Chapter 47: The Systemic Fungal Pathogens: Cryptococcus, Histoplasma, Blastomyces, Coccidioides, Paracoccidioides

The fungi discussed in this chapter cause a variety of infections, each ranging in severity from subclinical to progressive, debilitating disease. Some of these species are dimorphic, growing in the infectious mold form in the environment but switching to a round, yeast-like form in infected tissues. They differ from the opportunistic fungi in their ability to cause disease in previously healthy persons. However, the most serious infections still occur in patients with compromised immune systems. With the exception of Cryptococcus neoformans, each of these fungi is restricted to geographic niches corresponding to the environmental habitats of the mold form of the species. None of these infections is transmitted from human to human. The major features of the systemic pathogens are summarized in Table 47–1.

CRYPTOCOCCUS NEOFORMANS AND CRYPTOCOCCUS GATTII

Cryptococcus species were first isolated from environmental sources more than a century ago, and they are now recognized as important human pathogens, especially in the setting of HIV infection. The most important clinical manifestation of cryptococcal disease is a life-threatening meningitis in immunocompromised patients.

Found throughout the world, Cryptococcus species grow primarily as a budding yeast 4 to 6 μm in diameter. The most characteristic feature of these cells is a large polysaccharide capsule (Figure 47–1), often extending the overall diameter of these cells to 25 μm or more. Cryptococcal species are basidiomycetes, a group of fungi that includes the mushrooms as well as many agricultural pathogens. The Cryptococcus genus contains two pathogenic species complexes, C neoformans and the more recently recognized C gattii. Each has multiple serotypes, subspecies, or varieties.

The cryptococcal capsule is a unique feature among pathogenic fungi that is composed of a complex polysaccharide polymer. The major components of the capsule are glucuronoxylomannan and glucuronoxylomannogalactan, together referred to as GXM. Capsule production is repressed under environmental growth conditions, and it is stimulated by the human physiologic conditions found in tissues and in culture on some laboratory media. Capsular material is also secreted into the surrounding environment, serving to suppress the activity of nearby immune cells. GXM is so potently immunosuppressive that it has been used to treat autoimmune conditions such as rheumatoid arthritis in experimental trials.

In addition to the capsule, extracellular products include urease, phospholipases, and laccase enzymes. Laccases catalyze an important step in the production of an antioxidant melanin pigment. All of these fungal products specifically help to create conditions more favorable for fungal survival. The cryptococcal cell wall is made up of glucans, chitin, and proteins anchored to mannan or other cell wall elements. This fungal-specific structure is the site of capsule attachment at the cell surface. The cell wall also acts as an interface between the microbe and the host or external environment.

At either 25°C or 37°C, yeast colonies grow in culture within 2 to 3 days. These yeast cells can easily be identified as Cryptococcus be the presence of a capsule or one of the secreted products such as urease or melanin. When two strains of opposite mating type are co-incubated in the laboratory on specialized media, the teleomorph (sexual) forms can develop with hyphae and basidiospores. Although these mating structures are not observed during infection, there is strong evidence for sexual recombination occurring in the environment.

CRYPTOCOCCOSIS

EPIDEMIOLOGY

Cryptococcus neoformans can be isolated from environmental samples throughout the world, particularly in soil contaminated with bird droppings and decaying vegetable matter. One common environmental niche is the hollowed-out areas of trees, where laccases are likely involved in the degradation of wood. The infectious form is felt to be either desiccated yeast cells or basidiospores stirred up from these sites and subsequently inhaled. The less common species C gattii was once felt to be restricted to tropical and subtropical area, but it has recently been isolated from patients and the environment near the US Pacific Northwest (British Columbia, Washington, and Oregon).

Cases of symptomatic cryptococcal infections in immunologically normal people are very rare, although it is well known that most people are exposed to this fungus early in childhood. This suggests that cryptococcal species are well controlled by the immune system after initial infection. Cryptococcosis in immunocompromised patients occurs primarily in those with defects in T-lymphocyte function, particularly in patients with AIDS, in whom it is the most common systemic fungal infection. Recent data estimated that more than 500 000 deaths occur each year in AIDS patients due to this infection. In countries with well-developed antiretroviral therapy programs, the incidence of cryptococcal disease has markedly declined in recent years. However, this infection remains an important clinical issue in other immunocompromised populations. Life-threatening disease can occur in patients with no known immune defects, although many clinicians believe that poorly characterized immune disorders may explain the majority of these infections. Person-to-person transmission has not been documented, with most cases likely resulting from reactivation of dormant foci of remote infections, similar to tuberculosis.

