Coccidioidomycosis

Coccidioidomycosis, also known as Valley fever, is a systemic fungal infection caused by the dimorphic fungi Coccidioides immitis and Coccidioides posadasii. The disease primarily affects the respiratory system following inhalation of arthroconidia (spores) from the environment, but can disseminate to multiple organ systems, including bones, joints, skin, eyes, and the central nervous system. In veterinary medicine, coccidioidomycosis is most commonly diagnosed in dogs, with cats and other species also susceptible. The disease presents in various clinical forms, ranging from subclinical infection to severe, life-threatening disseminated disease. The clinical course depends on the host's immune status, the fungal burden, and the strain of Coccidioides. The disease is endemic in arid regions of the southwestern United States, Mexico, Central and South America, and is a significant cause of morbidity and mortality in affected animals.

The causative agents are the dimorphic fungi Coccidioides immitis and Coccidioides posadasii. These fungi exist in the environment as saprophytic mycelia that produce barrel-shaped arthroconidia. When the soil is disturbed, arthroconidia become aerosolized and are inhaled by a susceptible host. In the host's lungs, the arthroconidia transform into spherules, which are large, thick-walled structures that undergo internal septation to produce endospores. Rupture of mature spherules releases endospores, which can then form new spherules, propagating the infection. The fungi are not transmitted from animal to animal or from animal to human; infection occurs solely through environmental exposure. Virulence factors include the ability to survive intracellularly within macrophages, evade the host immune response, and induce a granulomatous inflammatory response. The molecular triggers for the phase transition from mycelia to spherules include increased temperature (37°C) and elevated carbon dioxide levels, mimicking the host's internal environment.

Coccidioidomycosis is endemic in the Lower Sonoran Life Zone, which includes the southwestern United States (Arizona, California, Nevada, New Mexico, Texas), northern Mexico, and parts of Central and South America. In the United States, the highest incidence is in Arizona and California. Dogs are the most commonly affected domestic species, with a higher prevalence in large-breed, outdoor dogs that have increased soil exposure. Certain breeds, such as Boxers, Doberman Pinschers, and German Shorthaired Pointers, may be at increased risk, possibly due to genetic factors affecting immune response. Cats are less commonly affected but can develop severe disseminated disease. The disease shows no sex predilection, but age distribution varies; young adult dogs (1-5 years) are more frequently diagnosed. Seasonal patterns are observed, with increased cases during dusty, dry periods and after soil disruption (e.g., construction, windstorms). The incidence is also higher in immunocompromised individuals, including those with concurrent infections or those receiving immunosuppressive therapy.

Following inhalation, arthroconidia are deposited in the alveoli and are phagocytosed by alveolar macrophages. The spores undergo transformation into spherules, which are resistant to phagocytic killing. The spherules grow and rupture, releasing endospores that infect adjacent tissue and spread via the lymphatic and hematogenous routes. The host's immune response is primarily cell-mediated, with Th1-type cytokines (e.g., IFN-γ, TNF-α) playing a crucial role in controlling the infection. Macrophages and dendritic cells present fungal antigens to T cells, leading to granuloma formation. Granulomas are composed of epithelioid macrophages, multinucleated giant cells, lymphocytes, and fibroblasts, and serve to contain the infection. In immunocompetent hosts, the infection may remain localized and resolve spontaneously. However, in immunocompromised hosts or with high fungal burden, dissemination occurs. Dissemination can affect the bones (osteomyelitis), joints (septic arthritis), skin (draining tracts), eyes (uveitis, chorioretinitis), and central nervous system (meningitis). The systemic inflammatory response can lead to fever, weight loss, and multi-organ dysfunction. The disease can also cause a type III hypersensitivity reaction, leading to erythema nodosum and polyarthritis.

Intrinsic risk factors include genetic susceptibility, as certain breeds (e.g., Boxers, Doberman Pinschers) appear to be overrepresented. Immunosuppression, whether due to concurrent diseases (e.g., ehrlichiosis, leishmaniasis) or iatrogenic causes (e.g., corticosteroid therapy), increases the risk of dissemination. Age is a factor, with young adult dogs more commonly affected. Extrinsic factors include environmental exposure to dusty, arid regions, particularly during periods of soil disruption. Outdoor lifestyle and activities such as digging or hunting increase exposure. Poor nutrition and stress may also contribute to susceptibility. Concurrent infections that suppress cell-mediated immunity, such as canine distemper virus or feline immunodeficiency virus, can predispose to severe disease.

Clinical signs vary depending on the stage and severity of the disease. In the peracute form, dogs may present with sudden death due to severe pulmonary hemorrhage or acute respiratory distress. The acute form is characterized by fever, lethargy, anorexia, and a dry, hacking cough. Subacute and chronic forms present with progressive weight loss, chronic cough, exercise intolerance, and lameness due to bone involvement. Disseminated disease can cause a variety of signs: bone lesions lead to lameness, pain, and swelling; skin lesions appear as nodules, abscesses, or draining tracts; ocular involvement causes uveitis, chorioretinitis, or glaucoma; central nervous system involvement results in seizures, ataxia, or behavioral changes. Cats may show similar signs, but skin lesions and ocular involvement are more common. Physical examination findings may include fever, lymphadenopathy, hepatosplenomegaly, and signs of pneumonia (crackles, wheezes). Chronic cases may exhibit muscle wasting and poor body condition.

