Pythium insidiosum is an aquatic oomycete that causes severe gastrointestinal pathology in a range of hosts in the tropic and subtropic climates. Based on ribosomal RNA gene sequence data, members of the Class Oomycetes are phylogenetically distinct from the Kingdom Fungi and are more closely related to algae than to fungi. The Oomycetes differ from fungi in two important properties (i. e., cell wall and cell membrane composition). Chitin is an essential component of the fungal cell wall, but it is generally lacking in the oomycete cell wall. Oomycetes also differ from fungi in that ergosterol is not a principal sterol in the oomycete cell membrane. This difference may explain why ergosterol-targeting drugs like itraconazole are less effective in the medical treatment of pythiosis.
The infective state of Pythium insidiosum is thought to be the motile zoospore, which is released into stagnant water in warm environments and likely causes infection either by encysting in the skin or by being ingested into the gastrointestinal tract. Ingested zoospores encyst and adhere to the gastric, jejunal, and colonic epithelium with a polarity oriented toward the submucosa for rapid tissue penetration after germ tube eruption. Pythium induces a chronic pyogranulomatous response in the gastrointestinal tract and mesenteric lymph nodes. The gastric outflow tract and ileocolonic junction are the most frequendy affected portions of the gastrointestinal tract, and it is not uncommon to find two or more segmental lesions in the same patient. Inflammation in affected regions is typically centered on the submucosa, with variable mucosal ulceration and occasional extension of disease through serosal surfaces, resulting in adhesion formation and peritonitis.
Weight loss, vomiting, diarrhea, and hematochezia are the most important clinical signs. Physical examination often reveals emaciated body condition and a palpable abdominal mass. Signs of systemic illness such as lethargy and depression are not typically present unless intestinal obstruction, infarction, or perforation occurs.
Ileocolonic wall thickening, obliteration of the normal layered appearance, and regional lymphadenopathy are common ultrasonographic features of canine intestinal pythiosis. Of course, these findings cannot be readily differentiated from those associated with intestinal malignancy. Definitive diagnosis requires histologic demonstration or immunohistochemical staining of the organism, positive enzyme-linked immunosorbent assay or polymerase chain reaction assays, or a combination of these techniques. The histologic findings associated with pythiosis generally are characterized by eosinophilic granulomatous to pyogranulomatous inflammation with fibrosis. Affected tissue typically contains multiple foci of necrosis surrounded and infiltrated by neutrophils, eosinophils, and macrophages. Discrete granulomas composed of epithelioid macrophages, plasma cells, multinucleate giant cells, and neutrophils and eosinophils may also be observed. Pythium zoospores may be cultured directly from affected tissue in antibiotic-containing (e. g., streptomycin, ampicillin) media. More recently, sensitive and specific enzyme-linked immunosorbent assay and polymerase chain reaction assays have been developed for the accurate diagnosis of pythiosis in dogs.
Aggressive surgical resection remains the treatment of choice for pythiosis in dogs. Because it provides the best opportunity for long-term cure, complete resection of infected tissue should be pursued whenever possible. Segmental lesions of the gastrointestinal tract should be resected with 3 to 4 cm margins whenever possible. Medical therapy for the oomycetes has not been very promising. This may relate to the absence of ergosterol (cell membrane target of most currently available antifungal drugs) in the oomycete cell membrane. Clinical and serologic cures have been obtained in a small number of dogs after therapy with amphotericin B lipid complex (2 to 3 mg / kg every other day, administered to a cumulative dose of 24 to 27 mg / kg) or itraconazole (5 mg / kg every 12 hours for 6 to 9 months).
Unfortunately, most dogs with gastrointestinal pythiosis are not presented to the veterinarian until late in the course of the disease, when complete excision is not possible. The anatomic site of the lesion (e. g., pylorus, ileocolic sphincter) may also prevent complete excision. Consequently, the prognosis is usually grave in most animals.