PATHOGENESIS

After being inhaled, cryptococci reach the alveoli, where production of the polysaccharide capsule is the prime determinant of virulence. Pulmonary infection is associated with the appearance of a subpopulation of very large (up to 100 μm) thick-walled forms called Titan cells, which are too large to be phagocytosed. Production of the GXM capsule is induced in the tissue milieu through sensing of multiple environmental signals (iron, pH, CO₂, glucose, and nitrogen). The capsule is antiphagocytic and has various other immunomodulating effects, such as downregulation of cytokines, interference with antigen presentation, inhibition of leukocyte migration, misdirecting of specific antibody responses, and delaying the development of T_H1 immune responses. These immune-suppressing effects may act at both a local and systemic level because cryptococci produce sufficient capsule that the GXM is readily detected in the blood and other body fluids. If engulfed by macrophages, C neoformans is able to survive and multiply by altering its metabolic pathways and by inducing melanin production, which interferes with oxidative killing mechanisms. This subversion of the first lines of innate immune defense may be what allows the organism to spread outside the lung.

The affinity of C neoformans for the central nervous system (CNS) is striking. Proposed explanations include crossing the blood–brain barrier inside macrophages (Trojan horse model) and the ability of laccase to convert the abundant catecholamines in the CNS to melanin. Similar to other neuropathogens, components on the microbial surface may also help to target C neoformans to the CNS by a specific interaction with proteins on the endothelial cells of the brain microvasculature.

Tissue reaction to C neoformans varies markedly between individuals. Often, there is a remarkable paucity of inflammatory cells at the site of infection despite the presence of numerous fungal forms. This is certainly consistent with an infection by a fungus that not only blocks its own phagocytosis but is able to downregulate multiple aspects of the immune response. Chronic, dormant infections are often characterized by granulomas, both in the lungs and in regional lymph nodes.

IMMUNITY

The capsule has many anti-inflammatory effects, but antibodies against capsular epitopes can develop during the course of infection. In fact, serological surveys found that most children in a metropolitan area displayed anti-Cryptococcus antibodies by 5 years of age. However, the mere presence of antibodies does not correlate with protection from subsequent or reactivated infection, so their role in immunity is not completely understood. Animal studies, and the strong clinical association of cryptococcosis with T-cell defects, indicate that T_H1-type immune responses are most important in the outcome of infection. Cryptococci phagocytosed by macrophages may not be immediately killed, and cytokine activation is needed to complete the clearing of the organisms. Immunodominant mannoproteins have been identified which use dendritic cells as the primary presenter to CD4+ T cells. In patients with cryptococcosis who have no known immune defects, it is often possible to detect subnormal T_H1 immune functions in laboratory testing. Clinical recovery in such cases is associated with return of these immune functions.

CRYPTOCOCCOSIS: CLINICAL ASPECTS

MANIFESTATIONS

Meningitis is the most commonly recognized form of cryptococcal disease. Unlike bacterial infections of the CNS, cryptococcal meningitis usually has a slow, insidious onset with relatively nonspecific findings until late in its course. Common presenting symptoms include intermittent headache, irritability, dizziness, and difficulty with complex cerebral functions, appearing over weeks or months. Behavioral changes have sometimes been mistaken for psychoses. Fever is usually, but not invariably, present. Seizures, cranial nerve defects, and papilledema may appear later in the clinical course, as may dementia and decreased levels of consciousness. A more rapid course may be seen in AIDS patients, and 5% to 15% of these patients may have symptomatic infection at some point. Although the onset of illness may be subacute, cryptococcal infection of the CNS is usually fatal if not recognized and treated.

Like many other pathogenic fungi that enter the host through the lung, most initial pulmonary infections are minimally symptomatic. Infections can be truly clinically inapparent, or they may manifest as a self-limited respiratory illness. However, cryptococcal pneumonia can be progressive and severe in immunocompromised patients. In either case, no clinical findings are sufficiently specific to suggest the etiology.

Dissemination of infection occurs almost exclusively in immunocompromised patients, sometimes targeting the skin and bones. Classically, cryptococcal skin lesions are papular or nodular, often with a central umbilication and remarkable for their lack of inflammation. The diagnosis is sometimes made when lesions are biopsied as suspected neoplasms.

There is evidence of differences in the disease spectrum of the two Cryptococcus species. Cryptococcus gattii is more likely to produce pulmonary infection and less likely to invade the CNS. Cryptococcus gattii infection has also been described more frequently in patients with no definable immunological defect. In the CNS, C gattii may cause more localized lesions (cryptococcomas) as opposed to the diffuse meningoencephalitis typical of C neoformans.

