Disease and Epidemiology

This thermally dimorphic fungus appears to be an inhabitant of soil and wood. Environments that are moist and shaded, have a high content of organic matter, and have slightly acidic pH seem to favor its growth. The ecologic niche of B. dermatitidis has been associated with water sources that change water levels. In highly endemic areas, shorelines may be an important reservoir for the fungus (Figure 47-2). Successful environmental isolations from rotting wood, decaying leaves, a beaver dam and lodge, a woodpile, twigs and roots, and tree bark have been reported. There may be an association of the organism with animal excreta; the fungus has been isolated from soil in chicken houses, from an abandoned mule stall, and from pigeon manure. Recovery of B. dermatitidis also has been documented from both the respiratory tract and feces of a dog with clinical disease, which suggests that yeast-phase cells may be recovered from the stool of dogs with pulmonary blastomycosis follow-ing transit through the gastrointestinal tract of swallowed infected sputum.

FIGURE 47-2 Natural history of the saprobic and parasitic life cycle of blastomycosis. Aerosolized mycelial fragments (1) or spores (2) from the environment are inhaled and deposited in alveoli of a susceptible host. The mature yeast form develops in the host (3–4) at 37° C.

(Courtesy Ashley E. Harmon.)

Several epidemiologic studies of canine blastomycosis have been conducted. Risk groups include sexually intact males ages 2 to 4 years, large breeds, especially sporting or hound, and young, geriatric, or immunocompromised dogs. Other risk factors are residence in the southeastern, south-central, and upper Midwestern regions of the United States and proximity to river valleys or lakes. The highest risk for blastomycosis is in late summer, autumn, and early winter, which may reflect suitable temperatures for mycelial growth in the environment. The seasonal distribution may also reflect increased outdoor activity of animals during these times, and heat stress and an increase in dusty conditions may represent additional risk factors. In areas with endemic blastomycosis, infections in dogs can signal an increased riskfor human infection. Whereas the prevalence of clinical versus subclinical disease in the dog is unknown, the incidence in dogs has been estimated to be approximately 10 times higher than in humans.

Zoonotic transmission of blastomycosis is uncommon. Percutaneous infection is most likely with the yeast phase. Cutaneous infection in humans has been reported as a result of bite wounds inflicted by dogs affected by B. dermatitidis, in which it is presumed that sputum was contaminated with yeasts. Veterinarians have developed disease after performing necropsies on infected dogs.

Man and dogs are the natural hosts of B. dermatitidis, but infection has been reported in a wide variety of species including domestic cats, horses, nonhuman primates, aquatic mammals, cattle, a ferret, wolves, a polar bear, a deer, a lion, a tiger, and a fruit bat. Two clinical forms of the disease exist: systemic or disseminated and cutaneous.

Prevalence of human blastomycosis in endemic areas may be as high as 0.5 to 4 cases per 100,000 individuals annually. Pneumonia and weight loss are the most common manifestations of disease. The acute pulmonary phase of blastomycosis may be subclinical or self-limiting, but if the disease progresses, pyogranulomatous inflammation develops in the lungs and at other sites. Blastomycosis has been reported in hunters and their dogs, suggesting that disease can develop in immunocompetent individuals.

Clinical signs associated with canine blastomycosis reflect the multisystemic nature of the disease and commonly include dyspnea, anorexia, depression, lameness, lymphadenopathy, or skin lesions. Ocular manifestations of blastomycosis (corneal opacity, uveitis, conjunctivitis, or blindness) occur in 30% to 40% of dogs with systemic disease. The prostate is often involved in males and theremay be orchitis, but the intestinal tract is usually unaffected. Abnormalities in cell-mediated immunity develop in dogs, and nearly 25% of infections are fatal.

The prevalence of blastomycosis is lower in cats than in dogs. The disease is similar to that in dogs, being characterized by pyogranulomatous inflammation of an isolated organ system or as a multiorgan systemic disease. The lungs and eyes are the most commonly affected sites, and many infected cats have pulmonary lesions regardless of the other organs involved. Clinical signs consist of nasal discharge, cough, lethargy, and weight loss.

Blastomyces dermatitidis is not a common equine pathogen—there have been only two reported cases of disease. Clinical findings in affected animals include pyogranulomatous pneumonia, pleuritis, peritonitis, and cutaneous abscessation.

Pathogenesis

Infection develops when spores or mycelial fragments, aerosolized from soil, are inhaled and deposited in alveoli. The fungal elements develop into yeast forms within pulmonary alveoli at body temperature and subsequently multiply asexually by budding. At this time, some animals may exhibit acute respiratory distress. If the spore inoculum is too large or the animal is immunocompromised, hematogenous or lymphatic dissemination results, with subsequent infection of the skin, bones, lymph nodes, eyes, central nervous system, or reproductive tract.

