Histoplasmosis in dogs

Fungi are a large family that includes mushrooms. They live in soil and organic material. Many types of fungi spread via airborne spores. Fungus spores, which resist heat and can live for long periods without water, gain entrance to the body through the respiratory tract or a break in the skin.

Fungal diseases can be divided into two categories. There are fungi that affect only the skin or mucous membranes, such as ringworm and thrush. In the other category, the fungus is widespread and involves the liver, lungs, brain, and other organs, in which case the disease is systemic.

Systemic fungal diseases are not common in dogs. They tend to occur in chronically ill or poorly nourished animals. Prolonged treatment with steroids and/or antibiotics may also change the dog’s pattern of resistance and allow a fungal infection to develop. Suspect a fungus when an unexplained infection fails to respond to a course of antibiotics.

Good hygiene is important when handling and caring for a dog with any fungal infection. The risk to humans is low, but these are difficult diseases to treat.

Histoplasmosis in dogs

This disease is found in the central United States near the Great Lakes, the Appalachian Mountains, Texas, and the valleys of the Mississippi, Ohio, and Missouri rivers. These areas have nitrogen-rich soil that facilitates growth of the causative fungus (Histoplasma capsulatum). Spores are found in soil contaminated by the dung of bats, and chickens and other birds. Spores are breathed in by dogs, people, or other animals.

In most cases, histoplasmosis is subclinical or inapparent, occasionally producing a mild respiratory infection. There is an acute intestinal form, however, that attacks the small bowel and colon. The principal signs are weight loss and intractable diarrhea. A systemic form is characterized by fever, weight loss, vomiting, muscle wasting, coughing, enlargement of the tonsils and other lymph nodes, as well as involvement of the liver, spleen, bone marrow, eyes, skin, and, rarely, the brain.

The diagnosis is made by chest X-ray, blood studies, and identification of the histoplasma organism in cytology, biopsy, or culture specimens.

Treatment: Oral anti-fungal drugs of the imidazole group, including ketoconazole, itraconazole, and fluconazole, are particularly effective in treating histoplasmosis that is not life-threatening. In dogs with severe infections, amphotericin B is often combined with one of the imidazoles. Amphotericin B is potentially damaging to the kidneys.

Antifungal therapy requires many months of drug use after the symptoms disappear. The disease will reappear if longterm suppression is not maintained. Anti-fungal drugs can be toxic and require close veterinary management.