DIAGNOSIS

Typical CSF findings in cryptococcal meningitis are increased intracranial pressure, pleocytosis (usually more than 100 white blood cells/mm³) with predominance of lymphocytes, and depression of glucose levels. In some cases, one or all of these findings may be absent, yet cryptococci are still isolated on culture. Encapsulated yeast cells (diagnostic of C neoformans infection) are demonstrable in CSF in approximately 50% of cases by mixing centrifuged CSF sediment with India ink and examining the mixture under the microscope (Figure 47–1). Experience is necessary to avoid confusion of lymphocytes with cryptococci. Cryptococcus neoformans stains poorly or not at all with routine histologic stains; thus, it is easily missed unless special fungal stains are used (Figure 47–2).

For the isolation of C neoformans by culture, the volume of CSF sampled is important. The number of organisms present may be small enough to require a substantial volume of fluid (more than 30 mL) to yield a positive culture. The detection of cryptococcal capsular antigen in the CSF is the most sensitive and specific way to make the diagnosis of cryptococcal meningitis. This test can also be performed on serum where it is especially useful in diagnosing infection in immunocompromised patients due to higher levels of circulating organisms. The cryptococcal antigen test is performed by latex agglutination or enzyme immunoassay, and its quantitation has prognostic significance. A rising antigen level can indicate progression, and a declining titer is a favorable sign.

TREATMENT

The primary treatment for serious cryptococcal infections includes induction therapy with Amphotericin B plus flucytosine, followed by an extended consolidation course of fluconazole. Azoles alone can often be used in non-CNS disease; therefore, it is very important to perform lumbar punctures and CSF analysis in all patients with cryptococcal infection to determine if the CNS is involved. Although 75% of persons with meningitis respond to this treatment, many patients suffer relapses after antifungal therapy is stopped, requiring repeated courses of therapy. In patients with AIDS-associated cryptococcosis, reconstitution of the immune system with antiretroviral therapy is very important to prevent recurrent infection. One half of patients with a microbiological cure have some kind of residual neurologic damage.

HISTOPLASMA CAPSULATUM

Histoplasma capsulatum is one example of a thermally dimorphic fungus (Figure 47–3B), microorganisms that change growth form depending on the ambient temperature. Common features of this group of fungi include the fact that most are restricted to particular geographic locations (regions of endemicity). Most of these fungi enter patients through the lung where they establish initial infection. Immune competent patients typically spontaneously resolve infections due to these fungi, but the infections can rarely become chronic or disseminated.

Histoplasma capsulatum grows in a round, yeast-like phase in tissue and in cultures incubated at 37°C. However, when incubated at lower temperatures, such as those most commonly encountered in the environment, Histoplasma species grow as a filamentous mold where it is a saprophyte in soil.

The yeast forms are small for fungi (2-4 μm) and reproduce by budding (blastoconidia). The mycelia are septate and produce microconidia and the diagnostic structure called the tuberculate macroconidium because of its thick wall and radial, finger-like projections (Figure 47–3A). Growth is obtained on blood agar, chocolate agar, and Sabouraud’s agar, but may take many weeks. The designation H capsulatum is actually a misnomer, because no capsules are formed. It comes from the halos seen around the yeasts in tissue sections, which are caused by a shrinkage artifact of routine histologic methods.

HISTOPLASMOSIS

EPIDEMIOLOGY

Histoplasma capsulatum grows in soil under humid climatic conditions, particularly soil containing bird or bat droppings. Inhalation of the mold microconidia, which are small enough (2-5 μm) to reach the terminal bronchioles and alveoli, is believed to be the mode of infection. The organism is particularly prevalent in certain temperate, subtropical, and tropical zones, and endemic areas are present in all continents of the world except Antarctica. The largest and best defined is the US region drained by the Ohio and Mississippi rivers (Figure 47–4). More than 50% of the residents of states in this area show radiologic evidence of previous infection. In some locales, up to 90% demonstrate delayed-type hypersensitivity to Histoplasma antigens, suggesting that they have been infected at some point in the past. Point source outbreaks of histoplasmosis have occurred after the inhalation of large amounts of fungi following disturbances of bird roosts, bat caves, and soil at construction sites. Persons in endemic areas whose employment (agriculture, construction) or avocation (spelunkers) brings them in contact with aerosolized microconidia are at increased risk. The infection is not transmitted from person to person. Disease is more common in men, but there are no racial or ethnic differences in susceptibility.

PATHOGENESIS

Once infection is established, H capsulatum cells live primarily in the lymph nodes, spleen, bone marrow, and other elements of the reticuloendothelial system. Therefore, this is an example of a microorganism that has adapted to intracellular growth within phagocytic macrophages. Other examples include M tuberculosis and C neoformans.