Blastomyces dermatitidis evokes a strong inflammatory response in the host, characterized by an influx of neutrophils and mononuclear phagocytes. Yeast forms of B. dermatitidis may be located both inside and outside phagocytes at inflammatory foci, and replicate in normal macrophages. Macrophages are activated as part of the induced immune response, and delayed-type hypersensitivity develops within several weeks after theinitial infection. Antifungal antibodies are also produced, but protective immunity resides within the cell-mediated arm of the response.

The organism possesses several key virulence attributes. Conversion from the mycelial to the yeast form confers an important survival advantage on the fungus. Whereas mycelia are readily phagocytosed and killed by neutrophils, yeasts appear resistant to neutrophils and mononuclear phagocytes during the early inflammatory process. The large size and thick cell wall of the yeast most likely contribute to this resistance.

Several constituents of the fungal cell wall are important virulence determinants. The major surface protein, termed WI-1, is a 120 kD adhesin that promotes attachment of spores to nonphagocytic cells and possibly to the extracellular matrix within the respiratory tract. It is somewhat of a conundrum that WI-1 appears also to promote recognition and killing of spores by phagocytes at the site of lung inflammation. Upon conversion of B. dermatitidis to the yeast phase, however, WI-1 expression is altered, minimizing recognition, ingestion, and killing of the yeasts by phagocytes. Because macrophages do not recognize the yeast cells, they freely invade tissues and disseminate via the bloodstream. Recent findings suggest that WI-1 modulates host immunity through blocking tumor necrosis factor-α (TNF-α) production by phagocytes, which further potentiates pulmonary infection. Shedding of WI-1 may also permit immune evasion by binding complement and opsonins, saturating macrophage receptors, impeding binding and phagocytosis of yeasts, and neutralizing phagocyte cytotoxic molecules. BAD-1, another adhesin, suppresses phagocyte proinflammatory responses of TNF-α. β-Glucan is a further cell wall constituent that initiates complement activation; canine complement enhances adherence of B. dermatitidis yeast to macrophages, and this adherence facilitates proliferation.

Diagnosis

In most animals, blastomycosis can be presumptively diagnosed by cytologic evaluation, which is based on finding the characteristic yeast form in preparations from fine-needle aspirates or touch preparations, and recovery of the fungus from clinical materials. Specimens suitable for disease diagnosis are exudates from draining tracts and lesions, transtracheal washes, fluid from the anterior chamber of the eye, prostatic fluid, and lymph node aspirates. The yeast ranges from 5 to 20 μm in diameter, is unencapsulated, and has a thick refractile, double cell wall (Figure 47-3).

FIGURE 47-3 Smear from a foot lesion of blastomycosis demonstrating budding yeast.

(Courtesy Public Health Image Library, PHIL #479, Centers for Disease Control and Prevention, Atlanta, date unknown.)

Blastomyces dermatitidis does not remain viable for lengthy periods in clinical materials, so fungal cultures should be initiated immediately when blastomycosis is suspected. The yeast form, and in some strains the mycelial form, is sensitive to cycloheximide, so fungal media with and without cycloheximide should be used. Generally the fungus matures within 2 weeks, but it is recommended to hold cultures for 8 weeks before discarding them as negative. At 25° to 30°C, the mycelial form is quite variable. Colonies range from flat and glabrous to cottony white or tan, and may have concentric rings (Figure 47-4). Visible spicules, consisting of aerial mycelia, may be evident within 1 week. Older cultures become brownish tan with a reverse tan pigment. The mycelial form is extremely infectious and great caution should be exercised when handling cultures. Plates should be examined in a biological safety cabinet.

FIGURE 47-4 Plate with mycelial growth of Blastomyces dermatitidis.

(Courtesy Public Health Image Library, PHIL #490, Centers for Disease Control and Prevention, Atlanta, date unknown, Libero Ajello.)

Because B. dermatitidis is a thermally dimorphic fungus, demonstration of conversion from the mycelial to the yeast form is required for definitive identification. After isolation of the mold, colonies should be plated onto an enriched medium such as brain-heart infusion agar with 5% sheep or bovine blood and incubated at 37° C for several weeks. Yeasts are waxy, wrinkled, and tan to cream. Some strains fail to convert in vitro, and it may be necessary to examine these strains with a specific oligonucleotide probe or by way of an exoantigen test to confirm the identity of the organism.

The yeast cells are the primary fungal structures found in tissue, although occasional mycelia may be observed. In hematoxylin-and-eosin–stained tissues, it may be difficult to see the forms. Use of fungal stains will aid in the visualization process.

