Histoplasmosis in Dogs

By 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.

"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."

In dogs, evidence suggests that prolonged exposure to high levels of Histoplasma organisms increases the risk of contracting the disease. Most dogs that develop this disease are young, large breed dogs. In one large study, there appeared to be an increased risk of this disease occurring in Pointers, Weimaraners, Brittany spaniels, and other sporting or working breeds. This is likely the result of greater environmental exposure in hunting breeds of dogs rather than an actual breed predisposition.

What are the signs of histoplasmosis?

The signs of histoplasmosis are variable and often not specific. Non-specific signs typically include mild fever, depression, weight loss, and loss of appetite. If the lungs are the primary site of infection, the dog may have labored breathing or a chronic cough.

If the intestinal tract is involved (which is more common in cases involving dogs), signs may include persistent diarrhea, tarry or bloody stools, or straining when passing a bowel movement. 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 (painful joints and/or lameness).

How is histoplasmosis diagnosed?

Your veterinarian may suspect that this disease is the cause of your dog'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 non-specific, 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 may not confirm a diagnosis of histoplasmosis, only 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.

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 fluid 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 dogs and have relatively few side effects when compared to the medications being used several years ago.

"Dogs may require several months (at least six months in most cases) of therapy to successfully treat this disease."

Dogs may require several months (at least six months in most cases) of therapy to successfully treat this disease. To minimize the chance of toxic side effects and to monitor your dog's response to treatment, it will be necessary to repeat the blood tests and X-rays at regular intervals.

What is the prognosis for my dog?

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 disease. A dog in poor condition or with advanced disease is less likely to survive. Dogs 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.

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

Am I at risk of infection from my dog?

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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