Like tuberculosis and cryptococcosis, the initial infection with H capsulatum occurs in the lungs after inhalation of infectious conidia. These fungal spores convert to the yeast form after germinating in the host. While in the lungs, Histoplasma cells attach to integrin and fibronectin receptors on professional phagocytes such as neutrophils and macrophages, allowing them to be readily engulfed. Histoplasma capsulatum is able to survive inside of these cells, resisting the oxidative burst and inhibiting phagosome–lysosome fusion. Key features in this survival and multiplication are the ability of H capsulatum to capture iron and calcium from the macrophage and to modulate phagolysosomal pH. The acidic pH required for optimal killing effect in the lysosome is elevated by H capsulatum toward the less effective more neutral pH range (pH 6.0-6.5).

The initial pulmonary infection of histoplasmosis generally spreads to regional lymph nodes, resulting in a primary focus of paired lung/lymphatic lesions similar to the Ghon complex of tuberculosis. Also like tuberculosis, most cases never advance beyond the primary stage, leaving only a calcified node and pulmonary calcifications as evidence of infection. As in tuberculosis, viable fungal cells may remain in these old lesions and reactivate later, particularly if the person becomes immunocompromised. The extent of systemic spread of H capsulatum within macrophages to other body sites during primary infection is unknown, but it is likely highly variable and dependent on host-specific immune factors.

Pathologically, histoplasmosis is characterized by granulomatous inflammation with associated necrosis. Even with special fungal stains, H capsulatum may be difficult to detect within these infected foci, making a precise pathological diagnosis challenging. Extrapulmonary spread of histoplasmosis occurs primarily in immunocompromised patients, primarily involving the reticuloendothelial system with resulting enlargement of the liver and spleen. In patients with compromised immunity, numerous organisms within macrophages may be found in these organs, in lymph nodes, bone marrow, or even peripheral blood (Figure 47–5).

IMMUNITY

Infection with H capsulatum is associated with the development of adaptive cell-mediated immunity, and this can be demonstrated by a positive delayed-type hypersensitivity skin test to H capsulatum mycelial antigens. Infection is believed to confer long-lasting immunity against new infection, primarily mediated by T_H1-weighted immune responses. In experimental infections, macrophages activated by T-lymphocyte–derived cytokines are able to inhibit intracellular growth of H capsulatum and thus control the disease. Neither B cells nor antibody seem to play a large role in preventing reinfection. Immunocompromised persons, particularly those with T-lymphocyte–related defects, are unable to stop growth of the organism and tend to develop progressive, disseminated disease. Populations at particular risk for developing disseminated histoplasmosis include patients with advanced AIDS as well as those being treated with tumor necrosis factor-alpha inhibitors.

HISTOPLASMOSIS: CLINICAL ASPECTS

MANIFESTATIONS

Most cases of H capsulatum infection in normal hosts are minimally symptomatic, often causing mild fever and cough for a few days or weeks. Mediastinal lymphadenopathy and subtle pulmonary infiltrates may be seen on X-rays. More severe cases are most often characterized by chills, malaise, chest pain, and more extensive lung infiltrates, all of which usually resolve without specific therapy. However, most people who resolve the primary infection merely have scattered calcified granulomas in their lungs as X-ray evidence of healed histoplasmosis. Rarely, residual pulmonary nodules due to histoplasmosis may continue to enlarge over a period of years, causing a differential diagnostic problem with pulmonary neoplasms. Progressive pulmonary disease can mimic pulmonary tuberculosis, with sputum production, night sweats, weight loss, and even the development of lung cavities. The clinical course of pulmonary histoplasmosis may infrequently be chronic and relapsing, with symptoms lasting for several months to years.

Disseminated histoplasmosis generally appears as a nonspecific febrile illness in an immunocompromised patient, with enlargement of reticuloendothelial organs. The CNS, skin, gastrointestinal tract, and adrenal glands may also be involved. Painless ulcers on mucous membranes are a common finding; however, given their painless nature, these lesions must be actively sought by clinicians in order to help make the diagnosis. The course of disseminated histoplasmosis is typically chronic, with manifestations that depend on the organs involved. For example, chronic bilateral adrenal failure (Addison disease) may develop when the adrenal glands are affected.

DIAGNOSIS

In most forms of pulmonary histoplasmosis, the diagnostic yield of direct examinations or culture of sputum is low. In disseminated disease, blood culture or biopsy samples of a reticuloendothelial organ are the most likely to contain Histoplasma. Of these cultures, bone marrow culture has the highest yield. Because of their small size, the yeast cells are difficult to see in potassium hydroxide (KOH) preparations, and their morphology is not sufficiently distinctive to be diagnostic. Selective fungal stains such as methenamine silver demonstrate the organism but may not differentiate it from other yeasts. Hematoxylin and eosin (H&E)-stained tissue or Wright-stained bone marrow often demonstrates the organisms in their intracellular location in macrophages (Figure 47–5). Specimens must be examined carefully under high magnification. Identification of culture isolates requires demonstration of the typical conidia and dimorphism. Nucleic acid probes have been developed for culture confirmation.

