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Histoplasmosis
Histoplasmosis is a noncontagious infection caused by Histoplasma capsulatum fungus, which is found worldwide. It lives in the soil as a mycelium (vegetative body of a fungus) and in the body as a yeast (one-cell fungi). The fungus produces fruiting bodies in the form of micronidia and macronidia. 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 (starling) and bat excrements (caves).
Infection occurs via inhalation of the micronidia and macronidia with dissemination via blood and lymphatics to become systemic. The fungal infection either is cleared or the organism continues to reproduce inside the cells and spreads throughout the body via lymphatic and blood circulation. It has been reported in dogs, cats, horses. It can potentially affect any species.
Signs
Incubation period is 12 to 16 days during which the micronidia 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. Signs of the disease such as depression, fever, poor appetite are common, as well as chronic diarrhea, intestinal blood loss, anemia, and weight loss.
Diagnosis and Treatment
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.
Histoplasmosis has become a serious problem in immunocompromised human individuals, such as AIDS patients who develop a disseminated and often rapidly progressive clinical course.
References
- Scott, Miller, Griffin. Small Animal Dermatology
- Greene CE. Infectious Diseases of the Dog and Cat
- Rippon JW. Medical Mycology
- Scott DW. Large Animal Dermatology
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