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Pythiosis in Dogs

Harry Cridge 1

Background
Pythiosis is an infectious disease caused by Pythium insidiosum, an aquatic oomycete found in tropical and subtropical regions, commonly associated with freshwater habitats. In the United States, it predominantly affects the Southeast and Gulf Coast but is spreading geographically. The disease primarily manifests in either gastrointestinal or cutaneous forms in dogs, rarely both concurrently. Accurate diagnosis and management are critical due to similarities with other fungal diseases that have different prognoses and treatment requirements.

Methods
This review compiles clinical experiences, retrospective analyses, and findings from case reports and series on canine Pythiosis. Diagnostic modalities discussed include cytology, histopathology with special stains (e.g., Gomori methenamine silver), serologic assays, culture, and molecular techniques such as PCR and MALDI-TOF. Imaging techniques including abdominal ultrasound, CT, and radiographs are also evaluated. Treatment protocols include surgical resection and various multi-agent medical therapies.

Results
Gastrointestinal Pythiosis often presents with severe segmental thickening or mass-like lesions in the pyloric or small intestine, with associated lymphadenopathy. Cutaneous forms appear as nonhealing wounds with ulcerated nodules and draining tracts, often involving the extremities and perineum. Diagnosis is confirmed by demonstrating hyphae through cytology/histopathology aided by GMS staining and molecular tools. Surgical resection with wide margins is considered curative in some cases, especially when combined with antifungal therapy. Medical management with itraconazole, terbinafine, mefenoxam, and prednisone has shown promise in both cutaneous and gastrointestinal forms. Response to therapy is monitored by clinical signs, imaging, and declining antibody titers. Approximately 50% survival has been reported with combined modalities.

Limitations
The review emphasizes that much of the treatment data is derived from retrospective studies and case reports rather than controlled clinical trials. The efficacy of medical regimens is difficult to quantify due to multiagent protocols. Furthermore, pharmacokinetics of some treatments, such as mefenoxam, remain inadequately studied. Diagnosis may be hindered by sampling errors or suboptimal tissue handling, and endoscopic biopsies can miss submucosal lesions.

Conclusions
Pythiosis in dogs, once considered uniformly fatal, now has a potentially favorable prognosis when aggressively treated. Early diagnosis, surgical resection when feasible, and the use of multiagent medical therapy significantly improve outcomes. Further research, particularly prospective and pharmacokinetic studies, is needed to optimize treatment protocols and understand long-term disease management.

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