Chapter 39: Chlamydia

Members of the genus Chlamydia are obligate intracellular bacteria which lack peptidoglycan in their cell wall. Three of the nine species cause disease in humans. Chlamydia trachomatis is the most important human pathogen as a major cause of genital infection and conjunctivitis. A chronic form of C trachomatis conjunctivitis, called trachoma, is the leading preventable cause of blindness in the world. Chlamydophila pneumoniae and Chlamydophila psittaci are respiratory pathogens. Our knowledge of biology and pathogenesis of these bacteria is based primarily on the study of C trachomatis.

BACTERIOLOGY

Chlamydia trachomatis are round cells between 0.3 and 1 μm in diameter depending on the stage in the replicative cycle (see below). Their envelope is of the gram-negative type including an outer membrane that contains lipopolysaccharide and proteins. A major difference is that chlamydiae lack the thin peptidoglycan layer between the outer membrane and the plasma membrane. Although there is no detectable peptidoglycan in chlamydial cells, genomic studies have demonstrated an almost complete set of genes for peptidoglycan synthesis. The outer membrane includes a major outer membrane protein (MOMP) which is immunogenic. Chlamydia are obligate intracellular parasites because they rely on the host cell for key amino acids and energy generating metabolites like ATP. Among bacteria only the mycoplasmas have a smaller genome.

DNA homology between C trachomatis, C psittaci, and C pneumoniae is less than 30%, although rRNA sequence analysis suggests they share a common origin. The three species share a common group antigen. Their major differential features are shown in Table 39–1. Chlamydia trachomatis has three, each with a different tissue tropism. Biovars A-C infect ocular epithelial cell and cause trachoma; biovars D-K target urogenital epithelial cells and cause nongonococcal urethritis (NGU), mucopurulent cervicitis, and inclusion conjunctivitis; and biovars L₁-L₃ infect genital colorectal tissues and cause lymphogranuloma venereum (LGV).

REPLICATIVE CYCLE

The replicative cycle of chlamydiae is illustrated in Figure 39–1. It involves two major forms of the organism: a small, hardy infectious form termed the elementary body (EB), and a larger fragile intracellular replicative form called the reticulate body (RB). The EB is a metabolically inert form that neither expends energy nor synthesizes protein. The cycle begins when the EB attaches to the plasma membrane of susceptible target cells and induces its own endocytosis. This is accomplished in part by the secretion of a preformed translocated actin recruiting protein (Tarp) which induces actin cytoskeletal rearrangements in the target cell. Utilizing stores of ATP the EB then begins the process of converting to the replicative RB. With inhibition of lysosomal fusion in the host cell, the organism forms its own membrane-bound vesicle called the inclusion. After RBs increase in number, the process reverses and the RBs reorganize and condense to yield multiple EBs. They are then released by exocytosis, extrusion of intact inclusions, or cell lysis to infect adjacent cells. The efficiency of this cycle is optimized by a chlamydial protease-like activity factor (CPAF) which regulates cellular apoptosis signals. In the growth phase apoptosis is inhibited, but at the release stage cell death proceeds. Both Tarp and CPAF are injected by secretion systems (type III). Tarp is injected across the plasma membrane, CPAF across the inclusion membrane. A variant in the overall replicative cycle is called the persistent state in which the EBs and RBs become dormant but are still able to resume multiplication. This state can be induced by some cytokines (IFN-γ), nutrient restriction, and interestingly, penicillin. As indicated earlier, Chlamydia lack the peptidoglycan target of penicillin but still have a set of genes for its synthesis.

CHLAMYDIA TRACHOMATIS DISEASE

EPIDEMIOLOGY

Chlamydia trachomatis causes disease in several sites, primarily the conjunctiva and genital tract. In its various forms, this infection is one of the most frequent in the world with an estimated 100 million new cases each year. Humans are the sole reservoir. Inclusion conjunctivitis is seen among population groups in which the strains causing C trachomatis genital infections are common. Chlamydia trachomatis also causes a common form of neonatal conjunctivitis when the newborn comes in direct contact with infective cervical secretions of the mother at delivery.

