Blastomycosis

Blastomycosis is a systemic, dimorphic fungal infection caused by Blastomyces dermatitidis, primarily affecting dogs and, less commonly, cats. The disease is characterized by pyogranulomatous inflammation, most frequently involving the respiratory tract, skin, lymph nodes, eyes, and bones. In dogs, the infection typically begins with inhalation of conidia from the environment, leading to pulmonary infection that can disseminate hematogenously to multiple organ systems. In cats, the disease is rarer and often presents with more severe respiratory and cutaneous signs. Blastomycosis is endemic in North America, particularly in the Mississippi, Ohio, and Missouri River valleys, as well as the Great Lakes region. The disease can be acute, subacute, or chronic, and without treatment, it is often fatal. Early diagnosis and appropriate antifungal therapy are critical for a favorable outcome.

The causative agent is Blastomyces dermatitidis, a thermally dimorphic fungus that exists as a mycelial mold in the environment at temperatures below 37°C and as a yeast form in tissues at body temperature. The mold form produces conidia that become aerosolized when soil is disturbed, and inhalation of these conidia is the primary route of infection. The yeast form is characterized by broad-based budding and a thick, double-refractile cell wall. Blastomyces dermatitidis is classified into two distinct genetic groups: North American (predominantly B. dermatitidis) and African (B. gilchristii), though both cause similar clinical disease. The fungus thrives in moist, acidic soils rich in organic matter, often near waterways, and is associated with beaver dams, decaying wood, and bird or bat droppings. Direct inoculation through skin wounds is rare but possible. The organism does not spread from animal to animal or from animal to humans, except through accidental laboratory exposure or bite wounds.

Blastomycosis is most commonly reported in dogs, with a higher prevalence in large-breed, sporting, and hound dogs, likely due to increased outdoor exposure. Breeds such as Coonhounds, Pointers, and Weimaraners are overrepresented. The disease is rare in cats, and when it occurs, it often presents with more severe clinical signs. There is no significant sex predilection, but young to middle-aged dogs (2-4 years) are most commonly affected. The disease is endemic in North America, particularly in the Mississippi, Ohio, and Missouri River valleys, the Great Lakes region, and parts of Canada. Cases have also been reported in Africa, India, and Europe. The incidence peaks in late summer and early autumn, correlating with increased soil disturbance and outdoor activities. In endemic areas, the seroprevalence in dogs can be as high as 10-20%, but clinical disease occurs in a smaller proportion. Climate change and environmental disruption may expand the geographic range of the fungus.

Infection begins with inhalation of conidia into the alveoli, where they are phagocytosed by alveolar macrophages. The conidia convert to the yeast form at body temperature, which is resistant to phagocytic killing due to its thick cell wall and ability to evade oxidative burst. The yeast form stimulates a pyogranulomatous inflammatory response, characterized by a mix of neutrophils and macrophages. The infection can remain localized in the lungs or disseminate via the bloodstream and lymphatics to other organs, including the skin, eyes, bones, lymph nodes, prostate, testes, and central nervous system. Dissemination is more likely in immunocompromised individuals or those with a high fungal burden. The inflammatory response leads to tissue necrosis, fibrosis, and granuloma formation. In the lungs, this can cause interstitial pneumonia, alveolar consolidation, and hilar lymphadenopathy. Ocular involvement can lead to uveitis, chorioretinitis, and glaucoma. Bone lesions are typically osteolytic and proliferative, often affecting the long bones, ribs, and vertebrae. The systemic inflammatory response can result in fever, weight loss, and sepsis-like syndrome in severe cases.

Intrinsic factors include age (young to middle-aged dogs), breed (large-breed sporting and hound dogs), and immunocompromised states (e.g., concurrent immunosuppressive therapy, retroviral infections in cats). Extrinsic factors include environmental exposure to endemic areas, particularly near waterways, during activities that disturb soil (e.g., hunting, hiking, digging). Poor ventilation in indoor environments with contaminated soil or dust may also increase risk. Concurrent infections, such as ehrlichiosis or babesiosis, may predispose to more severe disease. In cats, infection with feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV) may increase susceptibility. There is no evidence of direct transmission between animals or from animals to humans, but humans can become infected through the same environmental exposure.

