The Furry Critter Network

Pythiosis - Issue Description

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Pythiosis is caused by Pythium insidiosum and occurs most commonly in dogs and horses, but is also found in cats, cattle, and humans. The disease is typically found in young, large breed dogs. Pythium occupies swamps in late summer and infects dogs who drink water containing it. Pythium insidiosum is different from other members of the genus in that human and horse hair, skin, and decaying animal and plant tissue are chemoattractants for its zoospores.

Pythiosis occurs in areas with mild winters because the organism survives in standing water that does not reach freezing temperatures. In the United States it is most commonly found in the Gulf states, especially Louisiana, but has also been found in midwest and eastern states. It is also found in southeast Asia, eastern Australia, New Zealand, and South America.

It is suspected that pythiosis is caused by invasion of the organism into wounds, either in the skin or in the gastrointestinal tract. The disease grows slowly in the stomach and small intestine, eventually forming large lumps of granulation tissue. It can also invade surrounding lymph nodes.


Symptoms include vomiting, diarrhea, depression, weight loss, and a mass in the abdomen. Pythiosis of the skin in dogs is very rare, and appears as ulcerated lumps. Primary infection can also occur in the bones and lungs.


Pythiosis is suspected to be heavily underdiagnosed due to unfamiliarity with the disease, the rapid progression and morbidity, and the difficulty in making a diagnosis. Symptoms often appear once the disease has progressed to the point where treatment are less effective. As the organism is neither a bacterium, virus, nor fungus, routine tests often fail to diagnose it. In cytology and histology, the organism does not stain using Giemsa, H&E, or Diff-Quick, but the hyphae are outlined by surrounding tissue. GMS staining is required to identify the hyphae in slides, and highlights the lack of septation which helps distinguish the organism from fungal hyphae. Granulomatous inflammation with numerous eosinophils is suggestive that the hyphae are oomycetes rather than fungi, which are less likely to attract eosinophils. The symptoms are usually nonspecific and the disease may not be included in a differential diagnosis in human medicine, though it is familiar to veterinarians.

Biopsies of infected tissues are known to be difficult to culture, but can help narrow the diagnosis to several different organisms. A definite diagnosis is confirmed using ELISA testing of serum for pythiosis antibodies, or by PCR testing of infected tissues or cultures.

Due to the poor efficacy of single treatments, pythiosis infections are often treated using a variety of different treatments, all with varying success. Most successful treatments include surgery, immunotherapy, and chemotherapy.


Traditional treatment options for Pythium infected dogs include surgical resection of infected tissues and antifungal medications. Overall success when using one or more of these treatment options is only +/- 25%. Recently the USDA has approved an Immunotherapy treatment product. This product consists of purified proteins derived from Pythium Insidiosum. These proteins are injected into the infected patient in an effort to elicit an immune response which will kill the invading Pythium. In horses this treatment is successful in greater than 90% of cases, however, in canine cases the success rate is nearer 50%. Work continues on a new version of this product which will demonstrate a higher success rate in dogs.

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