned, stained, and examined microscopically for the pathognomonic Leishman–Donovan bodies. Material should also be cultured in liquid media. The leishmanin skin test becomes positive early in the course of the disease and remains so for life. Recently, it has been demonstrated that small numbers of Leishmania may be detected in tissue by NAA methods, and strains distinguished with probes to kinetoplast DNA. These techniques, though not widely available, permit direct, rapid, and specific diagnosis of all leishmanial infections.
Demonstration of Leishman–Donovan bodies or culture from tissue biopsy
Patients with small, cosmetically minor lesions that do not involve the mucous membrane may be carefully followed without treatment. Pentavalent antimonial agents and liposomal amphotericin B have proved to be effective chemotherapeutic agents for individuals with more consequential lesions. Recently, ketoconazole and itraconazole, alone or in combination with the previously mentioned agents, have been found to be effective in some forms of cutaneous leishmaniasis. Paromomycin has also proved to be useful. What has become clear is that what works for one form of cutaneal leishmaniasis may not work for another. Combinations of thermotherapy and drugs have also been tried. Bacterial superinfections are treated with appropriate antibiotics. Prophylactic measures include the control of the sandfly vector by use of insect repellents and fine mesh screening on dwellings.
Leishmania braziliensis causes a natural infection in the large forest rodents of tropical Latin America. Sandflies transmit the infection to humans engaged in military activities, road builders, opening jungle areas for new settlements, and others.
Rodent reservoir of L braziliensis
A primary skin lesion similar to oriental sore develops 1 to 4 weeks after sandfly exposure. Occasionally, it undergoes spontaneous healing. More commonly, it progressively enlarges, often producing large vegetating lesions. After a period of weeks to years, painful, destructive, metastatic mucosal lesions of the mouth, nose, and occasionally the perineum, appear in 2% to 50% of the patients. Sometimes, decades pass and the primary lesion totally resolves before the metastases manifest themselves. Destruction of the nasal septum produces the characteristic tapir nose. Erosion of the hard palate and larynx may render the patient aphonic. In blacks, the lesions are often large, hypertrophic, polypoid masses that deform the lips and cheeks. Fever, anemia, weight loss, and secondary bacterial infections are common. Mucosal lesions caused by other Leishmania species may be seen after visceral dissemination in AIDS patients.
Primary lesion metastasizes to oral and nasal areas
The diagnosis of mucocutaneous leishmaniasis is made by finding the organisms in the lesions as described for localized cutaneous leishmaniasis. Because the propensity to metastasize to mucocutaneous sites is specific to certain species and subspecies, precise identification of the responsible organism as described in the introduction is of clinical importance. The leishmanin skin test yields positive results, and most patients have detectable antibodies. As described for cutaneous leishmaniasis, it is now possible to provide a rapid, direct, species-specific diagnosis through the use of NAA methods and probes to kinetoplast DNA.
Treatment is accomplished with the agents described later in the chapter for kala azar. Advanced lesions are often refractory and relapse is common. Cured patients are immune to reinfection. Control measures, other than insect repellents and screening of dwellings, are impractical because of the sylvatic nature of the disease.
Detection of organisms as with cutaneous leishmaniasis
DISSEMINATED VISCERAL LEISHMANIASIS (KALA AZAR)
Kala azar is caused by L donovani and L infantum. Leishmania donovani is found in East Africa and the Indian subcontinent, whereas L infantum is found in Europe, North Africa, and Latin America. Its epidemiologic and clinical patterns vary from area to area. In Africa, rodents serve as the primary reservoir. Human cases occur sporadically, and the disease is often acute and highly lethal. In Eurasia and Latin America, the domestic dog is the most common reservoir. Human disease is endemic, primarily involves children, and runs a subacute to chronic course. In India, the human is the only known reservoir, and transmission is carried out by anthropophilic species of sandflies. The disease recurs in epidemic form at 20-year intervals, when a new cadre of nonimmune children and young adults appears in the community. There appears to be a high incidence of visceral leishmaniasis in patients with HIV infection. Presumably, HIV-induced immunosuppression either facilitates acquisition of the disease and/or allows reactivation of latent infection.
Marked geographic differences in reservoirs and disease severity
After the host is bitten by an infected sandfly, the parasites disseminate in the bloodstream and are taken up by the macrophages of the spleen, liver, bone marrow, lymph nodes, skin, and small intestine. Histiocytic proliferation in these organs produces enlargement with atrophy or replacement of the normal tissue.
