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Canine Pythiosis
Pythiosis is a severe and often fatal disease caused by the water mold Pythium insidiosum. There are 85 species of Pythium, most of which are parasites of plants, fish, and crustaceans. Pythium species belong to the class Oomycetes. The oomycetes, often referred to as water molds, have many characteristics in common with the fungi, but are more closely related to algae. Chitin, an essential component of the fungal cell wall, is generally lacking in oomycetes. Zoospore release is seasonal and associated with warm weather. Water lilies and other aquatic plants and submerged grasses, including rice plants, are thought to be normal hosts. The organisms have a strong attraction for horse skin and hair, human hair, and water-lily and grass leaves.
Young, male large-breed dogs are most often affected with reports of predilection in the German Shepherd dog and in outdoor working breeds such as Labrador Retrievers. Regardless of species and age, animals exposed to warm, standing fresh water are more likely to be in contact with the infectious zoospore and may have increased risk for disease. Reports of animals being infected with no known history of being near water suggests that animals can be inoculated by contact with resistant spores that form in wet soil and on grass.
In the United States, the infection is seen in warmer to temperate sections. In dogs pythiosis most often occurs along the Gulf coast and parts of California. There also have been reports from Missouri, Georgia, North Carolina, South Carolina, and Tennessee. Dogs become infected by ingesting, or swimming in contaminated water.
When animals with damaged skin come into contact with contaminated water or wet soil, zoospores enter the exposed tissue and encyst. In widespread disease, the organism usually penetrates the gastrointestinal tract and can spread to adjacent tissue such as pancreas, uterus, and lymph nodes.
Signs
Canine pythiosis most commonly occurs as gastrointestinal disease. Affected dogs often will have a history of upper gastrointestinal tract obstruction and may have a palpable abdominal mass. The stomach and small intestine are the most frequently recorded locations of the lesion, although any region of the alimentary tract can be affected. Clinical histories associated with the disease include loss of appetite, weight loss, vomiting, and diarrhea.3 The diarrhea may become bloody due to intestinal necrosis and ulceration. Cutaneous pythiosis in dogs typically causes nonhealing wounds and invasive masses that contain ulcerated nodules and draining tracts, most often involving the extremities, tail head, and neck.
Other organisms with similar morphology to Pythium species include fungi classified in the class Zygomycetes which cause Zygomycosis. Zygomycosis can also affect the alimentary tract of the dog.
Types of Canine Pythiosis
The cutaneous/subcutaneous form of pythiosis which affects skin and tissues is less frequent in dogs than the gastrointestinal form of the disease. This form of the disease is characterized by non-healing, tumor-like nodules with ulceration and pus-filled discharge. Disseminated or metastatic pythiosis occurs rarely in dogs and is thought to be triggered by use of corticosteroids or weakened state of the immune system. Lesions can be solitary or multiple and are usually confined to one area of the body, especially the legs, face, and tail head. Early lesions on the limbs may resemble acral lick dermatitis. Rarely, dogs develop locally invasive skeletal or disseminated infection.
Diagnosis
The diagnosis of oomycete infections can be difficult due to the clinical and histological similarity to fungal infections. Special expertise and considerable time are required for these laboratory procedures. Two ELISA tests are available for detection of antibodies in animals.
Treatment
Amputation of the affected limb may be indicated in dogs. Even with treatment, most cases are fatal. Antifungal chemotherapy agents (amphotericin B, ketoconazole, flucytosine) have been disappointing in the treatment of this disease.
Prevention
Recently, immunotherapy using a newly formulated vaccine has been successful in treating cutaneous pythiosis in horses and dogs.
References
- Oomycosis: Pythiosis in the Dog, Horse and Cat and Lagenidiosis in the Dog
- Muller & Kirk's small animal dermatology. George H. Muller, Danny W. Scott, Robert Warren Kirk, William Howard Miller, Craig E. Griffin
- Todd R. Tams. Handbook of small animal gastroenterology
- Stephen J. Ettinger, DVM, DACVIM and Edward C. Feldman, DVM, DACVIM. Textbook of Veterinary Internal Medicine
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