Overview of Histoplasmosis in Cats

Histoplasmosis is a non-contagious fungal infection of dogs, cats, other domestic and non-domestic animals, and humans. It is caused by inhalation of a fungus called Histoplasma capsulatum carried on dust with a primary infection in the lung.

Below is an overview of Histoplasmosis in Cats followed by in-depth information about the diagnosis and treatment of this condition. 

The factors that predispose cats to the disease aren’t clearly defined. Immaturity of the immune system of young cats and exposure to large amounts of the organism may be significant factors.

The impact of the disease on a particular cat is significant; treatment is expensive and prolonged, and the prognosis is poor.

Watch to Watch For

Acute weight loss/emaciation

Depression

Fever

Poor appetite

Anemia

Labored breathing

Enlarged liver

Enlarged spleen

Enlarged lymph nodes

Lameness

Skin disease

Diarrhea (the most common sign in dogs)

Eye disease

Diagnosis of Histoplasmosis in Cats

A definitive diagnosis of histoplasmosis requires detecting Histoplasma organisms in the tissues of cats and dogs showing clinical signs compatible with the disease. Several other tests are also recommended to gather information that supports a diagnosis of histoplasmosis.

Complete blood count

Chemistry panel and urinalysis

Radiographs

Complete ophthalmic exam

Rectal scrapings (dogs)

Bone marrow examination

Cytology of fine-needle aspirates

Biopsy

Serologic tests

Culture

Treatment of Histoplasmosis in Cats

Antifungal drugs

Supportive care

Home Care and Prevention

Provide good nutrition and administer all medications as prescribed.

No specific preventive measures exist regarding histoplasmosis.

In-depth Information on Histoplasmosis in Cats

Histoplasmosis is a non-contagious infection caused by the fungus Histoplasma capsulatum. The organism responsible for the disorder is a soil fungus that is widely distributed in the Midwestern and southern United States, especially in river valleys and plains. Within these endemic areas, some locales are more heavily contaminated with Histoplasma organisms than others.

In many instances, these variations have been correlated with proximity to bird habitats, which are often heavily contaminated because bird droppings serve as a good medium for the organism. Birds are not infected by the Histoplasma organism, although they can carry the organism on their feathers and spread the disease in this manner. Bat habitats can also be heavily contaminated with the organism, as bat guano is an excellent medium for Histoplasma growth. Unlike birds, bats can become infected with the organism.

Infection occurs when spores in the air are inhaled. The spores go to the lungs and cause infection of cells deep in the lungs. Most cats show a clinically unapparent pulmonary infection; the organism has infected the lungs, yet the cat shows no signs of being ill.

Occasionally, cats with pulmonary histoplasmosis shows signs of lung infection, such as fever, labored breathing, and coughing. The lung infection usually turns out to be benign and self limiting. If the unapparent respiratory infection extends and persists beyond the lungs and becomes systemic, a clinically apparent, often devastating form of the disease may develop. This has been referred to as “disseminated histoplasmosis,” and it carries a guarded or even grave prognosis.

It is uncommon for a clinically unapparent Histoplasma infection to disseminate from the lungs. When this happens, a large number of organs and body systems may be affected. In cats, the organs most often involved are the lungs, intestine, lymph nodes, liver, spleen and bone marrow. Systemic signs of illness such as depression, fever, and anorexia are common. The disseminated infection in cats most often results in chronic diarrhea, intestinal blood loss, anemia, and weight loss. In a few cats, infection of the bones, eyes, skin, and central nervous system may occur.        

Diagnosis can be tricky in cats because the prominent signs of fever, weight loss, depression, poor appetite and anemia suggest a generalized infectious or inflammatory process that is not specific for any organ in particular. Emaciation is a predominant and conspicuous feature. Cats may show labored or fast breathing, and this can be due to anemia, or due to lung disease. A big liver, big spleen, and enlarged lymph nodes may all be detected on physical examination. Lameness has been reported in several cats with disseminated histoplasmosis. Ulcerating skin nodules, and erosions in the mouth have been observed in cats as well. Ocular involvement, such as an ocular discharge, eyelid spasm, conjunctivitis, blindness, glaucoma, and inflamed or detached retina, are sometimes seen. Diarrhea, which is the most common feature of disseminated histoplasmosis in dogs, is rarely observed in cats. Cats will show anemia, inappetence, fever, enlarged lymph nodes, spleen, liver, and may show ocular involvement.