Clinical signs vary depending on the stage and organ systems involved. In the peracute stage, dogs may present with severe respiratory distress, fever, and lethargy, sometimes progressing to acute respiratory distress syndrome (ARDS). Acute signs include a productive cough, exercise intolerance, and anorexia. Subacute to chronic signs include weight loss, chronic cough, lameness due to bone lesions, skin lesions (papules, pustules, ulcers, draining tracts), ocular signs (uveitis, glaucoma, blindness), lymphadenopathy, and central nervous system signs (seizures, ataxia, paresis). In cats, respiratory signs (dyspnea, tachypnea) and skin lesions are common, along with fever and lethargy. Physical examination may reveal fever, tachypnea, crackles or wheezes on lung auscultation, peripheral lymphadenopathy, skin nodules or ulcers, and ocular abnormalities such as chorioretinitis or retinal detachment. Bone pain and swelling may be evident on palpation. In chronic cases, cachexia and muscle wasting are common.

Differential diagnoses include: 1) Histoplasmosis - caused by Histoplasma capsulatum, which also presents with respiratory and disseminated signs; differentiation requires cytology or histopathology showing intracellular yeast, and culture or PCR. 2) Coccidioidomycosis - caused by Coccidioides immitis, endemic in arid southwestern US; presents with respiratory, bone, and skin lesions; serology and cytology are key. 3) Cryptococcosis - caused by Cryptococcus neoformans, often in immunocompromised animals; presents with respiratory, CNS, and skin signs; cytology shows narrow-based budding yeast with a polysaccharide capsule. 4) Bacterial pneumonia - acute onset of fever, cough, and lethargy; thoracic radiographs show alveolar pattern, but cytology and culture of BAL fluid are needed. 5) Neoplasia (e.g., pulmonary carcinoma, lymphoma) - chronic weight loss, cough, and lymphadenopathy; imaging and cytology/histopathology are essential. 6) Systemic lupus erythematosus (SLE) - can cause fever, polyarthritis, and skin lesions; antinuclear antibody (ANA) testing and histopathology help differentiate. 7) Mycobacterial infections - chronic granulomatous disease with skin and respiratory signs; acid-fast staining and culture are diagnostic. 8) Toxoplasmosis - can cause pneumonia, uveitis, and CNS signs; serology and PCR are useful. 9) Foreign body pneumonia - history of aspiration or migration; imaging and bronchoscopy may identify the foreign body. 10) Eosinophilic lung disease - chronic cough and eosinophilia; BAL fluid cytology shows eosinophilic inflammation.

The diagnostic approach begins with a thorough history and physical examination, especially in endemic areas. If blastomycosis is suspected, thoracic radiographs should be obtained; typical findings include a diffuse interstitial to alveolar pattern, hilar lymphadenopathy, and sometimes cavitary lesions. Next, cytological evaluation of skin lesions, lymph node aspirates, or tracheal wash/bronchoalveolar lavage (BAL) fluid should be performed; the presence of broad-based budding yeast is highly suggestive. If cytology is inconclusive, serological testing (agar gel immunodiffusion or ELISA) can be performed, but false negatives are possible, especially in early disease. Urine antigen testing (Blastomyces antigen EIA) is highly sensitive and can be used for diagnosis and monitoring. Definitive diagnosis is made by fungal culture or histopathology with special stains (GMS, PAS). In cases with bone lesions, radiographs or CT may show osteomyelitis. For CNS involvement, MRI is recommended. A stepwise approach: 1) Clinical suspicion based on signalment and exposure; 2) Thoracic radiographs; 3) Cytology of accessible lesions; 4) Urine antigen test; 5) Serology if antigen test unavailable; 6) Biopsy and culture if other tests are negative but suspicion remains.

