Two main features of dog pythiosis are the cutaneous and subcutaneous lesions and the gastrointestinal involvement. Dogs acquire the infection through open wounds on the skin and mucous membranes. The fact that dogs frequently drink stagnant water and eat grass that may contain elements of P. insidiosum explains the high number of intestinal pythiosis cases in canines.
Canine gastrointestinal pythiosis is characterized by vomiting, weight loss, and sporadic diarrhea. Formation of hard gastrointestinal tumor-like masses, areas of mural thickness, and mucosal ulceration are common. The organism can spread to adjacent tissue such as pancreas, uterus, and mesenteric lymph nodes. Histopathologically, the mucosa shows ulceration, atrophy, and hyperplasia. Eosinophils, plasma cells, macrohages, epithelioid cells and giant cells are detected in those tissues. The hyphae of P. insidiosum, however, is difficult to detect. Special stain, such as silver stain are required to visualized the hyphae of this pathogen in the infected tissues.
The cutaneous and subcutaneous lesions are denuded of hair and perforated by fistulous sinus tract, that discharge a serosanguineous exudate. They are usually located on the dog’s tail, legs, thorax, and abdomen. The hard stony masses (kunkers) observed in equine pythiosis are not present in dogs with the disease. Microscopically, multifocal areas of necrosis with eosinophils and a moderate number of neutrophils and macrophages is the main feature of the infection. The hyphae of P. insidiosum are found in the center of eosinophilic microabcesses.
TREATMENT Because dog pythiosis is relatively a new disease in this specie, most small animal veterinary practitioners are not familiar with its clinical features. Most cases of cutaneous pythiosis are initially diagnosed and treated as bacterial or parasitic infections, whereas gastrointestinal pythiosis is often misdiagnosed as intestinal tumoral-like disease (neoplastic) and treated by surgery (removal of the tumoral masses).
SURGERY Because pythiosis lesions progress rapidly a quick diagnosis is essential for animal survival. Once the diagnosis of pythiosis have been established, wide surgical removal of the infected tissues is the only effective treatment for cutaneous, subcutaneous, and intestinal dog pythiosis.
DRUGS Antifungal therapy using amphotericin B, itraconazole, ketoconazole, or trebinafine has been unsuccessful in most cases.
IMMUNOTHERAPY Treatment with the therapeutic vaccine used to cure cases of equine pythiosis has been only 55% effective in dogs with pythiosis. This may be due in part that all vaccinated cases were dogs with chronic pythiosis more than two months old. The vaccine has yet to be evaluated in more cases of dog pythiosis to determine its true cure rate in this species. Immunotherapy has also been used in humans with the disease. About 55% of the treated human patients with pythiosis in Thailand, some of them with arterial involvement, did respond to this approach. This procedure was used once in a child with orbital pythiosis in the USA without success (unpublished data).