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Fungal Disease - Blastomycosis

Etiology

Blastomycosis is caused by inhaling the conidia of Blastomyces dermatitidis, a dimorphic fungus and environmental saprophyte. B. dermatitidis is found in moist, acidic, or sandy soil, primarily in North America along the Ohio, Mississippi, Missouri, St. Lawrence, and Tennessee Rivers; in southern Mid-Atlantic states; and in the southern Great Lakes region.

Predisposed animals

Blastomycosis is rare in cats and uncommon in dogs, with highest incidences reported in young, male, large-breed outdoor dogs, especially hounds and sporting breeds.

Pathogenesis

After inhalation, a lung infection is established that disseminates to lymph nodes, eyes, skin, bones, and other organs. Rarely, direct inoculation may result in localized skin disease, but cutaneous blastomycosis is more commonly a sign of disseminated disease.

Clinical signs

Nonspecific symptoms include anorexia, weight loss, and fever. Other symptoms, depending on the organ systems involved, may include exercise intolerance, cough, dyspnea, lymph adenomegaly, uveitis, retinal detachment, glaucoma, lameness, and CNS signs.

Lesions

May be found anywhere on the body but are most common on the face, nasal planum, and nail beds.

Cutaneous lesions include discrete subcutaneous abscesses and firm, proliferative, ulcerated masses with fistulous tracts that drain a serosanguineous to purulent exudate.

Differential diagnosis

Differentials include other fungal and bacterial infections, neoplasia, and foreign body reaction.

Diagnosis

Cytology (exudate, tissue aspirate): suppurative or pyogranulomatous inflammation with large, round, broad-based budding yeasts that have thick, refractile, double-contoured cell walls.

Dermatohistopathology: nodular to diffuse suppurative to (pyo)granulomatous dermatitis with large, thick, double-walled, broad-based budding yeasts.

Agar-gel immunodiffusion: detection of serum antibodies against B. dermatitidis; in early infection, test results may be negative.

Fungal culture (not needed to confirm diagnosis unless cytology and histopathology fail to reveal organism [submit to diagnostic laboratory because fungal cultures are highly infectious]): B. dermatitidis.

Radiography: pulmonary changes if lungs are involved; osteolytic lesions if long bones are involved.

PCR analysis, where available, may simplify the diagnosis.

Treatment

Long-term (minimum 2–3 months) systemic antifungal therapy should be administered and continued 1 month beyond complete clinical resolution.

The drug of choice is itraconazole (Sporanox). For cats, 5 mg/kg PO should be administered with food every 12 hours. For dogs, 5 mg/kg should be administered PO with food every 12 hours for 5 days, followed by 5 mg/kg PO with food every 24 hours.

Alternative therapies include the following:

  • » Fluconazole 5–10 mg/kg PO or IV q 24 hours
  • » Amphotericin B 0.5 mg/kg (dogs) or 0.25 mg/kg (cats) IV three times per week until a cumulative dose of 8–12 mg/kg (dogs) or 4–6 mg/kg (cats) is administered
  • » Amphotericin B lipid complex (dogs) 1.0 mg/kg IV three times per week until a cumulative dose of 12 mg/kg is administered

Prognosis: is good unless CNS or severe lung involvement is present. Regardless of the therapy used, approximately 20% of dogs relapse within 1 year of treatment because of premature discontinuation of therapy or the use of compounded medications; however, they usually respond to retreatment with itraconazole (Sporanox).


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