Parasites invade macrophages of reticuloendothelial system
KALA AZAR: CLINICAL ASPECTS
The majority of kala azar infections are asymptomatic; these become symptomatic years later during periods of host immunocompromise. Symptomatic disease most commonly manifests itself 3 to 12 months after acquisition of the parasite. It is often mild and self-limited. A minority of infected individuals develop the classic manifestations of kala azar. Fever, which is usually present, may be abrupt or gradual at onset. It persists for 2 to 8 weeks and then disappears, only to reappear at irregular intervals during the course of the disease. A double-quotidian pattern (two fever spikes in a single day) is a characteristic but uncommon finding. Diarrhea and malabsorption are common in Indian cases, resulting in progressive weight loss and weakness. Physical findings include enlarged lymph nodes and liver, massively enlarged spleen, and edema. In light-skinned persons, a grayish pigmentation of the face and hands is commonly seen, which gives the disease its name (kala azar, black disease). Anemia with resulting pallor and tachycardia are typical in advanced cases. Thrombocytopenia induces petechial formation and mucosal bleeding. The peripheral leukocyte count is usually less than 4000/mm3; agranulocytosis with secondary bacterial infections contributes to lethality. Serum IgG levels are enormously elevated, but play no protective role. Circulating antigen–antibody complexes are present and are probably responsible for the glomerulonephritis seen so often in this disease.
Delayed onset, recurrent fever, chronic disease, diarrhea
Severe systemic manifestations
Immune complex glomerulonephritis
The diagnosis of kala azar is made by demonstrating the presence of the organism in aspirates taken from the bone marrow, liver, spleen, or lymph nodes. In the Indian form of kala azar, L donovani is also found in circulating monocytes. The specimens may be smeared, stained, and examined for the typical Leishman–Donovan bodies (amastigotes in mononuclear phagocytes) or cultured in artificial media and/or experimental animals. As described for cutaneous leishmaniasis, a limited number of reference laboratories can provide a rapid, direct, species-specific diagnosis through the use of NAA and probes to kinetoplast DNA. Results of the leishmanin skin test are negative during active disease but become positive after successful therapy.
Demonstration of Leishman–Donovan bodies or culture
The mortality rate in untreated cases of kala azar is 75% to 90%. Treatment with pentavalent antimonial drugs lowers this rate dramatically. Initial therapy, however, fails in up to 30% of African cases, and 15% of those that do respond eventually relapse. Resistant cases are treated with the more toxic pentamidine, amphotericin B, or liposomal amphotericin B. Allopurinol and IFN-γ have proved to be useful adjunctive therapies in resistant cases. A new oral drug, miltefosine, has been shown to be very efficient and safe for both cutaneal and visceral leishmaniasis. Post-Kala azar dermal leishmaniasis, a condition marked by hypopigmented macules, papules, nodules, or facial erythema may appear many years after partial or even successful treatment of visceral leishmaniasis, particularly caused by L donovani. The lesions can be confused with those caused by leprosy. The lesions coincide with IFN-γ–producing cells causing skin inflammation as a reaction to persisting parasites in the skin. Patients need to be treated as those for visceral leishmaniasis. Control measures are directed at the Phlebotomus vector, with the use of residual insecticides, and at the elimination of mammalian reservoirs by treating human cases and destroying infective dogs.