Antibodies can be detected during and after infection, but their diagnostic usefulness in endemic areas is limited by false-negative results and cross-reactions in patients with blastomycosis. Rising antibody titers are suggestive of dissemination or relapse. The histoplasmin skin test has been useful in the past to document prior exposure, but the reagents are no longer commercially available. Isolation of the fungus in culture or clear histologic demonstration are often necessary to establish a firm diagnosis of histoplasmosis. The diagnosis of disseminated infection has been greatly aided by the development of a commonly used enzyme immunoassay (EIA) detecting a Histoplasma polysaccharide antigen. This test can be performed on blood or urine samples, and it detects more than 90% of cases of disseminated disease.

TREATMENT

Primary infections and localized lung lesions usually resolve without treatment. For mild disease localized to the lung, a systemic azole such as itraconazole is commonly used. For more severe or disseminated disease, initial therapy with amphotericin B is often followed by longer-term therapy with itraconazole. Azoles can be effective in an endemic area for prophylaxis of persons with a high risk of disease, including AIDS patients with low CD4 counts and other immunocompromised patients. The echinocandin class of antifungals is decidedly less effective against the endemic fungal pathogens, and these agents should not routinely be used for histoplasmosis.

BLASTOMYCES DERMATITIDIS

Blastomyces dermatitidis is a dimorphic fungus with some characteristics similar to those of Histoplasma. Growth develops in the yeast phase in tissues and in cultures incubated at 37°C. The yeast cells are typically larger (8-15 mm) than those of H capsulatum, with broad-based buds (blastoconidia) and a thick wall (Figure 47–6). The mold phase appears in culture at 25°C. Hyphae are septate and produce round to oval conidia sufficiently similar to the microconidia produced by H capsulatum to cause confusion between the two in young cultures. Although older cultures may produce chlamydoconidia, B dermatitidis produces no structure as distinctive as the tuberculate macroconidium of Histoplasma.

BLASTOMYCOSIS

EPIDEMIOLOGY

Blastomycosis is most commonly observed in geographic regions that overlap with those for histoplasmosis. In North America, this includes the upper Midwestern states and regions around the Great Lakes (Figure 47–4), but infections have been reported in Africa, the Middle East, and Europe as well. Unlike histoplasmosis, the causative agent of blastomycosis, B dermatitidis, is not strongly associated with bird or bat habitats. A specific skin test for blastomycosis is not routinely available, thus limiting epidemiological mapping of the endemic areas. It is assumed that inhalation of environmental microconidia is the means of infection, as it the case for most endemic fungal pathogens.

PATHOGENESIS

Primary infection occurs in the lungs after inhalation of aerosolized conidia, which develop in soil. Surface glucans and a glycoprotein adhesin (BAD1) have been identified, which bind the fungi to receptors on host cells, macrophages, and the extracellular matrix. A mixed inflammatory response results, which ranges from neutrophil infiltration to well-organized granulomas with giant cells. The organisms grow in tissue as large yeasts with thick double walls with blastoconidia attached. A significant difference from Histoplasma is that the yeast cells are primarily extracellular rather than residing within macrophages. This may be due to their relatively large size, but there is little to suggest that B dermatitidis shares the propensity for intracellular parasitism that is characteristic of H capsulatum.

IMMUNITY

The fungal cells activate the complement system by both the classical and alternative pathways, and antibodies directed against a glucan component of the cell wall have been identified. These antibodies decline as the primary infection resolves. As with other fungi, T_H1-mediated responses appear to be the most important determinant of immune-mediated clearance of infection. Macrophages activated by cytokines have enhanced capacity to kill B dermatitidis.

BLASTOMYCOSIS: CLINICAL ASPECTS

MANIFESTATIONS

Because mild cases of blastomycosis are difficult to diagnose, most infections are only recognized if they progress to more advanced or disseminated stages of the disease. Pulmonary infection is evidenced by cough, sputum production, chest pain, and fever. Hilar lymphadenopathy may be present, as may nodular pulmonary infiltrates with alveolar consolidation. This nonspecific clinical picture may mimic a pulmonary tumor, tuberculosis, or some other form of chronic pneumonitis. Skin lesions are among the most common manifestations of disseminated infection. In contrast to histoplasmosis, mucous membrane lesions are rarely observed in blastomycosis, with lesions developing more commonly on exposed skin often as “grouped microabscesses.” Given the chronicity of many untreated cases of cutaneous blastomycosis, the associated extensive necrosis and fibrosis may produce considerable disfigurement. Bone infection has features similar to those of other causes of chronic osteomyelitis. The urinary and genital tracts are the most commonly affected visceral sites; the prostate is especially prone to infection.