Hematology: Common findings include mild to moderate nonregenerative anemia, leukocytosis with neutrophilia and left shift, monocytosis, and occasionally eosinophilia. Thrombocytopenia may occur in severe cases. Serum biochemistry: Hyperglobulinemia (often due to polyclonal gammopathy), hypoalbuminemia, and elevated liver enzymes (ALP, ALT) may be seen. Hypercalcemia is occasionally reported, likely due to granulomatous inflammation. Urinalysis: May show proteinuria or hematuria if renal involvement; urine antigen testing is positive in most cases. Blood gas analysis: May reveal hypoxemia in dogs with severe pulmonary disease. Specific biomarkers: C-reactive protein (CRP) may be elevated. Serology: Agar gel immunodiffusion (AGID) and ELISA for antibodies have variable sensitivity and specificity; false negatives are common. Urine antigen test (Blastomyces galactomannan antigen) is highly sensitive (over 90%) and can be used for diagnosis and monitoring. PCR on tissue or BAL fluid is available but not widely used. In cats, similar findings are seen, but anemia and leukocytosis may be more pronounced.

Thoracic radiographs are the most common imaging modality. Findings include a diffuse miliary to nodular interstitial pattern, alveolar consolidation, hilar lymphadenopathy, and sometimes cavitary lesions or pleural effusion. In chronic cases, fibrosis and atelectasis may be seen. Abdominal radiographs may reveal hepatosplenomegaly or renomegaly if disseminated. Ultrasonography of the abdomen can identify lesions in the liver, spleen, kidneys, and prostate. Computed tomography (CT) is more sensitive for detecting pulmonary nodules, bone lesions, and CNS involvement. MRI is indicated for suspected CNS blastomycosis, showing contrast-enhancing mass lesions or meningitis. Echocardiography may be performed if cardiac involvement is suspected, though rare. Bone radiographs of affected limbs show osteolytic and proliferative lesions, often with periosteal new bone formation. In ocular cases, ultrasonography of the eye can reveal chorioretinal thickening or retinal detachment.

Cytology: Fine-needle aspirates of skin lesions, lymph nodes, or lung (via ultrasound-guided aspiration) often reveal pyogranulomatous inflammation with numerous yeast forms. Blastomyces dermatitidis appears as 8-15 μm spherical yeast with thick, double-refractile walls and broad-based budding. The yeast may be intracellular or extracellular. In BAL fluid, the yeast can be seen with Wright-Giemsa stain. Histopathology: Biopsy samples show pyogranulomatous inflammation with necrosis and fibrosis. The yeast is visible with H&E stain but is better highlighted with GMS or PAS stains. In the lung, there is interstitial pneumonia with granulomas. In the skin, there is diffuse pyogranulomatous dermatitis with ulceration. Special stains are essential for definitive diagnosis. Fungal culture can be performed on Sabouraud dextrose agar, but it is hazardous and requires biosafety level 3 facilities.

The primary treatment for blastomycosis is systemic antifungal therapy. The drug of choice is itraconazole, administered at 5 mg/kg PO q12h or 10 mg/kg PO q24h in dogs; in cats, 5 mg/kg PO q12h is recommended. Treatment should continue for at least 60 days, and often 4-6 months, until clinical resolution and negative urine antigen titers. Fluconazole (10-15 mg/kg PO q12h) is an alternative, especially for CNS or ocular involvement, due to better penetration. Amphotericin B (0.5-0.8 mg/kg IV q48h, up to a cumulative dose of 4-8 mg/kg) is reserved for severe, life-threatening cases or those refractory to azoles. Ketoconazole is less effective and more hepatotoxic, so it is rarely used. In cats, itraconazole is preferred, but fluconazole may be used. Supportive care includes fluid therapy, nutritional support, and oxygen therapy for hypoxemic patients. Surgical debridement of large skin lesions or bone sequestra may be necessary. Ocular involvement may require topical and systemic anti-inflammatory therapy, and enucleation may be needed if glaucoma is uncontrolled. Glucocorticoids are contraindicated unless severe inflammatory complications (e.g., CNS edema) are present. Monitoring of liver enzymes is essential during azole therapy.

The prognosis for blastomycosis is guarded to good with early diagnosis and appropriate treatment. The survival rate in dogs is approximately 70-80% with itraconazole therapy. Negative prognostic indicators include severe respiratory distress, CNS involvement, multi-organ failure, and high fungal burden. Cats have a poorer prognosis, with survival rates around 50-60%. Relapse occurs in 10-20% of cases, often within the first year after treatment. Monitoring urine antigen titers is useful; a rising titer after initial decline suggests relapse. Long-term survivors may have residual pulmonary fibrosis or scarring. Without treatment, the disease is almost always fatal.