Up to 90% mortality rate without treatment
The trypanosomes that comprise this group are all related to an ancestral Trypanosma brucei. They are morphologically identical, but vary in their disease-producing capabilities in animals and humans. The three subspecies, known as T brucei brucei, T brucei gambiense, and T brucei rhodesiense, can be distinguished by their biologic characteristics, host preferences, zymodeme types, and DNA hybridization patterns. Trypanosoma brucei only infects animals due to the presence of a lytic factor in human serum, while T brucei gambiense and T brucei rhodesiense give rise to West African and East African Sleeping Sickness in humans, respectively. All undergo similar developmental changes in the course of their passage between their insect (tsetse fly) and mammalian host. On ingestion by the tsetse fly (Glossina spp.) and after a period of multiplication in the midgut, the parasites migrate to the insect's salivary glands and assume the epimastigote form. After a period of time they are transformed into metacyclic trypomastigotes, rendering them infectious to mammals. When the fly again takes a meal, the parasites are inoculated with the fly's saliva. Newly emerged and young flies are more efficient transmitters of the disease than older flies. A highly variable surface glycoprotein (VSG) coat, which is acquired in the tsetse fly, accounts for this organism's ability to undergo a process of antigenic variation in its mammalian host. The parasite enters the bloodstream and trypomastigote stage parasites referred to as slender forms divide by longitudinal fission every 5 to 10 hours. For reasons independent of the host's immune response, multiplication eventually slows and some parasites of a dominant population of organisms assume a short, stumpy appearance. These forms have more of developed kinetoplast–mitochondrial complex and constitute the parasites that are infective to the tsetse fly. Near the end of the episode of parasitemia, both slender and stumpy types may be seen in a single blood specimen. Metacyclic trypomastigotes inoculated by a tsetse fly usually contain a population of organisms dominated by a distinctive antigenic type. After a period of time in the vertebrate host, usually a week or so, the antigenic variant type changes. This change is under the control of up to 1000 genes that have been identified in some strains of these organisms that can account for a change in the variant surface glycoprotein antigenic type. Each dominant population usually contains a few organisms that have already undergone antigenic change so that when the host responds immunologically to the dominant population there will be survivors that give rise to the next dominant population. Expression of individual genes largely appears to be controlled by the sequential duplication and subsequent transfer of each gene (expression-linked copy) to one or more areas of the genome responsible for gene expression. Genes located near expression loading sites and referred to as nonduplication activated genes also can give rise to new, and sometimes repeat, antigenic types.
Three recognized subspecies of T brucei
Epimastigote and trypomastigote forms develop in tsetse fly
Infectious trypomastigote form injected into the bloodstream of mammalian host from the fly's saliva
Antigenic variation of glycoprotein coat of trypomastigotes is due to shifting expression of preexisting genes
AFRICAN TRYPANOSOMIASIS (SLEEPING SICKNESS)
African trypanosomiasis is a highly lethal meningoencephalitis transmitted to humans by bloodsucking flies of the genus Glossina. It occurs in two distinct clinical and epidemiologic forms: West African or Gambian sleeping sickness and East African or Rhodesian sleeping sickness. Nagana, a disease of cattle caused by a closely related trypanosome, renders over 10 million square kilometers of Central Africa unsuitable for animal husbandry.
The tsetse fly, and consequently sleeping sickness, is confined to the central area of Africa between the continent's two great deserts, the Sahara in the north and the Kalahari in the south. The disease is also separated into West and East African forms and is loosely divided by the Rift Valley. Approximately 50 million people live in this area, and 10 000 to 20 000 acquire sleeping sickness annually. Because of the activity of many species of tsetse flies that transmit sleeping sickness and other trypanosome infections of animals, it has been estimated that an additional 100 000 000 cattle cannot be raised in this tsetse-infested area. Major outbreaks of human infection have been reported in several locations within the endemic area over the past two decades, partly as a result of the internecine wars in this area that have interrupted control programs. Although an estimated 20 000 Americans travel to endemic areas each year, less than two dozen cases of African trypanosomiasis have been diagnosed in Americans since 1967.
Tsetse fly confined to Central Africa
Riverine tsetse flies found in the forest galleries that border the streams of West and Central Africa serve as the vectors of the Gambian disease. Although these flies are not exclusively anthropophilic, humans are thought to be the major reservoirs of the parasite. The infection rate in humans is affected by proximity to water but seldom exceeds 2% to 3% in nonepidemic situations. Nevertheless, the extreme chronicity of the human disease ensures its continued transmission.
Humans major reservoirs of West African sleeping sickness; chronicity ensures maintenance
Rhodesian sleeping sickness, in contrast, is transmitted by flies indigenous to the great savannas of East Africa that feed on the blood of the small antelope and other ruminants inhabiting these areas. The antelope serves as a principal parasite reservoir, although human-to-human and cattle-to-human spread has been documented. Humans typically become infected when they enter the savanna to hunt or to graze their domestic animals. The Sudan is one country where both the Gambian and Rhodesian forms of sleeping sickness are still found. Continued civil strife and deforestation in other countries could change that picture. At present, there is little evidence of coinfections with African trypanosomes and HIV, possibly because the former is primarily rural in distribution and the latter is concentrated in cities and also because major immune responses to trypanosomes are largely antibody mediated and bypass T cells.