Trachoma, a chronic follicular conjunctivitis, afflicts an estimated 500 million persons worldwide and blinds 7 to 9 million, particularly in Africa. The disease is usually contracted in infancy or early childhood from the mother or other close contacts. Spread is by contact with infective human secretions, directly via hands to the eye or via fomites transmitted on the legs of flies.

The prevalence of chlamydial urethral infection in US men and women ranges from 5% in the general population to 20% in those attending sexually transmitted disease clinics. Approximately one-third of male sexual contacts of women with C trachomatis cervicitis develop urethritis after an incubation period of 2 to 6 weeks. The proportion of men with mild to absent symptoms is higher than in gonorrhea.

PATHOGENESIS

Chlamydiae have a tropism for columnar epithelial cells of the endocervix and upper genital tract of women (Figure 39–2), and the urethra, rectum, and conjunctiva of both sexes. Depending on the biovar a wide range of other cells may be infected including endothelium, smooth muscle, lymphocytes, and macrophages. Initial attachment is probably mediated by MOMP and possibly other outer membrane proteins followed by cellular invasion by the mechanisms described above. The LGV biovars can also enter through breaks in the skin or mucosa. Once the replication cycle is established, the primary injury is due to inflammation secondary to the release of proinflammatory cytokines such as interleukin-8 by infected epithelial cells. Chlamydial lipopolysaccharides probably also play an important role in initiation of the inflammatory process. This results in early tissue infiltration by polymorphonuclear leukocytes, later followed by lymphocytes, macrophages, plasma cells, and eosinophils. If the infection progresses further (because of lack of treatment and/or failure of immune control), aggregates of lymphocytes and macrophages may form in the submucosa; these can progress to necrosis, followed by fibrosis and scarring. The chronic progressive inflammation with scarring seen in trachoma is due to persistent or recurrent infections over many years beginning in childhood. In the later stages the process may be primarily immunopathologic. Live Chlamydia may not be present and inflammation can be triggered by C trachomatis antigens to which the patient has been sensitized.

IMMUNITY

Immunity to C trachomatis infections seems to take a long time to develop and even then is incomplete. Up to 50% of women with genital infection may still be shedding the organism a year later. The intracellular location and the prospect that low levels of cytokines may induce the persistent state are complicating features. T_H1 responses seem to be the most protective. T_H2 responses directed at MOMP may participate as well but antibody is also associated with immunopathologic injury in the chronic forms like trachoma.

CHLAMYDIA TRACHOMATIS: CLINICAL ASPECTS

MANIFESTATIONS

Eye Infections

Trachoma and inclusion conjunctivitis are distinct diseases of the eye that have some overlap in their clinical manifestations. Trachoma, a chronic conjunctivitis caused by C trachomatis biovars A, B, Ba, and C, is usually seen in less developed countries and often leads to blindness. Inclusion conjunctivitis, an acute infection commonly caused by biovars D to K, is usually not associated with chronicity or permanent eye damage. It occurs in newborns and adults worldwide.

Trachoma

Chronic inflammation of the eyelids and increased vascularization of the corneal conjunctiva are followed by severe corneal scarring and conjunctival deformities (Figure 39–3). Visual loss often occurs 15 to 20 years after the initial infection as a result of repeated scarring of the cornea.

Inclusion Conjunctivitis

Neonatal inclusion conjunctivitis usually presents as an acute, watery then mucopurulent eye discharge 5 to 12 days after birth. Infection occurs in roughly one-third of infants born vaginally to infected mothers. The infection is not prevented by prophylaxis with topical erythromycin or tetracycline. Untreated, it may persist for 3 to 12 months. Inclusion conjunctivitis is clinically similar in adults and is usually associated with concomitant genital tract disease. Diagnosis can be made quickly by demonstrating characteristic cytoplasmic inclusions in smears of conjunctival scrapings (Figure 39–4). In both neonates and adults, systemic therapy is preferred because the nasopharynx, rectum, and vagina may also be colonized and other forms of disease may develop, such as an infant pneumonia syndrome. More than 50% of all infants born to mothers excreting C trachomatis during labor show evidence of infection during the first year of life. Most develop inclusion conjunctivitis, but 5% to 10% develop the infant pneumonia syndrome. Chlamydia trachomatis accounts for about one-third to one-half of all cases of interstitial pneumonia in infants. The illness usually develops in a child between 6 weeks and 6 months of age and has a gradual onset. The child is usually afebrile, but develops difficulty in feeding, a characteristic staccato (pertussis-like) cough, and shortness of breath. The disease is rarely fatal, but may be associated with decreased pulmonary function later in life.