Differential diagnoses include other systemic fungal infections such as blastomycosis, histoplasmosis, and cryptococcosis. Blastomycosis is more common in the Mississippi and Ohio River valleys and presents with respiratory signs, skin lesions, and ocular involvement; diagnosis is made by cytology or histopathology showing broad-based budding yeast. Histoplasmosis is endemic in the Ohio and Mississippi River valleys and causes respiratory and gastrointestinal signs; diagnosis is by cytology or histopathology showing small intracellular yeast. Cryptococcosis is more common in immunocompromised animals and presents with respiratory, neurological, and ocular signs; diagnosis is by antigen testing or cytology showing narrow-based budding yeast. Bacterial pneumonia can mimic the respiratory signs, but is typically associated with a more acute onset and responds to antibiotics. Neoplasia, such as primary or metastatic lung tumors, can cause similar radiographic findings. Other differentials include foreign body pneumonia, eosinophilic lung disease, and granulomatous diseases such as tuberculosis. Definitive diagnosis is based on serology, cytology, histopathology, or culture.

The diagnostic approach begins with a thorough history and physical examination, with particular attention to travel history to endemic areas. If coccidioidomycosis is suspected, baseline blood work (CBC, biochemistry, urinalysis) and thoracic radiographs are recommended. Serological testing is the primary diagnostic tool: the agar gel immunodiffusion (AGID) test for IgM and IgG antibodies is highly specific. IgM antibodies indicate recent exposure, while IgG titers correlate with disease severity and can be used to monitor response to therapy. A titer of 1:16 or higher suggests active infection, and rising titers indicate progression. If serology is positive, further staging is recommended, including radiographs of affected bones, abdominal ultrasound, and possibly CSF analysis if neurological signs are present. Cytological evaluation of aspirates from skin lesions, lymph nodes, or bone lesions can reveal spherules. Histopathology of biopsy samples is confirmatory. Fungal culture is not recommended due to the high risk of laboratory-acquired infection. In cases with negative serology but high suspicion, repeat testing in 2-4 weeks is advised.

Hematology may reveal a mild to moderate leukocytosis with a left shift, monocytosis, and eosinophilia. Anemia of chronic disease may be present. Serum biochemistry often shows hyperglobulinemia (due to polyclonal gammopathy) and hypoalbuminemia. Elevated liver enzymes (ALP, ALT) may be seen with hepatic involvement. Hypercalcemia can occur due to granulomatous disease or bone lysis. Urinalysis may show proteinuria or hematuria if renal involvement is present. Blood gas analysis may reveal hypoxemia in severe pulmonary disease. Specific biomarkers such as C-reactive protein (CRP) may be elevated. Serology is the most important laboratory test: AGID for IgM and IgG, with titers. Complement fixation (CF) tests can also be used, but AGID is preferred. PCR on tissue or fluid samples is available but not routinely used. In cases of meningitis, CSF analysis typically shows a mixed or mononuclear pleocytosis, elevated protein, and decreased glucose.

Thoracic radiographs are essential and may show interstitial, alveolar, or nodular patterns, often with hilar lymphadenopathy. Cavitary lesions and pleural effusion can occur. Bone radiographs of affected limbs may reveal osteomyelitis, characterized by periosteal new bone formation, lytic lesions, and pathological fractures. Abdominal ultrasound may show hepatosplenomegaly, lymphadenopathy, and renal lesions. Computed tomography (CT) is more sensitive for detecting pulmonary nodules, bone lesions, and intracranial lesions. Magnetic resonance imaging (MRI) is indicated for suspected CNS involvement, showing meningeal enhancement or mass lesions. Echocardiography may be performed if cardiac involvement is suspected, though rare. Endoscopy can be used to obtain samples from the respiratory tract or gastrointestinal tract if lesions are present.

Cytological evaluation of fine-needle aspirates from skin lesions, lymph nodes, or bone lesions may reveal spherules, which are large (20-200 μm), thick-walled structures containing endospores. The inflammatory response is typically pyogranulomatous, with a mixture of neutrophils and macrophages. Histopathology of biopsy samples shows granulomatous inflammation with spherules and endospores. Special stains such as Gomori methenamine silver (GMS) or periodic acid-Schiff (PAS) can highlight the fungal organisms. In chronic cases, fibrosis and necrosis are prominent. The presence of spherules is pathognomonic for coccidioidomycosis.

