Fungus Among Us, Part 2: histoplasmosis

Posted by Dr. Kim Smyth on Nov 28 2012

This week we’re taking a look at the most common systemic fungal infections that can affect dogs and cats. Part 1 of this series addressed blastomycosis, and today we’ll address histoplasmosis.

Histoplasmosis is the infection caused by the organism Histoplasma capsulatum, which is found in the United States as well as Central and South America. In the U.S., the soils of the Ohio, Missouri and the Mississippi river valleys are the most common places to find the Histoplasma capsulatum, as are soils that are rich in bat or bird feces.

Pets are infected by inhaling spores, so if your dog is sniffing alongside one of the above rivers, be careful! Once inhaled, the spores begin to reproduce, and then infection spreads to local lymph nodes and other organs, such as the liver, spleen, bone marrow, bones and eyes. Occasionally, the skin, gastrointestinal system, nasal cavity and central nervous systems can be affected. The incubation period of histoplasmosis is generally 12 to 16 days.

Unlike blastomycosis, cats are fairly susceptible to histoplasmosis, and the disease can be slowly progressive and non-specific in cats, making it difficult to diagnose. In both cats and dogs, infection can be localized to one system or organ, or spread throughout the body. It is believed that while histoplasmosis can result in life-threatening disease, there are also many animals that are exposed to the organism and clear it without treatment.

Clinical signs of histoplasmosis vary depending on whether disease is diffuse (spread throughout the body) or localized. Signs may include:

  • Jaundice, or yellowing of the skin and eyes
  • Coughing
  • Blindness
  • Enlarged lymph nodes
  • Lameness
  • Anorexia, or unwillingness to eat

Veterinarians can diagnose infection by taking a sample of bone marrow, lymph node or fluid collected from the lungs, and examining it under a microscope. Blood tests are another option, though they are unreliable in both cats and dogs.

The severity of the disease ranges from mild to life-threatening, and prognosis will vary accordingly. For pets with localized and mildly diffused disease, prognosis is good with long-term treatment, which involves systemic anti-fungals. It could still take a week or two to see clinical response. During this time, severely affected patients may need hospitalization for supportive care, like IV fluids and nutrition. Because medications do not treat the eyes well, ocular disease may not respond to therapy, so blindness could be permanent.

As with all of the fungal diseases we’re talking about this week, long-term therapy is key. Treatment will need to continue for at least one to two months after resolution of clinical signs, making for a significant financial and time commitment. But with persistence at home, your pet will have the best chance to get back on her paws.