Savanna antelopes are reservoirs of East African trypanosomiasis; humans infected incidentally
PATHOGENESIS AND IMMUNE RESPONSIVENESS
Multiplication of the trypomastigotes at the inoculation site produces a localized inflammatory lesion. After the development of this chancre, organisms spread through lymphatic channels to the bloodstream, inducing a proliferative enlargement of the lymph nodes. The subsequent parasitemia is typically low grade and recurrent. Replicating organisms of the dominant antigenic type continuously produce surface glycoproteins. Much of this is shed from the parasite's surface and serves as a T-cell–independent antigen to directly stimulate B cells to produce antibody. The antibody produced in this fashion is IgM which can bind to the organism, leading to its destruction by lysis and opsonization. The trypomastigotes disappear from the blood, reappearing 3 to 8 days later as a new dominant antigenic variant arises. The recurrences gradually become less regular and frequent, but may persist for weeks to years before finally disappearing. During the course of parasitemia, trypanosomes localize in the small blood vessels of the heart and central nervous system (CNS). This localization results in endothelial proliferation and a perivascular infiltration of plasma cells and lymphocytes. In the brain, hemorrhage and a demyelinating panencephalitis may follow.
Local chancre at the site of inoculation and lymphadenitis
Intermittent parasitemia with antigenic shifts
Parasites localize in blood vessels of heart and CNS with local vasculitis
The mechanism by which the trypanosomes elicit vasculitis is uncertain. The infection stimulates a massive, nonspecific polyclonal activation of B cells, the production of large quantities of IgM (typically 8-16 times the normal limit), and the suppression of other immune responses. Most of this reaction represents specific protective antibodies that are ultimately responsible for the control of the parasitemia. Some, however, consist of nonspecific heterophile antibodies, antibodies to DNA, and rheumatoid factor. Antibody-induced destruction of trypanosomes releases invariant nuclear and cytoplasmic antigens with the production of circulating immune complexes. Many authorities believe that these complexes are largely responsible for anemia and vasculitis seen in this disease.
High levels of IgM include specific and nonspecific antibodies
Immune complexes may cause anemia and vasculitis
AFRICAN TRYPANOSOMIASIS (SLEEPING SICKNESS): CLINICAL ASPECTS
The trypanosomal chancre appears 2 to 3 days after the bite of the tsetse fly as a raised, reddened nodule on one of the exposed surfaces of the body. With the onset of parasitemia 2 to 3 weeks later, the patient develops recurrent bouts of fever, tender lymphadenopathy, skin rash, headache, and impaired mentation. In the Rhodesian form of disease, myocarditis and CNS involvement begin within 3 to 6 weeks. Heart failure, convulsions, coma, and death follow in 6 to 9 months. Gambian sleeping sickness progresses more slowly. Bouts of fever often persist for years before CNS manifestations gradually appear. Spontaneous activity progressively diminishes, attention wavers, and the patient must be prodded to eat or talk. Speech grows indistinct, tremors develop, sphincter control is lost, and seizures with transient bouts of paralysis occur. In the terminal stage, the patient develops a lethal intercurrent infection or lapses into a final coma. Recent studies have shown great variability in the progression of both the Rhodesian and Gambian forms of the disease.
Raised red papule on exposed surface
Parasitemic manifestations 2 to 3 weeks later
Late CNS involvement
A definitive diagnosis is made by microscopically examining lymph node aspirates, blood, or cerebrospinal fluid for the presence of trypomastigotes (Figure 53–6). Early in the disease, actively motile organisms can often be seen in a simple wet mount preparation smear; identification requires examination of an appropriately stained smear. If these tests prove negative, the blood can be centrifuged and the stained buffy coat examined. Inoculation of rats or mice can also prove helpful in diagnosing the Rhodesian disease. The patient may also be screened for elevated levels of IgM in the blood and spinal fluid or specific trypanosomal antibodies by a variety of techniques. A card agglutination test for trypanosomiasis (CATT), which can be performed on fingerstick blood, can provide serologic confirmation within minutes. Subspecies-specific DNA probes may eventually prove useful for the identification of organisms in clinical specimens.
African sleeping sickness. Trypanosoma brucei in a routine blood smear. (Reproduced with permission from Nester EW: Microbiology: A Human Perspective, 6th edition. 2009.)
