Histoplasmosis is a rare disease caused by the dimorphic fungus Histoplasma capsulatum. Dimorphic fungi exist in two forms, in the environment, they exist as filamentous fungi (moulds), but once they are inhaled into their host they convert to a yeast form. Histoplasmosis infects many mammals including cats, dogs, and humans and is distributed in many regions worldwide with endemic regions in Mississippi, Missouri, Ohio River Valleys and the mid-Atlantic states and can also be found in Latin America, Africa, Australia and parts of East Asia.
The fungus thrives in humid and moist conditions and is especially prevalent in soil which has been contaminated with bird or bat droppings. Infection occurs when cats are exposed to the fungus in soil, the fungal spores are inhaled or ingested.
Humans are not at risk of catching the disease from infected cats although they can become infected via the same environmental exposure.
How do cats become infected with histoplasmosis?
Cats become infected when they inhale or ingest the fungal spores (known as microconidia) from the environment. Once inside the cat, they are engulfed by macrophages where they continue to replicate. From this point on, the disease may go one of three ways. It may remain in the lungs, disseminate through the body to various organs or affect the skin.
These three forms are listed below:
Primary pulmonary histoplasmosis - This is the most common form of histoplasmosis. The spores are inhaled into the lungs where they cause localised symptoms.
Disseminated histoplasmosis - This is the most lethal form of histoplasmosis. Instead of being remaining in the lungs, the infection spreads throughout the body in the blood and lymph. Affected areas include liver, eyes, bones, skin, spleen and gastrointestinal tract. This form of histoplasmosis is seen most commonly in immunocompromised cats.
Disseminated histoplasmosis may be further broken down by the organs involved.
Most cats who develop disseminated histoplasmosis are co-infected with FIV or FeLV.
What are the symptoms of histoplasmosis in cats?
The lungs are the initial site of infection, and non-specific respiratory symptoms may primarily occur. The severity of symptoms can vary depending on the immune status of the cat as well as the number of microconidia inhaled and the form of histoplasmosis your cat has developed. Some infected cats may have no symptoms at all or may only cause symptomatic disease at a later stage when the cat's immune system is weakened.
Symptoms are more typically seen in severely affected cats, and will depend on where the infection is located but may include:
- Pale mucous membranes
- Labored breathing
- Abnormal lung sounds
- Enlarged spleen
- Enlarged lymph nodes
- Enlarged liver
- Lameness due to bone infection
- Ocular involvement may lead to uveitis, retinal detachment, blepharitis or conjunctivitis
- Organ dysfunction may vary depending on the affected organs
- Neurologic dysfunction
- Subcutaneous (under the skin) weeping nodules (lumps)
How is histoplasmosis diagnosed?
Diagnosing histoplasmosis can be challenging because the cat will often display vague symptoms with many possible causes. Diagnosis is based on evaluative testing and may include:
- Complete blood counts, biochemical profile, and urinalysis to evaluate organ function and the overall health of your cat. Low platelets (thrombocytopenia) may be present in cats with the disseminated form. If there is liver involvement, elevated enzymes may be seen on the biochemical profile.
- Chest x-rays may reveal a pattern in the lungs consistent with histoplasmosis. Abdominal x-rays which may reveal an enlarged spleen or liver.
- Fine needle aspirate or biopsy of subcutaneous nodules, bone, liver, spleen, lungs or lymph nodes.
- Blood smears.
- Rectal scrapings.
- Blood test to check for the presence of antibodies. This test may be negative to antibodies in immunocompromised cats.
Cytology. A definitive diagnosis can be made by finding the fungus from samples.
Your veterinarian may also want to run tests for FIV and FeLV if your cat has the disseminated form of histoplasmosis.
How is histoplasmosis treated?
Mild cases of histoplasmosis may not require any treatment. Where treatment is indicated, your cat will be put on oral antifungal drugs such as Itraconazole or ketoconazole. Your cat will need to remain on medication for between 4-6 months and occasionally as long as 12 months.
More serious cases may require in-hospital care, including tube feeding and IV fluids.
The prognosis for cats with pulmonary histoplasmosis is fair to good, however, in the case of disseminated histoplasmosis, it is poor to grave.