Histoplasmosis in cats is the second most prevalent systemic fungal disease in domestic felines. It primarily occurs in the central United States, specifically in the Mississippi, Missouri, and Ohio river valleys. While histoplasmosis is most commonly reported in Oklahoma, Texas, Virginia, and Louisiana, it has been documented on every continent except Antarctica. Cats become infected with Histoplasma capsulatum, the fungus responsible for histoplasmosis, by inhaling or, less frequently, ingesting infectious spores present in soil contaminated with bat and bird droppings. Although outdoor cats are more prone to encountering such contaminated soil, histoplasmosis can affect both indoor and outdoor cats. Indoor cats may be exposed to infection through sources like potted plants and unfinished basements. Additionally, cats infected with Feline Leukemia Virus (FeLV) have a slightly higher likelihood of developing histoplasmosis compared to FeLV-negative cats.
Related: FIV – Feline Immunodeficiency Virus in Cats
What is Histoplasmosis in Cats?
Histoplasmosis is a chronic fungal infection caused by the soil-dwelling organism known as Histoplasma capsulatum. This condition, which is not contagious, affects both humans and animals, although it is predominantly observed in dogs and cats rather than other species.
The fungus thrives in warm and damp surroundings, commonly residing in soil contaminated with bird or bat droppings. Histoplasmosis has a global distribution, with its prevalence varying across different regions. In the United States, it is most commonly found in the Mississippi, Missouri, and Ohio River valleys, while in Canada it is predominantly seen in the southern Great Lakes region and along the St. Lawrence Seaway.
Infection
The primary mode of transmission for this infection in both humans and animals is the inhalation of fungal spores. These spores travel to the lower respiratory tract, where they reproduce and initiate a localized infection. Alternatively, the organism can enter the body through the mouth and establish an infection in the intestines. The fungal infection may remain confined to the lungs or intestines, or it can disseminate to other parts of the body via the bloodstream or lymphatic system, resulting in a generalized or systemic infection. In cases of generalized infection, the fungus can establish itself in various locations such as lymph nodes, spleen, liver, bone marrow, skin, or eyes.
When it comes to cats, most individuals affected by this disease are typically under four years of age, although cats of any age can be susceptible. There is no specific breed predisposition for this disease in cats. Evidence suggests that prolonged exposure to high levels of Histoplasma organisms increases the risk of contracting the disease.
Symptoms and Diagnosis
The symptoms of histoplasmosis can vary and are often nonspecific to this particular disease. In cats, common signs include mild fever, lethargy, reduced appetite, and weight loss, which can also be associated with other feline ailments. If the lungs are primarily affected, respiratory distress such as coughing or difficulty breathing may be observed. In cases where the intestinal tract is the main site of infection, the only noticeable symptoms may be appetite loss and weight loss. If the disease has spread throughout the body, your veterinarian may identify enlarged liver, spleen, and/or lymph nodes, ulceration in the throat or intestines, skin lesions, eye infections, or lameness due to joint involvement.
Your veterinarian might suspect histoplasmosis as the underlying cause of your cat’s illness if they were in an area where the disease is prevalent one to two months before becoming sick. Since the symptoms of histoplasmosis can be vague and nonspecific, your veterinarian will recommend a series of screening tests, including X-rays, urinalysis, and blood tests. The results of these tests will help support a preliminary diagnosis of histoplasmosis.
Testing can be performed on blood and urine samples to detect a component of the fungal cell wall. However, there are limitations to this test: it may yield false-positive results for other fungal species, confirming only a fungal infection rather than specifically histoplasmosis, and it can produce false-negative results if the infection is localized to one area of the body, such as the lungs.
To conclusively diagnose histoplasmosis, cytology and/or histopathology are required. Cytology involves examining cells under a microscope, which can be done in the veterinarian’s office using material obtained from draining wounds or aspirated from nodules or lymph nodes. If cytology results are inconclusive, it may be necessary to collect biopsy samples for histopathological analysis. Histopathology involves the microscopic examination of cells within a tissue sample, which is sent to a veterinary pathologist for diagnostic evaluation.
Treatment
Yes, it is possible for pets to survive histoplasmosis, although not all will. The good news is that cats generally tolerate the newer antifungal medications, such as fluconazole (Diflucan®) and itraconazole (Itrafungol® and Sporanox®), better than the medications used in previous years, with fewer side effects. Treating this disease in cats typically requires several months of medication, often continuing for 4-6 months. Regular veterinary check-ups, including blood tests and X-rays if the lungs are affected, will be necessary during the treatment period.
Histoplasmosis in cats: Prognosis
The prognosis cannot be determined before starting treatment, but X-ray and blood test results help assess the extent of the disease. Cats in poor overall condition or with advanced disease have a lower likelihood of survival. Cats with limited lung involvement tend to respond better to treatment compared to those with intestinal or systemic involvement. If the fungus has spread to the eyes or nervous system, eliminating the infection becomes more challenging due to the natural defensive barriers in these tissues.
In all cases, treatment is prolonged. Patients in poor condition often require hospitalization for intensive treatment until they stabilize, and a combination of medications may be necessary to effectively control the infection.
Since each case is unique, your veterinarian will provide guidance on your cat’s individual prognosis based on clinical signs and the results of diagnostic testing.