DIAGNOSIS

Direct demonstration of typical large yeasts with broad-based buds (blastoconidia) in KOH preparations of infected tissue is the most rapid means of diagnosis (Figure 47–6). Biopsy specimens also have a high yield, and the organisms are visible in histopathology samples stained with either H&E or special fungal stains. Blastomyces dermatitidis grows in the clinical microbiology laboratory on routine fungal media, but cultures may take as long as 4 weeks. Conidia are not particularly distinctive, and demonstration of thermal dimorphism and typical yeast morphology is essential to avoid confusion with other fungi. A DNA probe is particularly useful in differentiating cultures from Histoplasma. Serological tests are available but are less sensitive than those for other fungal pathogens.

TREATMENT

As with histoplasmosis, itraconazole and other mold-active azoles may be used for mild to moderate disease. Amphotericin B is indicated for more serious or disseminated infections. Fluconazole or voriconazole may be used in meningitis. As with other systemic mycoses, response to treatment is slow, and relapse is common.

COCCIDIOIDES IMMITIS AND COCCIDIOIDES POSADASII

Coccidioides species are dimorphic fungi commonly encountered within North America in the desert regions of the southwestern United States and northern Mexico. In contrast to Blastomyces and Histoplasma species that grow in the body as budding yeast-like cells, the tissue form of Coccidioides species is a large (12-100 μm), round-walled spherule (Figure 47–7A). This structure is quite distinctive and unique among the pathogenic fungi. Its formation takes place in a process illustrated in Figure 47–8. Spherule development requires simultaneous invagination of the fungal membrane (plasmalemma) and production of new cell wall to form the large multicompartmental structure. The compartments differentiate into uninucleate structures called endospores, each with a thin wall layer. Multiple endospores develop within each spherule and the entire structure is surrounded by an extracellular matrix. The spherule eventually ruptures, releasing 200 to 300 endospores (Figure 47–9), each of which can differentiate into another spherule.

In the environment, Coccidioides grows as a mold under harsh conditions in sandy alkaline soil with high salinity, such as those in arid, desert regions. In the laboratory, Coccidioides growth becomes visible in 2 to 5 days. The hyphae are septate and produce thick-walled, barrel-shaped arthroconidia (Figure 47–7B) in about 1 week. Mature arthroconidia readily separate from the hyphae and survive for long periods in the environment. When airborne, they are the infectious particles in nature. Arthroconidia can be converted to spherules in the laboratory, but only under very specialized conditions.

As with other fungi, the application of modern genotyping methods has led to name splitting within the Coccidioides genus. The original C immitis isolated in California’s San Joaquin Valley appears to be a distinct clone, and most strains from elsewhere in the Americas belong to another species (C posadasii). Because there are no differences in disease between the two species, and since clinical laboratories cannot easily distinguish between them, the more clinically familiar name C immitis will be used to refer to both species here.

COCCIDIOIDOMYCOSIS

EPIDEMIOLOGY

Coccidioidomycosis is the most geographically restricted of the systemic mycoses because C immitis grows only in the alkaline soil of semiarid climates known as the Lower Sonoran life zone (Figure 47–4). These areas are characterized by hot, dry summers, mild winters with few freezes, and annual rainfall of about 10 inches during brief rainy seasons. Ecologic “islands” with these conditions are found scattered throughout Central and South America. The primary endemic zones in the United States are in Arizona, Nevada, New Mexico, western Texas, and the arid parts of central and southern California. Three unrelated cases in the eastern half of Washington State could give this zone its most northern extension. The area between the Cascade and Rocky Mountains is also dry and arid, but prolonged winter freezes make it less hospitable for Coccidioides species. Persons living in the endemic areas are at high risk of infection, and positive skin test rates of 50% to 90% occur in long-time residents of highly endemic areas. Coccidioidomycosis is not transmissible from person to person.

Infection cannot typically be acquired without at least visiting an endemic area, although some interesting examples have been recorded in which the endemic zone itself “paid a visit” and resulted in infections. In 1978, a storm originating in Bakersfield, California (endemic zone), carried a thick cloud of dust all the way to San Francisco. This weather event was followed by cases of coccidioidomycosis in persons who had never left the Bay Area. Similarly, infection in a patient who had never left the southeastern United States was epidemiologically associated with exposure to pre-processed cotton grown in Arizona. In 1992, a tenfold increase in disease in California followed an unusually wet winter in which the storms created a drought–rain–drought pattern just right for growth of the mold (and wildflowers). When the Sonoran Desert blooms, a Coccidioides arthroconidium “crop” is not far behind. The incidence of coccidioidomycosis is increasing in the United States primarily because of the influx of populations into the sunbelt states where C immitis is endemic. Ninety percent of new cases of coccidioidomycosis occur in California or Arizona.

