Introduction | Causes & Prevention | Symptoms & Signs | Diagnosis & Test | Treatment Options

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Introduction

Histoplasmosis is a systemic fungal infection that can affect companion animals and people. It is caused by exposure to the soil-dwelling fungus, Histoplasma capsulatum.

How Histoplasmosis Affects Cats

Histplasmosis can affect cats of any breed, age or gender, although typically it is seen in younger animals. Early on, infected cats become depressed, have difficulty breathing, become anorexic and lose weight. The respiratory tract rather than the gastrointestinal tract is the target of this infection in cats. They also can develop a cough, lameness, abnormal eye redness, eye swelling and discharge, and diarrhea. As the fungus disseminates throughout the cat's body, the liver, spleen and bone marrow can become affected. Enlarged lymph nodes, a high fever and significantly increased respiratory effort also are common. Histoplasmosis can be fatal in a short matter of weeks. Fortunately, this infection can be diagnosed by fungal culture (diagnosis is not necessarily simple) and usually can be treated successfully with an extended course of oral anti-fungal medications.

Causes & Prevention

Cause of Histoplasmosis in Cats

Histoplasmosis is caused by exposure to Histoplasma capsulatum. This fungal organism is found in moist soils organically rich in bird and bat manure and is endemic in many temperate regions of the world. In the United States, it is most commonly found in the Mississippi, Missouri and Ohio River basins, although it has been reported in Tennessee, Texas, California, the southeastern states and the Great Lakes region, as well. Cats become infected by either inhaling (or less commonly, ingesting) fungal spoors, which infiltrate the lower airways (or intestinal lining), germinate and become yeast, reproducing by a process called "budding." The cat's immune system responds to the infection by mobilizing certain cells that envelop the yeast, which continue to replicate. At this point, affected cats typically show only respiratory signs. However, over time, the fungi can disseminate throughout the cat's body through either the lymphatic or general circulatory systems.

Preventing Histoplasmosis

The best way to prevent cats from developing this disease is to prevent their exposure to the fungal spores. This entails avoiding outdoor activities in known endemic areas (temperate climate with moist soil rich in organic material containing bird and/or bat feces). There currently is no vaccine available against Histoplasma capsulatum.

Special Notes

The prognosis for cats with histoplasmosis ranges from good to guarded depending upon how far the disease has progressed at by the time it is diagnosed. Again, with prompt recognition of clinical signs and proper treatment, the infection almost always can be resolved successfully.

While companion animals and people are susceptible to infection from this microorganism, it is not known to spread from animals to people, or vice versa. However, people can inhale the organism from the same source and at the same time as their cats or dogs, and can become simultaneously infected.

Symptoms & Signs

Histoplasmosis is a systemic fungal infection that can affect companion animals and people. It is caused by exposure to a soil-dwelling fungus called Histoplasma capsulatum. This organism is found in soils organically rich in bird and bat manure and is endemic in many temperate regions of the world, including many areas of the United States. Cats infected with this organism often display signs of disseminated histoplasmosis, a form of the disease which occurs when the fungus invades multiple tissues and organ systems.

Symptoms of Histoplasmosis in Cats

There is no known feline breed predilection for contracting this infection. Importantly, many infected cats never develop outward signs of disease.
Most clinically affected cats are less than 4 years of age, and according to some reports females may be overly represented. Cats are affected primarily in their respiratory tract, but bone, bone marrow, liver, spleen, skin and lymph nodes can also become involved. Cats initially show nonspecific signs of the infection, such as depression, respiratory difficulty, inappetence and weight loss. Cats also can develop a cough, lameness, abnormal eye redness, eye swelling and discharge, and diarrhea. Cats normally do not show the significant gastrointestinal involvement that dogs do. As the fungus disseminates throughout a cat's body, the liver, spleen and bone marrow may become involved. Enlarged lymph nodes, a high fever and significantly increased respiratory effort typically are found upon physical examination by the attending veterinarian.
If you notice any of the above signs in your cat, especially if you live in a high-risk geographical area, take your cat to a veterinarian as quickly as possible. Histoplasmosis can be fatal if treatments are not begun promptly. Fortunately, this fungal infection can be diagnosed through fungal culture (although diagnosis is not simple) and usually can be treated successfully with an extended course of antifungal oral medications.

Treatment Options

Histoplasmosis is a serious, systemic fungal disease which occurs in cats that ingest or inhale a soil-dwelling organism called Histoplasma capsulatum. This fungus is endemic in many areas, and not all cats that are exposed will develop clinical disease. The infection can be difficult to diagnose and can be fatal. Thankfully, once it is diagnosed, it is normally successfully - and permanently - treatable. The treatment is expensive, but the prognosis is good for cats without significant lung involvement and damage before treatment begins.

Treating Histoplasmosis in Cats

The goal of treatment is to completely clear the infection from the cat. The duration of treatment and the selection of appropriate anti-fungal drugs must be decided upon by a veterinarian, because the treatment protocols may differ in cases of disseminated versus mild disease. Speaking generally, anti-fungal azole medications are used to combat this infection. They normally need to be administered orally, twice a day, for at least 4 to 6 months depending upon the severity of the cat's condition and its response to treatment. In severe cases where hospitalization is necessary, additional intravenous medications may be added to the protocol, together with oxygen, fluids and nutritional support. The azole drugs can cause gastrointestinal side effects (vomiting, diarrhea, lethargy and abdominal pain) and normally should be given with meals. Anti-fungal medications are expensive, but less so in cats than in dogs because of cats' smaller size.
Affected cats should remain on a high-quality, palatable diet with free access to fresh water to ensure adequate nutritional support and hydration. Regular visits to the veterinarian for assessment of progress are important and may include repeated chest radiographs for cats with significant lung involvement.

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