Pythiosis in the Dog

Has your dog been losing weight lately, exhibiting chronic vomiting, had diarrhea, or hematochezia (blood in the feces)? Alternatively, does your dog have an ulcerated, draining skin lesion that just won’t heal? Is your dog a sports breed like a Labrador retriever, or does he love the water? Do you live in an area with a warmer climate? Then your dog could be suffering from an infection called Pythiosis.

Pythiosis is an infection caused by the organism Pythium insidiosum. Although typically misclassified as a fungus because it produces hyphal structures, P. insidiosum is actually a pathogenic “water mold” and a member of the Oomycetes class in the zoological classification system.   

The organism Pythium is believed to gain entry into the body by becoming encysted in damaged skin or the gastrointestinal mucosa after being swallowed. In the dog, P. insidiosum may cause gastrointestinal or cutaneous lesions. The cutaneous and gastrointestinal forms of the disease are rarely encountered together in the same individual. The Labrador retriever is the most commonly affected breed due to their popularity and fondness for water.

The characteristic lesions produced by pythiosis are eosinophilic and pyogranulomatous inflammation. The inciting organism is often not detected on cytology or histopathology. Diagnosis is characteristically made on the basis of the histological appearance of the lesion or by serology. Serology is highly sensitive and specific for the detection of P. insidiosum antibodies and is available at various veterinary school laboratories. On histological examination these lesions will have multiple areas of necrosis which are surrounded and infiltrated by various white blood cells trying to combat the infection, such as neutrophils, eosinophils, and macrophages.

Skin lesions will be ulcerative in appearance, often with exudate (discharge) emerging from various draining tracts. With skin lesions, aggressive surgical resection is the treatment of choice. When cutaneous lesions are confined to a single leg amputation should be recommended.

Dogs with the gastrointestinal form of pythiosis will have severe thickening of one or more portions of the GI tract that may include the stomach, small intestine, colon, rectum, or in rare cases even the esophagus. The resulting pathology will result in anorexia (no appetite), vomiting, diarrhea (sometimes bloody), and abdominal straining. Extensive weight loss may be evident.

Laboratory findings may be normal or the dog may exhibit any of the following: eosinophilia (a rise in the number of a certain type of white blood cell (WBC) called an eosinophil - these WBCs are usually found in greater numbers with allergic and parasitic infections), anemia, hyperglobulinemia (high levels of globulin in the blood), hypoalbuminemia (low levels of the blood protein albumin), and in rare cases
hypercalcemia (high calcium levels). Abdominal contrast studies and ultrasounds may reveal a thickening of the GI tract, an abdominal mass, or mesenteric lymph node enlargement. In advanced cases an abdominal mass may be palpated on routine examination.

The treatment of choice for gastrointestinal lesions is complete surgical resection as soon as possible. Although mesenteric lymph nodes may be enlarged, they are rarely infected with the organism. Lymph nodes may be removed when possible, or a biopsy may be taken for greater prognostic information.

A Pythium fungal protein extract, which may be thought of as a vaccine of sorts, is available for injection following surgical removal of all visible lesions. The extract is given one month following surgical recovery. Injections of the protein extract are given as a series of three beginning on day one, with follow up injections given on day seven and twenty-one after surgery. The goal of vaccination is to stimulate the body’s response or immune system fight the organism. Additional injections may become necessary and are typically given on an as needed basis. Serology should be conducted at 3-month intervals to assess the level of success with treatment. A rising antibody titer would necessitate the use of additional Pythium protein injections or would indicate that further surgery is necessary.

It is hard to culture P. insidiosum from infected tissues. The organism is extremely sensitive to cold, and samples should be sent at room temperature to a laboratory within 24 hours for culture.

Pythium infections are often fatal even with intensive treatment. Following GI resection and a series of protein injections, chances for long term survival may be 50% at best. The earlier the treatment is started the better the prognosis.

References:

Bonagura, John and David Twedt, Editors. Kirk’s Current Veterinary Therapy. Saunders/Elsevier 2009. Pp. 1268 - 69.

Glass, Bob. Case Conversations. December 2008.

Kuhnt, Leah. "State of Alabama Dept of Agriculture". Case Conversations. November 2008.

Mendoza, Leo. Michigan State University, Telephone Conversations. December 2008.

Newton, Joseph. Auburn University. Pathobiology Diagnostic Service. Case Conversations. December 2008.