Most veterinary and human medical students hear the phrase, "When you hear hoof beats, think horses not zebras," at least once during their training. In other words, common diseases are just that … common, so always first rule out the stuff we see frequently before moving on to the more exotic differential diagnoses.

This is wise advice, but every now and then we do actually run across a zebra. The disease pythiosis falls into this category. Most reports of pythiosis in veterinary medicine involve dogs or horses, although cats, cattle and other animals (including humans) can also be affected.

Pythiosis is caused by infection with a microorganism called Pythium insidiosum. This is a weird germ. It is closely related to fungi like yeast but also has some similarities to algae. It lives in warm, relatively stagnant bodies of water, so it shouldn’t be too surprising that most cases are diagnosed in animals that have drank from or waded/swum in ponds and the like. Pythiosis is seen primarily in tropical and subtropical regions of the world. In the U.S., the Gulf Coast is the "hot zone," so to speak, although the disease has also been diagnosed in more temperate climates like California, Kentucky, North Carolina, Missouri, Indiana, Washington and Wisconsin, and may be moving into these and surrounding areas.

The symptoms of pythiosis depend on where the infection is localized. Animals with the cutaneous form of the disease present with nasty, non-healing skin lesions and nodules that get worse despite treatment with antibiotics and other traditional therapies. When animals are infected through drinking contaminated water, they develop gastrointestinal signs like vomiting, diarrhea, and weight loss. None of the symptoms of pythiosis are unique to this disease, so a definitive diagnosis must be based on finding the organism in a tissue sample and/or blood work that confirms exposure to the P. insidiosum.

Treating pythiosis is not easy. Some combination of aggressive surgical removal of affected tissues, long term treatment with antifungal medications, and immunotherapy holds the greatest chance of success, but the prognosis is not good even with appropriate treatment. Therefore, prevention is key.

Even though pythiosis is a "zebra," add it to your list of good reasons (e.g., Giardia and leptospirosis) to keep your dogs, horses, and other pets out of stagnant bodies of water.

Dr. Jennifer Coates

Image: JuliaSha / via Shutterstock