Histoplasmosis is a noncontagious infection caused by a dimorphic fungus Histoplasma capsulatum which is found worldwide. It lives in the soil as a mycelium and in the body as a yeast (one-cell fungi). In the USA, most cases occur in the central part in the Ohio, Missouri, and Mississippi River Valleys (same region as Blastomyces). Surveys indicate that most people and canine inhabitants of endemic areas become infected. The organism prefers areas with moist, humid conditions and soil containing nitrogen-rich organic matter such as bird and bat excrements. Infection occurs via inhalation of the fruiting bodies of the fungus. A wide variety of mammalian species can be affected, and cats may be more susceptible to infection than dogs. As with most systemic fungal diseases, animals younger than 4 years old are at an increased risk, but any age can be affected. The respiratory system is likely the primary route of infection in cats, humans, and dogs, but the GI system may also be an important route in the dog.

After inhalation or ingestion, conidia transform from the mycelial phase to the yeast phase and are phagocytized by cells of the macrophage monocyte system, where they grow as facultative intracellular organisms. The organism then spreads via blood and lymphatics resulting in systemic disease. The lungs, GI system, lymph nodes, liver, spleen, bone marrow, eyes, and adrenal glands are common organs affected in dogs; lungs, liver, lymph nodes, eyes, and bone marrow are most commonly affected in cats.

Incubation period is 12 to 16 days during which the fruiting bodies convert to the yeast phase and replicate. The disseminated form of histoplasmosis mainly affects lungs, GI tract, liver, spleen, and bone marrow. Ocular signs can also be present. If the dog's skin is affected, ulcers, nodules, sinus tracts, and oral lesions usually develop. Depression, fever, poor appetite are common, as well as chronic diarrhea, intestinal blood loss, anemia, and weight loss.

Diagnosis requires identification of the fungus in the blood or tissues. Disseminated histoplasmosis can be difficult to treat, requiring a long course of combination drug therapy. Prevention of disease is based upon avoidance of areas with heavy bird and bat fecal contamination. Small areas of fungal contamination can be disinfected with 3% formalin solution. Pulmonary histoplasmosis may have a self-limiting clinical course, but antifungal treatment is still recommended because there is significant potential for chronic dissemination. Dogs or cats with disseminated histoplasmosis usually die without treatment. Treatment is similar to that for blastomycosis. The duration of treatment depends on the severity of the infection and the response of the animal.

histoplasmosis - a disease caused by inhalation of Histoplasma capsulatum fungus
mycelium - vegetative body of a fungus
yeast - one-cell fungi
conidia - asexual spores of fungi borne on hyphae
hypha - one of the filaments composing the mycelium of a fungus


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  5. Stephen J. Ettinger, DVM, DACVIM and Edward C. Feldman, DVM, DACVIM. Textbook of Veterinary Internal Medicine



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