Fungal Diseases 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.

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.


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.

Coccidioidomycosis (Valley Fever)

This is the most severe and life-threatening of the systemic fungal diseases. Coccidioidomycosis is found in dry, dusty parts of the southwestern United States, and in California and neighboring Mexico. (Note that coccidioidomycosis is not the same disease as coccidiosis, a disease caused by intestinal protozoa.)

Infection occurs by inhaling spores. Most cases are subclinical or inapparent. A severe form affects the lungs and produces acute pneumonia. If the disease becomes systemic, it may involve the long bones (most common), liver, spleen, lymph nodes, brain, and skin. Affected dogs will often have a chronic cough, weight loss, lameness, and fever.

The diagnosis is made by identifying the organism (Coccidioides immitis)in cytology, biopsy, or culture specimens.

Treatment: Coccidioidomycosis can be treated effectively using one of the imidazole group of antifungal agents (as described for Histoplasmosis). Prolonged treatment for up to a year is required to try to prevent recurrence. However, relapses are common.