Trypomastigotes sought in lymph node aspirates, blood, and cerebrospinal fluid
Animal inoculation may be required in Rhodesian disease
Lumbar puncture must always be performed before initiation of therapy for sleeping sickness. If the specimen reveals evidence of CNS involvement, agents that penetrate the blood–brain barrier must be included. Unfortunately, the most effective agent of this type is a highly toxic arsenical, melarsoprol (Mel B). Although this agent occasionally produces a lethal hemorrhagic encephalopathy, the invariably fatal outcome of untreated CNS disease warrants its use. The ornithine decarboxylase inhibitor, eflornithine (DFMO) appears capable, when used alone, or in combination with suramin, of curing CNS disease caused by T brucei gambiense without the serious side effects associated with melarsoprol. Unfortunately, it is very expensive and is only variably effective in T brucei rhodesiense infections. If the CNS is not yet involved, less toxic agents, such as suramin, pentamidine, or eflornithine, can be used. In such cases, the cure rate is high and recovery complete.
Selection of drugs dependent on whether CNS is involved
Without CNS involvement, recovery often complete
Although a variety of tsetse fly control measures, including the use of insecticides, deforestation, and the introduction of sterile males into the fly population, have been attempted, none has proved totally practicable. The tsetse fly is larviparous and carries a larva within its body until mature and ready to pupate. This means flies have a better chance of survival. In addition, adults are strong fliers. Similarly, eradication of disease reservoirs by the early detection and treatment of human cases and the destruction of wild game has had limited success. Attempts to develop effective vaccines are currently underway but are complicated by the antigenic variability of the trypanosomes. A degree of personal protection can be achieved with insect repellents and protective clothing. Although prophylactic use of pentamidine was once advocated, enthusiasm for this treatment has waned.
Neither vector or reservoir control has been successful
The trypomastigotes of T cruzi are smaller than those of T brucei and typically assume a C shape when seen in the peripheral circulation. Their developmental cycle differs in several respects from that of T brucei. Most significant, T cruzi does not multiply in the blood stream. The circulating trypomastigotes must invade tissue cells, lose their flagella, and assume the amastigote form before binary fission can occur. Continued multiplication as amastigotes and epimastigotes in intracellular nests leads to distention and eventual rupture of the tissue cell. Released trypomastigotes regain the bloodstream. This new generation of trypomastigotes may invade other host cells, thus continuing the mammalian cycle. Alternatively, they may be ingested by a feeding reduviid and develop into epimastigotes within its midgut. On completion of the invertebrate cycle, the parasites migrate to the hindgut and are discharged as infectious metacyclic trypomastigotes when the reduviid defecates in the process of taking another blood meal. This process can recur at each feeding for as long as 2 years. Infection in the new host is initiated when the trypomastigotes contaminate either the feeding site or the mucous membranes.
Mammalian cycle with nondividing extracellular trypomastigotes and dividing intracellular amastigotes and epimastigotes
Invertebrate cycle produces trypomastigotes in bug
Trypanosoma cruzi comprises a number of strains, each with its own distinct geographic distribution, tissue preference, and virulence. These strains may be distinguished from one another with specific antisera and by differences in their isoenzyme and DNA restriction patterns. All are somewhat morphologically similar. In blood specimens, the trypomastigotes can be distinguished from those of T brucei by their characteristic C or U shape, narrow undulating membrane, and large posterior kinetoplast. Trypanosoma cruzi does not undergo antigenic variation.
AMERICAN TRYPANOSOMIASIS (CHAGAS DISEASE)
American trypanosomiasis is a disease produced by T cruzi and transmitted by true bugs of the family Reduviidae. Clinically, the infection presents as an acute febrile illness in children and a chronic heart or gastrointestinal malady in adults.
Chagas disease affects 10 to 15 million people in a geographic area extending from Mexico to southern Argentina, producing death in 50 000 annually. Within these areas, it is the leading cause of chronic heart disease, accounting for 25% of all deaths in the 25- to 44-year age group. Transmission occurs primarily in rural settings, where the reduviid can find harborage in animal burrows and in the cracked walls and thatch of poorly constructed buildings. This large (3 cm) insect leaves its hiding place at night to feed on its sleeping hosts. Its predilection to bite near the eyes or lips have earned this pest the nicknames of “kissing bug” and “assassin bug.” Most new infections in these areas occur in children. Infections can also be acquired transplacentally and through blood transfusions or organ transplantations.
