Histoplasmosis in Cats

By Malcolm Weir, DVM, MSc, MPH; Krista Williams, BSc, DVM; Cheryl Yuill, DVM, MSc, CVH

What is histoplasmosis?

Histoplasmosis is a chronic, non-contagious fungal infection caused by the soil-dwelling fungus Histoplasma capsulatum. This fungus is found globally and may infect both humans and animals. However, histoplasmosis is uncommon to rare in all but dogs and cats.

The fungus thrives in warm moist environments and is commonly found in soil that contains bat or bird feces (including chickens, pigeons, starlings, and blackbirds). In the United States, histoplasmosis is most prevalent along the Mississippi, Missouri, and Ohio  river valleys, while in Canada it is found along the southern Great Lakes, along the St. Lawrence River, and in Alberta.

How does a person or an animal get this disease?

Humans and animals most commonly get this infection by inhaling fungal spores, which then reach the lower airways where they multiply and set up a localized infection. The organism may also enter the body through the mouth, and from there set up an infection in the intestines. The fungal infection may remain localized in the lungs or the intestines, or it may spread to other parts of the body through the bloodstream or lymphatic system, causing a generalized or systemic infection. With a generalized infection, the fungus may become established in the lymph nodes, spleen, liver, bone marrow, skin, or eyes.

Most cats that develop this disease are less than four years old; however, cats of any age may be affected. There is no breed predilection for this disease in cats. Evidence suggests that prolonged exposure to high levels of Histoplasma organisms increases the risk of contracting the disease.

What are the signs of histoplasmosis?

The signs of histoplasmosis are variable and often not specific. In cats, the typical symptoms include mild fever, depression, loss of appetite, and weight loss, which are signs associated with many other ailments. If the lungs are the primary site of infection, the cat may show signs of respiratory distress such as coughing or difficulty breathing. If the intestinal tract is the primary site of infection, the only symptoms may be loss of appetite and weight loss. If the disease has become more generalized, your veterinarian may detect enlargements in the liver, spleen, and/or lymph nodes, evidence of ulceration in the throat or intestinal tract, draining skin lesions, infection in the eyes, or joint involvement causing lameness.

How is histoplasmosis diagnosed?

Your veterinarian may suspect that this disease is the cause of your cat's illness if the pet was in an area where this disease is prevalent about 1-2 months before becoming ill. Since the signs of histoplasmosis are often vague and nonspecific, your veterinarian will recommend a series of screening tests, including X-rays, urinalysis, and blood tests. Results of these tests help support a tentative diagnosis of histoplasmosis.

"Cytology and/or histopathology are required to diagnose histoplasmosis conclusively."

Certain blood and urine tests may detect a component of the fungal cell wall. There are some limitations to this test: it will cross-react with other fungal species so it may not confirm a diagnosis of histoplasmosis, only a fungal infection. The test can also be falsely negative if the infection is walled off in one area of the body such as the lungs or the eyes.

Cytology and/or histopathology are required to diagnose histoplasmosis conclusively. Cytology, the microscopic examination of cells, may be performed in the veterinarian's office using material that is draining from an open wound or is aspirated from a nodule or lymph node. If the results of cytology are inconclusive, it may be necessary to collect biopsy samples for histopathologic analysis. Histopathology is the microscopic examination of cells within a tissue sample. A tissue sample is obtained and sent to a veterinary pathologist for this diagnostic test.

Can the disease be treated? 

Yes, although not all pets will survive. Fortunately, newer anti-fungal medications such as fluconazole (brand name Diflucan®) and itraconazole (brand names Itrafungol® and Sporanox®) are well tolerated by most cats and have few side effects when compared to the medications being used several years ago.

Your cat will require several months of medication to successfully treat this disease. In most cases, treatment will need to be continued for 4-6 months. Your cat will require regular re-checks with your veterinarian including blood tests and X-rays if the lungs are affected.

What is the prognosis for my cat?

There is no way to determine the prognosis before starting treatment, although the results of X-rays and blood tests help determine the extent of the disease. A cat in poor condition or with advanced disease is less likely to survive. Cats with disease limited to the lungs tend to respond better to treatment than those with disease of the intestinal tract or other body systems. If the fungus has invaded the eyes or nervous system, it is much harder to eliminate the infection because of the natural defensive barriers in these tissues.

In all cases, treatment is prolonged. Patients that are in poor condition usually need to be hospitalized for intensive treatment until they stabilize, and it may be necessary to use a combination of medications to successfully get the infection under control. Supportive care such as IV fluids, oxygen therapy, and bronchodilators to help with breathing may all be needed.

Since each case is different, your veterinarian will advise you on your individual cat's prognosis based on the clinical signs and the results of diagnostic testing.

Am I at risk of infection from my cat?

Studies on this fungus have shown that once an animal is infected, the organism enters a different form or phase that does not appear to be infectious to other animals or to humans. However, common sense dictates that strict hygiene should be followed when handling any draining lesions. Handlers should wear protective gloves and wash their hands thoroughly after contact with an infected animal.

In most cases, the infected pet does not need to be segregated from the owner or from other household pets. The true risk of infection to others probably comes from sharing the same environment where the original infection occurred (the soil). Because the H. capsulatum organisms may be present near your home, you should advise your family physician of your pet's diagnosis.

If anyone in your family falls into one of the following categories, consult your physician:

  • Infants or small children
  • Organ transplant patients
  • Chemotherapy patients
  • HIV/AIDS patients
  • Elderly family members
  • Anyone with a known immune system disease
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