Pythiosis is a relatively uncommon fungal-like infection causing cutaneous or subcutaneous, gastrointestinal, respiratory or multisystemic disease in many species of animals including humans. Horses are most commonly infected, and the devastating tumor-like nodular skin masses seen in these cases are likely to be remembered long after the actual name of the organism—Pythium insidiosum—is forgotten. The extremely rapid rate of growth of these lesions and the generally fatal outcome in these cases makes remembering this disease crucial for equine practitioners since early recognition and appropriate treatment are the only hope for survival for infected horses.
An increasing problem
Pythium insidiosum is referred to as an aquatic fungi or water mold, but, although it has some characteristics in common with typical molds, it is phylogenetically distinct. It was first identified in 1901 and has caused problems throughout North, Central and South America, the Caribbean Islands, Australia, the Pacific Islands and Asia. (It is interesting that tropical conditions support pythiosis, but to date no cases have been reported in Africa).
Pythiosis has been called a number of names throughout the world, from swamp cancer, Florida horse leeches and summer sores to bursautee. This lack of scientific or descriptive terminology reflects the lack of knowledge about this disease.
Photo 1: A lateral view of an aggressive pythiosis lesion in a Quarter Horse mare. The initial irritation on this horse's lower chest just caudal to the elbow looked like a minor scrape or puncture and was initially treated as a nonresponding wound.
Recently, however, new research and better diagnostic methodologies seem to indicate that pythiosis, and infection by another member of the same class of organisms—Lagenidium—might be responsible for an increasing number of infections in horses and other species. Bob Glass, an allergy specialist and owner of Pan American Veterinary Labs, has been investigating pythiosis for years.
Photo 2: The same horse in Photo 1 seen from a three-quarters view. As this lesion continued to grow, it was treated with antibiotics, corticosteroids and various topical products without response. Pythiosis was diagnosed through a blood test, and vaccine treatment was initiated but, as of publication, this mare has failed to change her allergic response into a repairative one.
"Although we've been interested in Pythium, Lagenidium and the hundreds of other related species for years, it has typically been a small number of researchers looking at a small number of confirmed cases," says Glass. "Better diagnostic tests and increased awareness have brought us many more cases, and these diseases seem to be on the rise, so we are now making more rapid strides in our research." Ten years ago, Pan American Veterinary Labs recorded fewer than 10 cases of pythiosis in dogs per year. Currently, they are identifying about 20 cases per month.