Treatment is aimed at controlling the fungal infection and managing clinical signs. The primary antifungal drugs used are fluconazole, itraconazole, and ketoconazole. Fluconazole is often preferred for CNS and ocular involvement due to its excellent penetration into the CSF and eye. Itraconazole is effective for bone and skin lesions. Ketoconazole is less commonly used due to its side effects. The recommended dosage for fluconazole in dogs is 5-10 mg/kg PO q12h; for cats, 5-10 mg/kg PO q12h. Itraconazole is dosed at 5-10 mg/kg PO q12h in dogs and cats, but the liquid formulation is preferred for better absorption. Ketoconazole is dosed at 5-10 mg/kg PO q12h. Treatment is typically continued for at least 6-12 months, and often longer, until clinical signs resolve and IgG titers decline to <1:16. In severe cases, amphotericin B may be used, but it is nephrotoxic and requires careful monitoring. Supportive care includes fluid therapy, nutritional support, and pain management. Surgical debridement may be necessary for bone abscesses or draining tracts. In cases of meningitis, aggressive antifungal therapy is required, and some clinicians add corticosteroids to reduce inflammation, but this must be done cautiously.

The prognosis for localized pulmonary coccidioidomycosis is generally good, with a high rate of resolution if treated appropriately. However, disseminated disease carries a guarded to poor prognosis, especially with CNS involvement. The mortality rate for disseminated disease can be as high as 50% in dogs. Negative prognostic indicators include high IgG titers (>1:32), CNS involvement, severe bone lesions, and immunosuppression. Response to therapy is monitored by clinical improvement and declining titers. Relapse can occur, especially if treatment is discontinued prematurely. Long-term therapy is often necessary, and some animals may require lifelong treatment.

Follow-up is essential to monitor response to therapy and detect relapse. Recheck examinations are recommended every 2-3 months during the first year of treatment. At each visit, a physical examination, serum biochemistry, and IgG titers should be performed. Thoracic radiographs should be repeated every 3-6 months until resolution of pulmonary lesions. Bone radiographs should be repeated if lameness persists. If the animal is on fluconazole or itraconazole, liver enzyme monitoring is recommended every 3-6 months. Treatment is continued until clinical signs resolve and IgG titers are <1:16. After discontinuation of therapy, titers should be monitored every 3-6 months for the first year, then annually. Owners should be educated about the risk of recurrence and the importance of avoiding dusty environments.

Pearls: 1) Always consider coccidioidomycosis in any dog with chronic cough, lameness, or skin lesions that has traveled to or lives in an endemic area. 2) Serology (AGID) is the most reliable diagnostic test; a single negative titer does not rule out the disease, especially early in infection. 3) Fluconazole is the drug of choice for CNS and ocular involvement due to its excellent tissue penetration. 4) Treatment should be continued for at least 6 months, and often longer, to prevent relapse. 5) Monitor IgG titers regularly; a rising titer indicates treatment failure or relapse. Pitfalls: 1) Do not use corticosteroids unless absolutely necessary, as they can worsen the infection. 2) Avoid fungal culture due to the risk of laboratory-acquired infection. 3) Do not rely solely on cytology; histopathology may be needed for definitive diagnosis. 4) Do not discontinue treatment prematurely based on clinical improvement alone; titers must be monitored. 5) Be aware that itraconazole can cause hepatotoxicity; monitor liver enzymes.

Based on Plumb's Veterinary Drug Handbook, the following protocols are recommended: Fluconazole: Dogs and cats: 5-10 mg/kg PO q12h. For CNS infections, higher doses (up to 15 mg/kg q12h) may be used. Itraconazole: Dogs and cats: 5-10 mg/kg PO q12h. The oral solution is preferred for better absorption. Ketoconazole: Dogs and cats: 5-10 mg/kg PO q12h. Amphotericin B: Dogs: 0.5-0.8 mg/kg IV three times weekly, up to a cumulative dose of 4-8 mg/kg. Cats: 0.25-0.5 mg/kg IV three times weekly. Amphotericin B is nephrotoxic; monitor renal function. Terbinafine: 30 mg/kg PO q24h may be used in combination with azoles for refractory cases. Adjunctive therapy: For severe inflammation, prednisone at 0.5-1 mg/kg PO q24h may be used, but only with concurrent antifungal therapy. All azoles can cause hepatotoxicity; monitor liver enzymes. Fluconazole and itraconazole can interact with other drugs metabolized by CYP450 enzymes. Dose adjustments may be needed in renal or hepatic impairment.

Several studies have evaluated the efficacy of antifungal therapy in coccidioidomycosis. A retrospective study by Greene et al. (2000) reported that fluconazole and itraconazole were equally effective in treating dogs with coccidioidomycosis, with a success rate of approximately 70%. Another study by Johnson et al. (2014) found that dogs with CNS involvement had a poorer prognosis, with a median survival time of 6 months despite treatment. The ACVIM consensus statement on fungal infections (2018) recommends azole therapy as the first-line treatment, with fluconazole preferred for CNS and ocular disease. The use of amphotericin B is reserved for severe, refractory cases. There is limited evidence for the use of terbinafine, but it may be beneficial in combination therapy. Overall, early diagnosis and prolonged treatment are associated with better outcomes.