Coccidioidomycosis
(Valley Fever)
Coccidioidomycosis (Valley Fever) is a dustborne, noncontagious infection caused by the dimorphic fungus Coccidioides immitis. Infections are limited to arid and semiarid regions of the southwestern USA and to similar areas of Mexico and Central and South America. Although many species of animals, including people, are susceptible, only dogs are affected significantly. Placental infection leading to abortion and osteomyelitis have been described in horses. Ruminants and pigs may have subclinical infections with lesions restricted to foci in the lungs and to thoracic lymph nodes. Inhalation of fungal spores is the only established mode of infection, and spores may be carried on dust particles. Epidemics may occur when rainy periods are followed by drought, resulting in dust storms. Most bovine infections are contracted in dusty feedlots.
Clinical Findings and Lesions:
The disease varies from inapparent (cattle, sheep, pigs, dogs, cats) to progressive, disseminated, and fatal (dogs, nonhuman primates, cats, and people). Coccidioidomycosis is primarily a respiratory disease that ranges from self-limiting to chronic. Dissemination occurs in ~20% of canine infections, with many tissues, especially eyes, joints, and bone, affected. Clinical signs can vary greatly, depending on organ involvement and severity of infection. Dogs with disseminated disease may have chronic cough, anorexia, cachexia, lameness, enlarged joints, fever, and intermittent diarrhea. Dissemination to the skin with draining ulceration may occur, but primary infection through the skin is rare. Cats infected with C immitis most often present with dermatologic signs (draining skin lesions, subcutaneous granulomatous masses, abscesses), fever, inappetence, and weight loss. Less common clinical signs in cats include respiratory (dypsnea), musculoskeletal (lameness), neurologic, and ophthalmologic abnormalities. Approximately 50% of infected cats appear to have disseminated disease.
Gross lesions may be limited to the lungs, mediastinum, and thoracic lymph nodes, or may be disseminated to various organs. Lesions are discrete, variable-sized nodules with a firm, gray-white cut surface, and resemble those of tuberculosis (see Tuberculosis and other Mycobacterial Infections). The nodules are pyogranulomas composed of epithelioid and giant cells, and the center of some foci may contain purulent exudate and fungal organisms. Some lesions may have mineralized foci.
Diagnosis:
In endemic areas, coccidioidomycosis should be considered in dogs with chronic bronchopulmonary disease and when pulmonary nodules and enlarged lymph nodes are found on thoracic radiographs. The lesions are pyogranulomas that contain C immitis free in the exudate and in epithelioid and multinucleate giant cells. The organisms vary in size and appear as relatively large (20–80 μm, up to 200 μm) spherules with a double-contoured wall. The mature spherules (sporangia) contain endospores (sporangiospores) 2–5 μm in diameter. Diagnosis is established by demonstrating the spherules in tissues. Serum can also be tested by agar gel immunodiffusion (AGID) assays for detection of precipitin and complement-fixing antibodies. Currently, most commercial laboratories run AGID assays for IgG and IgM antibodies; results of these assays are specific but relatively insensitive. An enzyme immunoassay for Coccidioides galactomannan antigen useful in human infections has proved insensitive in dogs. A presumptive diagnosis can be made when serology is positive in an animal with consistent clinical signs. Attempts to culture the fungus should be restricted to those laboratories equipped to handle such dangerously infective cultures.
Treatment:
Disease is often self-limiting, but if chronic respiratory signs or multisystemic disease are present, longterm antifungal therapy is needed; with disseminated infection, treatment of at least 6–12 mo is typical. Fluconazole (2.5–10 mg/kg/day) is the most commonly used drug to treat disseminated or chronic respiratory infections. Ketoconazole (10–30 mg/kg/day) and itraconazole (10 mg/kg/day) are also commonly used to treat dogs with coccidioidomycosis but are more expensive and have a higher incidence of adverse effects. Amphotericin B may be the most effective antifungal drug, but it is highly nephrotoxic. It may be indicated in animals that either do not improve or are unable to tolerate the azole antifungals.
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