Commercially available serologic testing may assist in diagnosis. Complement fixation and immunodiffusion tests generally lack sensitivity. An antibody test based on the immunodominant antigen WI-1 has good sensitivity and specificity but is not widely available. Titers become high, decline during treatment, and persist for months. A highly sensitive and specific DNA probe assay for confirmation of the identity of clinical isolates has been reported.

Prevention and Control

The risk for exposure to blastomycosis remains small even in areas where the disease is endemic, and few public health recommendations have been developed for disease prevention.

Amphotericin B lipid complex has been used to treat blastomycosis in dogs where it has proven to be both safe and effective. Treatment with itraconazole is also effective.

Because blastomycosis may pose an occupational hazard for veterinarians or veterinary technicians, precautions should be taken to decrease the likelihood of exposure when handling animals or tissues with suspected or confirmed blastomycosis.

Disease and Epidemiology

Coccidioides spp. are residents of soil in certain arid and semiarid regions of North, Central, and South America. Endemic foci are located within the Lower Sonoran Life Zone in North America, which encompasses portions of Arizona, California, New Mexico, Texas, Utah, Nevada, and northern Mexico. Other endemic foci are in portions of Guatemala, Argentina, Paraguay, Colombia, and Venezuela. Climatic conditions in these regions are conducive to fungal propagation, with average rainfalls of 10 inches per year during a monsoon season, average summer temperatures of 100° F, and average winter temperatures of 35° F. The fungus can live indefinitely in alkaline soils where there is a high content of salt and carbonized organic material.

As a dimorphic fungus, Coccidioides spp. have an environmental or saprobic form and a parasitic tissue form. The mycelial form exists in the soil and is undetectable during the wet winter and spring, but actively grows in the upper layers of the soil during a hot, dry summer (Figure 47-5). Growth in the soil is directly affected by competition with other microorganisms. Penicillium janthinellum and Bacillus subtilis, which proliferate duringthe rainy season, appear to be important inhibitory organisms. Hyphae produce infective arthrospores that become aerosolized in the late summer and fall during dust storms or excavation. Winds may carry arthroconidia up to 400 miles from their point of origin. The fungus survivesin rodent burrows, and rodents can apparently distribute it in the environment.

FIGURE 47-5 Natural history of the saprobic and parasitic life cycle of coccidioidomycosis. Mold to spherule transition occurs in a susceptible host at 37° C (1–7). Arthroconidia in the environment (1) mature into mycelia, which fragment into arthrospores and are inhaled by the host.

(Courtesy Ashley E. Harmon.)

Naturally occurring coccidioidomycosis is seen most frequently in humans, dogs, horses, and llamas. Sporadic cases of disease have been reported in other species, including cattle, sheep, swine, cats, burros, desert rodents, a bottlenose dolphin, sea lions, sea otters, armadillos, a mountain lion, coyotes, chinchilla, Bengal tigers, nonhuman primates, and a snake.

In humans, more than half of the infections are asymptomatic. Clinical manifestations range from an influenza-like illness to severe pneumonia, and rarely, extrapulmonary disseminated disease. The last form is one of the most severe systemic mycoses, which may result in extensive tissue damage to the bones, joints, skin, and central nervous system. Coccidioidomycosis is uniformly fatal in the immunocompromised patient.

The high incidence of coccidioidomycosis in the dog may be a result of its propensity for sniffing or digging in the soil. Large and medium-sized male dogs are affected most commonly. Surveys of coccidioidomycosis in dogs indicate that 80% of dogs have primary pulmonary infection and 20% develop disseminated disease. Clinical signs noted at presentation vary widely and include dyspnea, anorexia, cough, weight loss, lameness, draining tracts, abscesses, lymphadenopathy, meningitis, and intermittent diarrhea. Extrapulmonary dissemination occurs most frequently to the bone and skin. A primary cutaneous form of the disease has been rarely reported and may have been preceded by trauma or wounds.

Pulmonary coccidioidal granuloma in cattle was first reported in 1937. Although frequently noted in slaughtered animals in the southwestern United States, the disease appears to be mild and self-limiting in ruminants, being confined to the bronchial and mediastinal lymph nodes. The lesions have been confused with those of tuberculosis. Infection is seen more often in feedlot animals than in those on pasture.

Unlike other herbivorous animals, horsesand llamas may develop disseminated coccidioidomycosis. Equine coccidioidomycosis may have a gender-related predilection for females, and although a variety of breeds were affected, Arabians may be overrepresented. Abortion, mastitis, osteomyelitis, nasal granuloma, meningitis, pneumonia, and a visceral form (hepatic, splenic, and renal involvement) have been reported in the horse. Llamas appear to be highly susceptible to Coccidioides spp. Respiratory, dermatitis, osteomyelitis, meningitis, and polyperiarthritis syndromes have been recognized in this species.