What Is Histoplasmosis?
Histoplasmosis is an infection caused by Histoplasma capsulatum, a fungus that grows in soil rich in bird or bat droppings. Pets breathe in tiny spores from disturbed soil, old barns, chicken coops, caves, or areas under roosting birds. In the US it is most common in the Ohio, Mississippi, and Missouri River valleys, though cases appear in many states.
Both dogs and cats can be infected. In cats it is one of the most common deep fungal infections, and indoor-only cats are not fully protected because spores can be carried in on dust, potted plants, or shoes. Once inhaled, the fungus can stay in the lungs or spread through the body to the gut, liver, spleen, bone marrow, eyes, skin, and bones.
Why it matters
Histoplasmosis can look like cancer, inflammatory bowel disease, or other fungal infections. Telling your vet where your pet lives and roams helps them test for it early.
Signs Owners Notice
- Weight loss, poor appetite, and low energy that build over weeks
- Fever that does not respond to antibiotics
- In dogs: chronic large-bowel diarrhea with mucus or blood and straining
- Coughing or faster breathing; see dog coughing for other causes
- In cats: breathing changes, cloudy or red eyes, skin bumps, or limping
- Pale gums from anemia in more advanced cases
Diagnosis
Your vet will usually run bloodwork, chest X-rays, and abdominal ultrasound. The most direct way to confirm the infection is finding the yeast-like organisms inside cells on a sample from a lymph node, rectal scraping, bone marrow, skin nodule, or other affected tissue. A urine or blood antigen test is also widely used, though it can cross-react with blastomycosis.
Histoplasmosis is one of several regional fungal infections. Blastomycosis overlaps in geography but more often targets the lungs, skin, and eyes of dogs; valley fever is found in the desert Southwest; cryptococcosis commonly affects the nose of cats; and nasal aspergillosis mainly affects the nasal passages of dogs.
Treatment and Outlook
- Antifungal medicine: itraconazole by mouth is the usual treatment, given for at least four to six months and continued after signs resolve.
- Severe cases: injectable antifungals in hospital may be used first for very sick pets.
- Supportive care: fluids, nutrition support, and oxygen if breathing is affected.
- Monitoring: regular rechecks and liver tests, and repeat antigen testing to help decide when to stop treatment.
Pets with infection limited to the lungs or caught early usually have a fair to good outlook. Widespread disease, especially in very sick cats or dogs with severe gut involvement, has a more guarded prognosis. Relapses can happen, so finishing the full course matters. Long antifungal treatment can be costly, so ask about generic or compounded options.
Practical Tips for US Pet Owners
- In endemic areas, keep pets away from chicken coops, bird roosts, caves, and old barns.
- Do not let pets dig in soil under roosting birds or bats.
- Tell your vet about travel to the Ohio or Mississippi River valleys if your pet becomes ill.
- Find a nearby practice through veterinary clinic listings.
When to see a vet
Book a visit for weight loss, fever, or diarrhea lasting more than a week. Seek urgent care if your pet is breathing hard or has sudden eye changes.
Key Takeaways
- Histoplasmosis is a fungal infection inhaled from soil enriched by bird or bat droppings.
- It can affect the lungs, gut, eyes, skin, and bones of dogs and cats.
- Months of antifungal treatment usually control it, especially when caught early.
FAQ
Can my pet give me histoplasmosis?
No. It is not spread from pets to people or between pets. People and pets catch it from the same environment.
Can indoor cats get histoplasmosis?
Yes. Spores can enter homes on dust, soil, or shoes, so indoor cats in endemic areas can still be infected.
How long does treatment last?
Usually at least four to six months, and often longer, until signs and test results have settled.