Chagas disease in South and Central America
“Kissing bug” feeds at night in rural areas
In addition to humans, a number of wild and domestic animals, including rats, cats, dogs, opossums, racoons, and armadillos, serve as reservoirs for Chagas disease. The close association of many of these hosts with human dwellings tends to amplify the incidence of disease in humans and the difficulty involved in its control.
Other wild and domestic animal reservoirs amplify transmission
Organ transplantation and transfusion-related infections are rapidly increasing problems in urban settings within endemic areas. Recrudescence of the latent infection is increasingly seen in immunosuppressed individuals, including patients with HIV infections. More effective blood bank screening provides hope that transmission of this disease will be substantially curtailed in the near future.
An estimated 50 000 infected Latin American immigrants are currently living in the United States. Because T cruzi has been found in both vertebrate and invertebrate hosts in the southern United States, there is a possibility of sustained transmission of this organism within this country. Although serologic evidence suggests that the acquisition of human infection in this area is not uncommon, clinically apparent autochthonous cases have been rare. The majority of these acquired the infection through blood–blood transfusions.
Multiplication of the parasite at the portal of entry stimulates the accumulation of neutrophils, lymphocytes, and tissue fluid, resulting in the formation of a local chancre or chagoma. The subsequent dissemination of the organism with invasion of tissue cells produces a febrile illness that may persist for 1 to 3 months and result in widespread organ damage. Any nucleated host cell may be involved, but those of mesenchymal origin, especially the heart, skeletal muscle, smooth muscle, and ganglion neural cells, are particularly susceptible. Cell entry is facilitated by binding to host cell fibronectin; a 60-kDa T cruzi surface protein (penetrin) appears to promote adhesion. After penetration, the trypomastigote escapes the phagosome via the production of a pore-forming protein, transforms to the amastigote form, and multiplies freely within the cytoplasm to produce a pseudocyst, a greatly enlarged and distorted host cell containing masses of organisms (Figure 53–7). With the rupture of the pseudocyst, many of the released parasites disintegrate, eliciting an intense inflammatory reaction with destruction of surrounding tissue. The development of an antibody-dependent, cell-mediated immune response leads to the eventual destruction of the T cruzi parasites and the termination of the acute phase of illness.
Chagas disease. A. Acute myocarditis with atrophic myofibers separated by inflammatory cells. B. Trypanosoma cruzi amastigotes clustered in myofiber from the same case. (Reproduced with permission from Connor DH, Chandler FW, Schwartz DQ, et al: Pathology of Infectious Diseases. Stamford CT: Appleton & Lange, 1997.)
Local chancre at the site of inoculation
Entry to mesenchymal cells facilitated by fibronectin-binding surface protein
Pore-forming protein aids escape from phagosome
Pseudocysts formed from cytoplasmic multiplication in host cells
Parasitic antigens released during this acute phase may bind to the surface of tissue cells, rendering them susceptible to destruction by the host's immune response. It has been suggested by some that this results in the production of antibodies that cross-react with host tissue, initiating a sustained autoimmune inflammatory reaction in the absence of systemic manifestation of illness. In the heart, this reaction leads to changes in coronary microvasculature, loss of muscle tissue, interstitial fibrosis, degenerative changes in the myocardial conduction system, and loss of intracardiac ganglia. In the digestive tract, loss of both ganglionic nerve cells and smooth muscle results in dilatation and loss of peristaltic movement, particularly of the esophagus and colon.
Damage to heart may have an autoimmune mechanism
Ganglionic and smooth muscle cells lost in digestive tract
AMERICAN TRYPANOSOMIASIS (CHAGAS DISEASE): CLINICAL ASPECTS
Serologic studies suggest that only one-third of the persons newly infected with Chagas disease develop clinical illness. Acute manifestations, when they occur, are seen primarily in children. They begin with the appearance of the nodular, erythematous chagoma 1 to 3 weeks after the bite of the reduviid. If the eye served as a portal of entry, the patient presents with Romaña sign: reddened eye, swollen lid, and enlarged preauricular lymph node. The onset of parasitemia is signaled by the development of a sustained fever; enlargement of the liver, spleen, and lymph nodes; signs of meningeal irritation; and the appearance of peripheral edema or a transient skin rash. In a small percentage of symptomatic patients, heart involvement results in tachycardia, electrocardiographic changes, and occasionally arrhythmia, enlargement, and congestive heart failure. Newborns may experience acute meningoencephalitis. Clinical manifestations persist for weeks to months. In 5% to 10% of untreated patients, severe myocardial involvement or meningoencephalitis leads to death.