Genital Infections

The clinical spectrum of sexually transmitted infections with C trachomatis is similar to that of Neisseria gonorrhoeae. Chlamydia trachomatis can cause urethritis and epididymitis in men and cervicitis, salpingitis, and a urethral syndrome in women. In addition, three biovars of C trachomatis cause LGV, a distinctly different sexually transmitted disease (Table 39–1).

Chlamydia trachomatis urethritis is manifested by dysuria and a thin urethral discharge. Infections of the uterine cervix may produce vaginal discharge but are usually asymptomatic. Ascending infection in the form of salpingitis and pelvic inflammatory disease (PID) occurs in an estimated 5% to 30% of infected women. The scarring produced by chronic or repeated infection is an important cause of sterility and ectopic pregnancy.

Lymphogranuloma venereum is a sexually transmitted infection caused by C trachomatis strains L₁, L₂, or L₃. It occurs principally in South America, Africa, Southeast Asia, India, and Caribbean countries. The clinical course is characterized by a transient genital lesion followed by multilocular suppurative involvement of the inguinal lymph nodes (Figure 39–5). The primary genital lesion is usually a small painless ulcer or papule, which heals in a few days and may go unnoticed. The most common presenting complaint is inguinal adenopathy. Nodes are initially discrete, but as the disease progresses, they become matted and suppurative. The skin over the node may be thinned, and multiple draining fistulas develop. Systemic symptoms such as fever, chills, headaches, arthralgia, and myalgia are common. Late complications include urethral or rectal strictures and perirectal abscesses and fistulas. In homosexual men, LGV strains can cause a hemorrhagic ulcerative proctitis. Lymph nodes may need to be aspirated to prevent rupture.

DIAGNOSIS

Demonstration of C trachomatis by smear or culture requires the collection of epithelial (not inflammatory) cells from the site of infection (conjunctiva, urethra, cervix). Culture is carried out in specially treated cells in which chlamydial inclusions are detected by immunofluorescence. Results require incubation for 3 to 7 days. Direct fluorescent antibody (DFA) and immunoassay methods have also been developed. All these methods have now been replaced by the newest generation of nucleic acid amplification (NAA) tests. They are rapid, sensitive, specific, and for genital infections urine is a suitable specimen although less sensitive than direct genital samples. Culture or DFA are now reserved for pharyngeal and rectal specimens which for NAA tests might generate false positives. Nucleic acid amplification methods for genital Chlamydia infection are now combined with parallel tests for N gonorrhoeae.

Serodiagnostic methods have limited use in diagnosis because of the difficulty of distinguishing current from previous infection although detection of IgM antibodies against C trachomatis is helpful in cases of infant pneumonitis. Chlamydial serology is also useful in the diagnosis of LGV, where a single high complement fixation antibody titer (higher than 1:32) or a fourfold rise supports a presumptive diagnosis. The most satisfactory method for diagnosis of LGV is isolation of an LGV strain of C trachomatis from aspirated lymph nodes or tissue biopsies. In 80% to 90% of patients, the LGV complement fixation test is positive (titer higher than 1:64) shortly after the appearance of inguinal lesions.

TREATMENT

Strains of C trachomatis are susceptible to macrolides and tetracyclines. Azithromycin is the preferred therapy because it is given as a single oral dose for non-LGV C trachomatis infection. Doxycycline is an alternative for C trachomatis and is the drug of choice for treating LGV. For trachoma, a single dose of azithromycin is the treatment of choice.

