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Spring 1999 Newsletter

Histopathology Service
Caprine Arthritis Enchephalitis Virus
Pythiosis in Dogs
Disseminated Intravascular Coagulation
Infectious Pancreatic Necrosis Virus
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Pythiosis in Dogs

            Pythiosis is an infectious disease caused by a fungus-like organism, Pythiuminsidiosum, that naturally inhabits wetlands, ponds, and swamps.  In dogs, the disease usually is manifested by gastroenteritis or dermatitis.  Presumably dogs become infected by ingesting, or swimming in contaminated water.  Pythiosis is endemic in states that border the Gulf of Mexico, but has been diagnosed in dogs from southern Indiana (with no history of travel outside the state).  Dogs with gastrointestinal pythiosis often have a history of retrieving objects, such as sticks, from water and then chewing on them; young male retriever-type dogs are particularly at risk.  Dogs with open skin wounds are probably predisposed to acquiring cutaneouspythiosis.

            Gastrointestinal pythiosis is usually a fatal disease.  Pathologically, the organism is highly invasive and angiotropic, so the infection can be complicated by disruption of vascular supply.  The infection typically involves the full-thickness of the stomach or intestine and sometimes extends to adjacent organs.  For example, pythiosis of the pylorus or duodenum can extend to the pancreas, omentum, lymph nodes, or contiguous viscera.  The tissue response is typically a combination of macrophages, multinucleated giant cells, lymphocytes, plasma cells, and eosinophils.  Pythiumhyphae are usually found in necrotic foci, surrounded by inflammatory cells and sometimes coated by proteinaceous material that is probably derived from eosinophil granules.  In H&E-stained sections, this coating (termed a Splendore-Hoeppli reaction) allows for microscopic visualization of the outline of the otherwise non-staining Pythiumhyphae.  Necrotic foci and granulomas associated with these organisms may be grossly visible as yellow granules.  Extensive fibrosis often results in stenosis of the affected segment of gut producing clinical signs referable to the site of infection.  For example, pythiosis of the gastric pylorus often results in projectile vomiting, and pythiosis of the duodenum in obstruction.

            Lesions of cutaneouspythiosis usually develop on the limbs, tail, or face.  They are typically firm or spongy (depending on the degree of fibrosis) and ulcerated, often with a draining fistulous tract.  Clinically, these lesions may resemble acral lick granulomas.

            Because both the organism morphology and the inflammatory reaction are so characteristic, the diagnosis of pythiosis is usually made by microscopic examination of affected tissue(s).  A diagnostic biopsy sample of gastrointestinal tissue usually requires the submucosa (i.e. a biopsy of only the mucosa may not provide a diagnostic sample).  Cytology and histopathology of the sample is recommended.  The organism produces non-septatehyphae, 4 to 8 microns in diameter.  They are best visualized in histologic sections stained with Gomori'smethenamine silver (GMS).  Culture and immunohistochemistry are ancillary techniques that are usually reserved for research.

            Successful treatment of dogs with pythiosis is difficult.  Although morpho-logically resembing fungi, the cell wall - plasma membrane of Pythium - differs bio-chemically from that of fungi.  So it is essentially resistant to antifungal drugs.  The only treatment option available for a potential cure is surgery and the goal is complete resection of the affected tissue.  In the gastrointestinal tract, pythiosis clinically mimics an invasive carcinoma so aggressive surgical extirpation must be attempted.  But the prognosis is poor.  For cutaneouspythiosis involving extremity, amputation may be necessary.

- byYumi Yuasa, ECFVG

- edited by Evan Janovitz, DVM, PhD


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