Most infections asymptomatic; acute disease usually in children
Myocardial injury indicated by tachycardia and electrocardiographic changes
Chronic disease, the result of end-stage organ damage, is usually seen only in adulthood. Ironically, most patients with late manifestations have no history of acute illness. The most serious of the late manifestations is heart disease. Studies of asymptomatic, seropositive patients in endemic areas have shown that a significant proportion have cardiac abnormalities demonstrated by electrocardiographic, echocardiographic, or cineangiographic techniques, suggesting that Chagas cardiomyopathy is a progressive, focal disease of the myocardium and conduction system, leading eventually to clinical disease. This may present as arrhythmia, thromboembolic events, heart block, enlargement with congestive heart failure, and cardiac arrest. In some areas of rural Latin America, up to 10% of the adult population may show cardiac manifestations. In the United States, chagasic heart disease in immigrants is usually initially misdiagnosed as coronary artery disease or idiopathic dilated cardiomyopathy. Megaesophagus and megacolon, which are less devastating than the heart disease, are typically seen in more southern latitudes. This geographic variation in clinical manifestations is thought to be attributable to a difference in tissue tropism between individual strains of T cruzi. Megaesophagus leads to difficulty in swallowing and regurgitation, particularly at night. Megacolon produces severe constipation with irregular passage of voluminous stools. Trypanosoma cruzi brain abscess has been described in a small number of AIDS patients.
Chronic cardiomyopathy in adults leads to heart block and/or congestive heart failure
Dilatation of esophagus and colon seen in southern latitudes
The diagnosis of acute Chagas disease rests on finding the trypomastigotes in the peripheral blood or buffy coat, and their morphologic identification as T cruzi. The methods are similar to those described for diagnosis of African trypanosomiasis. If the results are negative, a laboratory-raised reduviid can be fed on the patient, then dissected and examined for the presence of parasites, a procedure known as xenodiagnosis. Alternatively, the blood may be cultured in a variety of artificial media or experimental animals. In the diagnosis of chronic disease, recovery of the organisms is the exception rather than the rule, and diagnosis depends on the clinical, epidemiologic, and immunodiagnostic findings. A variety of serologic tests are available; small numbers of false-positive results limit their usefulness, particularly when used as screening procedures in nonendemic areas. The recent production of specific recombinant proteins and synthetic peptides for use as antibody targets may improve the reliability of these procedures. Polymerase chain reaction techniques for the amplification of trypomastigote DNA are available.
Demonstration of trypomastigotes in peripheral blood
Xenodiagnosis involves allowing bugs to feed
Organisms difficult to recover in chronic disease
The role of treatment in Chagas disease remains unsettled. Two agents, nifurtimox and benznidazole, effectively reduce the severity of acute disease but appear to be ineffective in chronic infections. Both drugs must be taken for prolonged periods of time, may cause serious side effects, and do not always result in parasitologic cure. Allopurinol, a hypoxanthine oxidase inhibitor devoid of serious side effects, has recently been shown to be capable of suppressing parasitemia and reversing the serostatus of patients with acute disease. Additional studies to confirm these encouraging results are necessary.
Treatment may reduce acute disease
The reduviid vector can be controlled by applying residual insecticides to rural buildings at 2- or 3-month intervals. The addition of latex to the insecticide creates a colorless paint that prolongs activity. This approach has proven effective because larval instar stages of the kissing bug lack wings and, therefore, stay close to their source of blood. A strong initiative using this approach has been undertaken in the southern portion of South America. Fumigants can be used to prevent reinfection. Patching wall cracks, cementing floors, and moving debris and woodpiles away from human dwellings reduces the number of reduviids within the home. Transfusion-induced disease, a major problem in endemic areas, has been partially controlled by the addition of gentian violet to all blood packs before use or by screening potential donors serologically for Chagas disease. The large number of infected immigrants now entering nonendemic countries presents an increasing risk of transfusion-mediated parasite transmission in these areas as well. Cases of acute Chagas disease have been reported in the United States in immunosuppressed patients who received blood from donors unaware of their infection status; the resulting diseases were particularly fulminant. Immunodiagnostic tests for Chagas disease are neither readily available nor sufficiently specific for use in nonendemic areas; prevention will probably require deferral of blood donations from persons who have recently emigrated from endemic areas. Immunoprophylaxis is not available at present.
Control of reduviid bugs in rural homes most important measure