PREVENTION

Prophylaxis for infants using topical erythromycin or silver nitrate on the conjunctiva has limited effectiveness for Chlamydia because 15% to 25% of exposed infants still develop inclusion conjunctivitis. The primary approach to prevention of all forms of genital and infant C trachomatis infection comprises detection of this infection in sexually active individuals and appropriate treatment, including infected women with azithromycin late in pregnancy. For trachoma, corrective surgery may prevent blindness and is required for severe corneal and conjunctival scarring. Control of trachoma is directed toward prevention of continued reinfection during early childhood. Improvement in general hygienic practices is the most important factor in decreasing transmission of infection within families and, of course, one of the most difficult to implement on a broad scale.

EPIDEMIOLOGY

Human psittacosis (ornithosis) is a zoonotic pneumonia contracted through inhalation of respiratory secretions or dust from droppings of infected birds. It was initially described in psittacines, such as parrots and parakeets, but was subsequently shown to occur in over 100 avian species, including turkeys. The disease is usually latent in its natural host, but may become active, particularly with the stress of recent captivity or transport; Chlamydophila psittaci is then excreted in large amounts. Until recently, classified with the genus Chlamydia the closely related Chlamydophilia psittaci (and Chlamydophila pneumoniae) were splint off based on differences in ribosomal RNA sequence analysis.

Psittacosis in humans is seen mainly as an occupational hazard of poultry workers and bird fanciers, particularly owners of psittacine birds. Reported cases of human psittacosis in the United States decreased during the 1950s, in association with the use of antimicrobials in poultry feeds and quarantine regulations for imported psittacine birds. Currently, 100 to 200 cases of psittacosis are reported each year. Some strains of C psittaci are highly contagious and pose a hazard for laboratory workers processing specimens for C psittaci isolation. Its airborne infectious potential is enough for C psittaci to be placed on lists of potential bioterrorism weapons. Human-to-human transmission is rare.

CLINICAL DISEASE AND TREATMENT

The incubation period for psittacosis is 5 to 15 days. Psittacosis in humans is an acute infection of the lower respiratory tract, usually presenting with acute onset of fever, headache, malaise, muscle aches, dry hacking cough, and bilateral interstitial pneumonia. Occasionally, systemic complications such as myocarditis, encephalitis, endocarditis, and hepatitis may develop. The liver and spleen are often enlarged. The diagnosis of psittacosis should be suspected in any patient with acute onset of febrile lower respiratory illness who gives a history of close exposure to birds. Indeed, a history of bird exposure should be especially sought in patients who appear to have a bilateral pneumonia not proven to be caused by other agents. It must be remembered that spread can occur from both symptomatic and asymptomatic infections of birds. The specific diagnosis is usually made by demonstrating a fourfold rise in the titer of microimmunofluorescence antibody or a single IgM titer of higher than 1:16. Although C psittaci can be isolated from blood or sputum early in the disease, these methods are attempted only in specialized laboratories because of the risk of laboratory infection. Treatment with doxycycline (preferred), tetracycline, azithromycin, or erythromycin are effective if given early in the course of illness.

Chlamydophila pneumoniae has been shown to be as common a cause of community-acquired pneumonia that mirrors the features of M pneumoniae infection. Since this agent has been recognized as a cause of pneumonia for little more than a decade, its clinical features and disease mechanisms are still in development. It is estimated that 10% of pneumonia and 5% of bronchitis cases are due to this agent. Epidemiologic evidence indicates that infection occurs throughout the year and is spread between humans by person-to-person contact. Outbreaks of community-acquired pneumonia caused by C pneumoniae have been reported, as well as apparent nosocomial spread. Reinfections occur, and clinically evident C pneumoniae infection may be more evident in the elderly than in younger individuals. Most infections manifest as pharyngitis, lower respiratory tract disease, or both, and the clinical spectrum is similar to that of M pneumoniae infection. Pharyngitis or laryngitis may occur 1 to 3 weeks before bronchitis or pneumonia, and cough may persist for weeks. The diagnosis is established by serologic testing (microimmunofluorescence), culture, or NAA methods, but these tests are not widely available. Treatment with macrolides (azithromycin or clarithromycin), doxycycline, and fluoroquinolones is effective in ameliorating the signs and symptoms of C pneumoniae infection.