Coccidioides immitis is also a notorious cause of infection in laboratory workers. The high infectivity of cultured arthroconidia has caused it to be classified as a significant biohazard and potential bioweapon.

PATHOGENESIS

Inhaled arthroconidia are small enough (2-6 μm) to bypass the defenses of the upper tracheobronchial tree and lodge in the terminal bronchioles and alveoli. After infection, the typical incubation period before the onset of symptoms is 1 to 3 weeks. Human monocytes can ingest and kill some arthroconidia on initial exposure, although the outer portion of the wall of the arthroconidium has antiphagocytic properties, allowing them to persist into the early stages of spherule development. Surviving arthroconidia convert to the spherule stage, which begins its slow growth to a size that makes effective phagocytosis difficult. Although polymorphonuclear neutrophils are able to digest the spherule wall, their access appears to be restricted by the extracellular matrix surrounding it. The young endospores are released in packets that include the extracellular matrix derived from the parent spherule, which may protect them until they develop into new spherules.

Several proteases found in the conidial cell wall or in spherules have been proposed as C immitis virulence factors. In addition to their role in the fungal life cycle, some of these enzymes attack host substrates, such as collagen, elastin, and immunoglobulins, but no direct specific contribution to disease has been defined. Components of the spherule outer wall and a metalloproteinase found there have been linked to virulence in animals and to survival of developing endospores.

IMMUNITY

Lifelong immunity to coccidioidomycosis clearly develops in most of those who become infected. This immunity is associated with strong polymorphonuclear leukocyte and T_H1-mediated responses to coccidioidal antigens. In most cases, a mixed inflammatory response is associated with early resolution of the infection and development of a positive delayed-type hypersensitivity skin test. Progressive disease is associated with weak or absent cellular immunity and loss of delayed-type hypersensitivity to coccidioidal antigens. In most infected persons, the infection is controlled after mild or unapparent illness. The disease progresses when cell-mediated immunity and consequent macrophage activation do not develop. Such immune deficits may be a result of disease (AIDS) or immunosuppressive therapy, but progressive coccidioidomycosis may infrequently occur in persons with no known immune defects.

The central event in the host–pathogen interaction in coccidioidomycosis appears to be the reaction to arthroconidia or to endospores released from ruptured spherules. Arthroconidia can be phagocytosed and killed by polymorphonuclear leukocytes even before an adaptive immune response is mounted. The handling of endospores requires the additional participation of macrophages that do not become maximally effective until activated by cytokines produced by the T_H1 subsets. Prior to this, C immitis endospores may be able to impair phagosome–lysosome fusion in the phagocyte.

Humoral mechanisms are not known to play a major role in immunity to coccidioidomycosis. In fact, C immitis is resistant to complement-mediated killing, and levels of complement-fixing antibody are inversely related to the process of disease resolution. Persons with minimal objective indications of tissue involvement (eg, lesions and radiographs) have strong T-lymphocyte responses to C immitis antigens and little if any detectable anti-Coccidioides antibody. Those with disseminated disease and absent cellular immunity have high titers of antibody. Thus, the levels of antibody seem to indicate the degree of antigenic stimulation rather than any known contribution to resolution of the infection.

COCCIDIOIDOMYCOSIS: CLINICAL ASPECTS

MANIFESTATIONS

More than 50% of those infected with C immitis experience no symptoms, or the disease is so mild that it cannot be recalled when evidence of infection (serology, skin test) is discovered. Others develop malaise, cough, chest pain, fever, and arthralgia 1 to 3 weeks after infection. This illness, dubbed Valley Fever by the local San Joaquin Valley residents, lasts 2 to 6 weeks with few distinctive findings. The chest X-ray is usually clear or shows only hilar adenopathy. Red, inflamed skin nodules, known as erythema nodosum, may occur during the course of initial Coccidioides infection, particularly on the extremities. Occurring most frequently in women, these nodules may provide a clinical clue to the cause of infectious symptoms. In most cases, all clinical symptoms of Valley Fever resolve spontaneously, but often only after considerable discomfort and loss of productivity. In more than 90% of cases, there are no pulmonary residua. A small number of cases progress to a chronic pulmonary infection characterized by cavity formation and a slowly relapsing course that extends over years. Less than 1% of all primary infections and 5% of symptomatic cases disseminate to foci outside the lung.