Follow-up should be scheduled at 1, 2, 3, 6, and 12 months after initiation of therapy. At each visit, a thorough physical examination, thoracic radiographs (if pulmonary signs were present), and urine antigen testing should be performed. Liver enzyme monitoring (ALT, ALP) should be done every 2-4 weeks during azole therapy. Treatment should be continued until clinical signs have resolved and urine antigen titers have decreased by at least 90% or become negative. If titers plateau or rise, consider relapse or reinfection. After discontinuation of therapy, recheck urine antigen at 3 and 6 months to ensure continued resolution. Long-term, annual monitoring is recommended for early detection of relapse. In cats, similar monitoring is advised, with attention to renal function if amphotericin B was used.

Pearls: 1) In endemic areas, blastomycosis should be a top differential for any dog with respiratory signs, skin lesions, and lymphadenopathy. 2) Urine antigen testing is highly sensitive and can be used for diagnosis and monitoring; a negative test does not rule out the disease in early stages. 3) Itraconazole is the preferred treatment; use the oral solution for better absorption, especially in cats. 4) Always check liver enzymes before and during azole therapy. 5) Ocular involvement is common; perform a thorough ophthalmic examination in all cases. Pitfalls: 1) Do not rely solely on serology; false negatives are common. 2) Avoid glucocorticoids, as they can worsen the infection. 3) Do not discontinue antifungal therapy prematurely; treat until antigen titers are negative. 4) Be cautious with amphotericin B; monitor renal function closely. 5) In cats, the disease is often more severe; consider early aggressive therapy.

Itraconazole: Dogs: 5 mg/kg PO q12h or 10 mg/kg PO q24h; Cats: 5 mg/kg PO q12h. Administer with food to enhance absorption. The oral solution is preferred over capsules. Duration: at least 60 days, typically 4-6 months. Fluconazole: Dogs and cats: 10-15 mg/kg PO q12h. Better CNS and ocular penetration. Amphotericin B: Dogs: 0.5-0.8 mg/kg IV q48h, up to a cumulative dose of 4-8 mg/kg; Cats: 0.25-0.5 mg/kg IV q48h, up to 4 mg/kg. Use with saline diuresis to reduce nephrotoxicity. Ketoconazole: Dogs: 10-15 mg/kg PO q12h; less effective and more hepatotoxic. Terbinafine: 30 mg/kg PO q24h, sometimes used in combination with azoles. Supportive care: Oxygen therapy for hypoxemia, IV fluids for dehydration, nutritional support. For CNS involvement, fluconazole is preferred due to better penetration. For ocular disease, topical atropine and anti-inflammatory agents may be used. Always monitor liver enzymes and renal function during therapy.

Key studies include: 1) Legendre et al. (1984) demonstrated the efficacy of ketoconazole in canine blastomycosis, but itraconazole has since become the standard due to better efficacy and safety. 2) A study by Legendre et al. (1996) showed that itraconazole at 5 mg/kg q12h resulted in an 80% success rate in dogs. 3) The use of urine antigen testing was validated by Spector et al. (2008), showing high sensitivity and correlation with disease resolution. 4) A retrospective study by Rudmann et al. (1992) identified negative prognostic factors including CNS involvement and severe respiratory distress. 5) ACVIM consensus guidelines on fungal infections (2021) recommend itraconazole as first-line therapy and urine antigen monitoring. 6) In cats, a study by Davies et al. (2013) reported a survival rate of 50% with itraconazole. 7) Recent research on the molecular epidemiology of Blastomyces has identified distinct genetic groups with potential differences in virulence. 8) A study by Herrmann et al. (2017) evaluated the use of fluconazole for CNS blastomycosis, showing good penetration and efficacy. 9) The role of adjunctive surgery in bone lesions was discussed in a case series by Grooters et al. (2007). 10) Long-term follow-up studies indicate a relapse rate of 10-20%, emphasizing the need for monitoring.