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Fungal Disease - Histoplasmosis

Etiology

Histoplasmosis is a systemic disease caused by Histoplasma capsulatum, a dimorphic fungus and soil saprophyte. After conidia are inhaled or ingested, an infection is established in the lungs or gastrointestinal (GI) tract that then disseminates elsewhere.

H. capsulatum is found worldwide in temperate and subtropical areas. In the United States, the disease occurs most commonly along the Mississippi, Missouri, and Ohio Rivers.

Animal predisposition

Histoplasmosis is rare in dogs and uncommon in cats, with highest incidences reported in young adult animals.

Clinical signs

Skin involvement is rare, but multiple small nodules that ulcerate and drain or crust over have been reported. Nonspecific symptoms such as anorexia, depression, weight loss, and fever are typical.

Other symptoms in dogs and cats may include dyspnea, tachypnea, and ocular disease.

Lameness in cats and cough, diarrhea, icterus, and ascites in dogs may be seen.

Differential diagnosis

Differentials include other fungal and bacterial infections and neoplasia.

Diagnosis

Cytology (tissue aspirates): (pyo)granulomatous inflammation with numerous intracellular, small yeasts that have basophilic centers.

Dermatohistopathology: nodular to diffuse (pyo) granulomatous dermatitis with numerous intracellular yeasts. Special fungal stains may be needed for visualization of organisms.

Radiography: pulmonary lesions are often seen.

Fungal culture: submit to diagnostic laboratory because fungal cultures are highly infectious: H. capsulatum.

PCR analysis, where available, may simplify the diagnosis.

Treatment

Systemic antifungal therapy should be administered over the long term (minimum 4–6 months) and continued at least 2 months beyond complete clinical resolution.

Effective therapies include the following:

  • » Ketoconazole (dogs) 5–10 mg/kg PO with food q 12 hours
  • » Fluconazole 10 mg/kg PO q 12 hours
  • » Terbinafine 30–40 mg/kg PO q 24 hours
  • » Itraconazole (Sporanox) 10 mg/kg PO with food q 12 hours

For severe cases, a quicker response may be achieved by combining itraconazole (Sporanox) or fluconazole with amphotericin B 0.25 mg/kg (cats) or 0.5 mg/kg (dogs) IV three times per week, until a cumulative dose of 4–8 mg/kg (cats) or 5–10 mg/kg (dogs) is administered.

Prognosis: is fair to good for most cats. The prognosis is poor for severely debilitated cats and for dogs with GI or severe signs of disseminated disease.


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