There is a well-recognized but poorly understood predisposition to chronic infections among distinct patient populations. Disseminated disease is more common in men, as well as people from sub-Saharan Africa and southeast Asia, particularly the Philippines. Given the importance of CD4-mediated immunity in controlling coccidioidomycosis, patients with AIDS or transplants are also at particular risk for disseminated infection. Evidence of extrapulmonary infection almost always appears in the first year after infection. The most commonly involved sites are bones, joints, skin, and the central nervous system. Coccidioidal meningitis develops slowly with gradually increasing headache, fever, neck stiffness, and other signs of meningeal irritation. The CSF findings are similar to those in tuberculosis and other fungal causes of meningitis, such as C neoformans. Mononuclear cells predominate in the cell count, but substantial numbers of neutrophils and eosinophils are often present. If untreated, the disease is slowly progressive and fatal.

DIAGNOSIS

With enough persistence, direct examination of infected tissue can reveal diagnostic forms of C immitis. The thick-walled spherules are so large and characteristic (Figure 47–7A) that they are difficult to miss in wet mounts (KOH, calcofluor) or biopsy sections. Skin and visceral lesions are most likely to demonstrate spherules; however, these fungal forms are rarely seen in the CSF. Spherules released into expectorated sputum are often small (10-15 mm) and immature without well-developed endospores, thus difficult to visualize. In contrast, spherules stain well in histologic sections of infected tissue using either H&E or special fungal stains.

Culture of C immitis from sputum, visceral lesions, or skin lesions is not difficult, but must be undertaken only by those with experience and proper biohazard protection. Cultures of CSF are positive in less than half the cases of meningitis. Laboratories must be warned of the possibility of coccidioidomycosis to ensure diagnosis and prevent inadvertent laboratory infection. The latter is particularly significant outside the endemic areas, where routine precautions may not be in place. Identification requires observation of typical arthroconidia and confirmation using a DNA probe. Nucleic acid amplification procedures for direct detection are in development.

Serologic tests are particularly useful in the diagnosis and management of coccidioidomycosis (Figure 47–10). One half to three-quarters of patients with primary infection develop anti-Coccidioides IgM antibody during the first 3 weeks of illness. IgG antibodies (measured by complement fixation) appear in the third week or later, and their titer and duration depend on the extent of disease. IgG disappears with resolution or successful treatment, and it persists with continuing infection. In an appropriate clinical setting, the detection of specific antibodies can confirm the diagnosis of coccidioidomycosis, but their absence does not exclude it. In managing disseminated disease, the magnitude of the IgG titer is a measure of the extent of disease, and the direction of any change is often an indication of prognosis. For example, a high (>1:32) and rising titer indicates a poor prognosis. The presence of IgG in the CSF is also important in the diagnosis of coccidioidal meningitis because cultures are frequently negative. The coccidioidin skin test was also a useful tool to indicate prior exposure, but it is no longer routinely available.

TREATMENT

Primary coccidioidomycosis is self-limited, and no antifungal therapy is indicated except to reduce the risk of dissemination in patients with risk factors, such as immunocompromise and pregnancy. Itraconazole is preferred for acute or progressive pulmonary disease, with fluconazole as an alternative agent. Disseminated, extrapulmonary infection may require amphotericin B. Fluconazole is often favored as treatment for meningitis because of its enhanced CSF penetration. In cases of refractory meningitis, amphotericin B may be infused directly into the CSF. Unlike other forms of the disease that can be treated for cure, coccidioidomycosis involving the CNS often requires lifelong therapy.

Paracoccidioides brasiliensis is the cause of paracoccidioidomycosis (South American blastomycosis), a disease limited to tropical and subtropical areas of Central and South America. The organism is a dimorphic fungus, the most noteworthy feature of which is the production of multiple blastoconidia from the same cell. Characteristic 5 to 40 μm cells covered with budding blastoconidia may be seen in tissue or in yeast-phase growth at 37°C. This structure has a morphology reminiscent of a ship’s steering wheel, often referred to as a “Captain’s wheel” when seen in tissue. The disease manifests primarily as chronic mucocutaneous or cutaneous ulcers. The disfiguring ulcers spread slowly and develop a granulomatous mulberry-like base. Regional lymph nodes, reticuloendothelial organs, and the lungs may also be involved.

Little is known about the pathogenesis of paracoccidioidomycosis, although the route of infection is believed to be inhalation. Progression in experimental animals is associated with depressed T-lymphocyte–mediated immune responses. Paracoccidioidomycosis has a striking predilection for men, despite skin test evidence that subclinical cases occur at the same rate in both sexes. This may be related to the experimental observation that estrogens but not androgens inhibit conversion of mold-phase conidia to the yeast phase. Treatment is with sulfonamides, amphotericin B, and, more recently, the azole compounds.