If your horse doesn’t live in a tropical or subtropical region, you’ve most likely never had to worry about equine pythiosis, a relatively rare and sometimes fatal skin infection that causes tumorlike masses and ulcerated lesions in horses. But researchers have noted recently that the disease is now also affecting horses in cooler, drier climates–indicating that it’s becoming more widespread and is an affliction more horse owners need to be aware of.

According to Leonel Mendoza, MSc, PhD, an associate professor in the Department of Microbiology at Michigan State University, who recently completed a review on the disease, equine pythiosis is a rare infection caused by a funguslike microorganism called Pythium insidiosum. The infection, typically found in tropical and subtropical climates, has recently been diagnosed farther north than usual, appearing in drier states such as Arizona, California, and Washington state. The reason for the geographic spread of the disease remains unknown.

The causative agent resides in standing water or moist soil and finds its way into small wounds on a horse’s legs, lower abdomen, chest, or face, causing large, tumorlike masses that ulcerate. These itchy lesions contain necrotic (dead) tissue with a characteristic strong odor and draining tracts, and they contain stony masses called "kunkers" at their core.

Once considered a rare infection in the United States, pythiosis is a noncontagious disease with hundreds of cases reported annually in dogs and horses, and few cases in humans (one to two per year). Mendoza notes that there have been no reports of animal-to-animal or animal-to-human (or vice versa) transmission of the disease.

An increased interest in or recognition of the disease has accompanied its spread and some researchers have described the disease as "emerging." However, other authorities on the subject, including Mendoza, disagree, noting that the disease was described as early as 1884 in India.

Early Action Key to Successful Treatment

No breed, age, or sex predilection appears to exist, so all horses have a similar risk for contracting the disease. If pythiosis is suspected in a horse that begins to develop lesions in the described areas, veterinarians advise testing for the disease (via blood test or biopsy) as quickly as possible. Long-standing (chronic) cases are less likely to respond to treatment than cases that are caught promptly (generally within two weeks).

A treatment technique called immunotherapy (referred to the researchers as vaccination) is reportedly associated with a very high success rate. Researchers on one study reported that 72% of 18 horses recovered completely from pythiosis following vaccination (Mendoza notes, however, that most of these horses had undergone other treatment modalities in addition to the vaccination, including surgery to excise the lesion, or administration of various drugs such as antifungal agents). Thus, the chance of curing a horse with the vaccine alone remains unclear.

"Despite this, immunotherapy remains the first choice for the management of equine pythiosis in the USA," said Mendoza.

He noted that although P. insidiosum is not a fungus, veterinarians sometimes turn to antifungal agents as treatment; however, these drugs are not associated with a high success rate.

Researchers have noted that radical surgery, including limb amputation, is still the only treatment option in many cases, but many patients (about 55%) die despite surgical intervention.

Although a surefire preventive technique remains elusive, veterinarians suggest that practicing good wound care techniques and attempting to keep horses away from standing water or moist soil might aid in preventing infection.

The review, "Pythium insidiosum: an overview," was published in Veterinary Microbiology in November 2010. The abstract is available online.