Histoplasmosis in Cats
Dr. Arnold Plotnick
Histoplasmosis is a non-contagious infection caused by the fungus Histoplasma capsulatum. The organism responsible for the disorder is a soil fungus that is widely distributed in the Midwestern and southern United States, especially in river valleys and plains. Within these endemic areas, some locales are more heavily contaminated with Histoplasma organisms than others.
In many instances, these variations have been correlated with proximity to bird habitats, which are often heavily contaminated because bird droppings serve as a good medium for the organism. Birds are not infected by the Histoplasma organism, although they can carry the organism on their feathers and spread the disease in this manner. Bat habitats can also be heavily contaminated with the organism, as bat guano is an excellent medium for Histoplasma growth. Unlike birds, bats can become infected with the organism.
Infection occurs when spores in the air are inhaled. The spores go to the lungs and cause infection of cells deep in the lungs. Most cats show a clinically unapparent pulmonary infection; the organism has infected the lungs, yet the cat shows no signs of being ill.
Occasionally, cats with pulmonary histoplasmosis shows signs of lung infection, such as fever, labored breathing, and coughing. The lung infection usually turns out to be benign and self limiting. If the unapparent respiratory infection extends and persists beyond the lungs and becomes systemic, a clinically apparent, often devastating form of the disease may develop. This has been referred to as "disseminated histoplasmosis," and it carries a guarded or even grave prognosis.
It is uncommon for a clinically unapparent Histoplasma infection to disseminate from the lungs. When this happens, a large number of organs and body systems may be affected. In cats, the organs most often involved are the lungs, intestine, lymph nodes, liver, spleen and bone marrow. Systemic signs of illness such as depression, fever, and anorexia are common. The disseminated infection in cats most often results in chronic diarrhea, intestinal blood loss, anemia, and weight loss. In a few cats, infection of the bones, eyes, skin, and central nervous system may occur.
Diagnosis can be tricky in cats because the prominent signs of fever, weight loss, depression, poor appetite and anemia suggest a generalized infectious or inflammatory process that is not specific for any organ in particular. Emaciation is a predominant and conspicuous feature. Cats may show labored or fast breathing, and this can be due to anemia, or due to lung disease. A big liver, big spleen, and enlarged lymph nodes may all be detected on physical examination. Lameness has been reported in several cats with disseminated histoplasmosis. Ulcerating skin nodules, and erosions in the mouth have been observed in cats as well. Ocular involvement, such as an ocular discharge, eyelid spasm, conjunctivitis, blindness, glaucoma, and inflamed or detached retina, are sometimes seen. Diarrhea, which is the most common feature of disseminated histoplasmosis in dogs, is rarely observed in cats. Cats will show anemia, inappetence, fever, enlarged lymph nodes, spleen, liver, and may show ocular involvement.