Systemic Mycoses – Big Words for Little Fungi Part II: Histoplasmosis
By Luke Peterson, DVM, MS
Histoplasmosis is another fungus that can cause multiple forms of disease in dogs. Histoplasma capsulatum can be found in a large portion of the central and eastern United States (see map in figure 1) primarily through the Ohio and Mississippi river valleys with the highest region in southern Illinois, southern Indiana, western Tennessee, western Kentucky, southeastern Missouri, eastern Arkansas, northern Mississippi, and the northwestern corner of Alabama. The fungus resides in soil and is especially associated with areas containing bird or bat droppings. Pointers, Weimeraners, and Brittany Spaniels have an increased risk compared to other breeds. Exposure to the fungus does not always result in disease. Research conducted during post-mortem exams isolated the fungus from 36% (Kentucky) and 44% (Virginia) of otherwise healthy dogs displaying no disease.
The fungus produces spores which are released into the air when the soil is disturbed. Dogs and humans are at risk when those spores are inhaled into the lungs. The spores then transform into yeast and spread to lymph nodes and through the blood stream to various organ systems. The yeast form is not directly contagious from one dog to another or from dog to human. The length of time it takes for the disease to progress in severity varies based on the health of the dog in question. But studies have found a more aggressive form which takes 2-4 weeks to become fatal and occurs in about 10% of affected dogs compared to a slower chronic form which takes 2-20 months the other 90% of the time. Disease syndromes most commonly found are pulmonary, mediastinal lymphadenitis (this is infection/inflammation of the mediastinal lymph nodes which are located in the chest cavity), and progressive disseminated histoplasmosis.
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