 

Diagnosis In-depth

Complete blood count. There may be several findings on a complete blood count that support a diagnosis of histoplasmosis. Anemia is a common finding in dogs with histoplasmosis. The anemia is mostly due to suppression of red blood cell production by the infected bone marrow. In dogs, blood loss in the stool, due to severe Histoplasma infiltration of the intestinal tract, contributes to the anemia. Thrombocytopenia (a low platelet count) is commonly seen. There is no consistent pattern to the white blood cell count in affected dogs; the white count may be high, low, or normal.

Chemistry panel and urinalysis. Serum biochemical profiles often reveal a low albumin in dogs with the disease. Much of the low albumin is attributed to loss of albumin in the stool. Urinalysis shows no specific abnormalities.

Radiographs. X-rays of the chest often reveal a nodular pattern that is characteristic for fungal infections. Pulmonary lymph nodes are often visibly enlarged. Abdominal films may show an enlarged liver and spleen. X-rays of the bones in animals presenting with lameness may show various degrees of bone destruction and new bone formation.

Complete ophthalmic exam. An examination of the eye, especially the retina, may show abnormalities characteristic of fungal disease, lending further support to a diagnosis of histoplasmosis.

Rectal scrapings. A rectal scraping, obtained using a small metal spatula and stained and examined under a microscope often reveals Histoplasma organisms inside many of the cells. This allows a definitive diagnosis of the disease.

Bone marrow examination. Examination of stained bone marrow specimens often reveals cells containing the Histoplasma organism, allowing for a definitive diagnosis.

Cytology of fine-needle aspirates. Obtaining a tissue sample by aspirating it through a fine needle and spreading it onto a slide, staining, and examining under a microscope may reveal fungal organism and allow a definitive diagnosis. The liver, lung, spleen, and lymph nodes are good tissues to sample, and may be better than rectal scrapings in dogs that do not have signs of gastrointestinal involvement.

Biopsy. If examination of aspirated tissue cells is not diagnostic, examination of a biopsy specimen may be required. A specific type of inflammation is often seen in fungal infections and may be highly suggestive of the disorder, but finding actual fungal organisms may be difficult. Special fungal stains are often used to increase the chances of identifying the organism.

Serologic tests. Blood tests that look for antibodies against the organism may be performed. A positive test means that the animal was exposed to the organism. It does not necessarily mean that the animal is infected. Results of serologic tests are inconsistent and are not very useful in making a diagnosis.

Culture. Because of the large numbers of organism present in affected tissues, the Histoplasma organism can often be cultured from fine needle aspirates; however, the organism grows slowly and it may take 7 to 10 days before results are available. The organism, when grown in culture, also poses a significant hazard to human health, and laboratories involved need to be notified before submitting samples.

Therapy In-Depth

Treatment of disseminated histoplasmosis is difficult. It requires the use of antifungal agents and supportive therapy like adequate nutrition, hydration, and control of secondary bacterial infections.

Antifungal drugs. A number of antifungal drugs such as ketaconazole, itraconazole, amphotericin B have shown some efficacy against Histoplasma infection, either alone or in combination. These drugs have to be given for prolonged periods of time, and are often fairly expensive.

Supportive care. Intravenous fluids, good nutrition, and possible antibiotic therapy to control or prevent secondary bacterial infections may be necessary as part of the overall therapy for histoplasmosis.

Follow-up Care for Cats with Histoplasmosis 

Optimal treatment for your pet requires a combination of home and professional veterinary care. Follow-up can be critical, especially if your pet does not rapidly improve.

Administer all prescribed medications as directed. Alert your veterinarian if you are experiencing problems treating your pet

The prognosis for cats with disseminated histoplasmosis is guarded. Treatment is frequently unrewarding because the patients are often quite debilitated at the time of diagnosis, and the disease may already be fairly widespread. Newer antifungal drugs have improved the success rate of treatment.