Cause of Fungi
Histoplasmosis is a systemic fungal disease of dogs and cats caused by Histoplasma capsulatum. In the environment, H. histoplasma organisms are mycelial, saprophytic soil fungi. In infected tissue or when cultured at 30 to 37° C, the organism is a yeast. The fungus is endemic throughout most of the temperate and subtropic regions of the world. Most cases of histoplasmosis in the United States occur in the central states, with the geographic distribution following the Mississippi, Ohio, and Missouri Rivers.
Infection is probably via inhalation or ingestion of infective conidia from the environment. The respiratory system is thought to be the primary route of infection in cats and dogs, although the gastrointestinal tract may also be an important route in the dog. After inhalation or ingestion, conidia transform from the mycelial phase and are phagocytized by macrophages, where they grow as facultative intracellular organisms. Hematogenous and lymphatic dissemination results in multisystemic disease. Organisms can be disseminated to any organ system, but the lungs, gastrointestinal tract, lymph nodes, liver, spleen, bone marrow, eyes, and adrenal glands are the most common organs of dissemination in the dogs; lungs, liver, lymph nodes, eyes, and bone marrow are most commonly affected in cats. Cell-mediated immunity induces a granulomatous inflammatory response in most infections.
Dogs with gastrointestinal histoplasmosis typically have mild fever, anorexia, lethargy, weight loss, vomiting, diarrhea, hematochezia, and tenesmus. Cachexia is a common physical examination finding. Other historical and physical examination findings (dyspnea, cough, ascites, lameness, oropharyngeal ulcerations, chorioretinitis, neuropathy) will depend upon organ and tissue involvement. The full clinical spectrum of histoplasmosis infection is outlined in site.
Diagnosis of Fungi
Endoscopic examination usually reveals severe granulomatous inflammation of the large intestine. Organism identification is required for definitive diagnosis. The most common means of organism identification is cytology. Cytology from affected tissue reveals pyogranulomatous inflammation, often with numerous small, round to oval intracellular yeast cells (2 to 4 μm in diameter) characterized by a basophilic center and a light halo. Exfoliative cytology during colonoscopy is particularly useful in diagnosing the disease. Histopathology is helpful if cytology is nondiagnostic or inconclusive. Multiple endoscopic colonic biopsies are usually sufficient to diagnose the disease. The yeast form does not stain well with routine hematoxylineosin (H&E) stains, so special stains such as PAS and Gomori’s methenamine silver stain are often used to demonstrate organisms. Fungal culture from affected tissue can be used for diagnosis but is rarely needed in clinical cases. Currently available serologies have poor specificity and sensitivity.
Treatment of Fungi
Itraconazole (5 mg / kg orally, twice a day for 2 to 4 months) is considered the treatment of choice for feline histoplasmosis. In one study, itraconazole therapy cured histo-plasmosis infections in all eight study cats. Ketoconazole and amphotericin B have been described as the treatments of choice for canine histoplasmosis. With colonic involvement, additional gastrointestinal therapy may be useful in affected dogs (e. g., dietary modification, treatment for small intestinal bacterial overgrowth, direct antidiarrheal therapy). Corticosteroids may-have been used successfully in the treatment of airway obstruction secondary to hilar lymphadenopathy in chronically infected dogs.
Prognosis of Fungi
There may be important species differences in prognosis, although the paucity of reports, especially of prospective clinical trials, makes it difficult to generalize. It would seem that the prognosis is guarded in dogs but fair to good in cats.