Definition
An infectious disease primarily affecting the skin or GI tract of dogs and cats. It is caused by the aquatic pathogen Pythium insidiosum, an organism in the class Oomycetes.
Pathophysiology
- The infective form of P. insidiosum is thought to be the motile biflagellate zoospore, which is released into warm water environments and is chemotactically attracted to damaged tissue and animal hair. Animals are likely infected when they enter or ingest water that contains infective zoospores.
- P. insidiosum is considered a pathogenic rather than opportunistic organism because immune suppression is not a prerequisite for infection.
- In the GI tract, P. insidiosum infection causes chronic pyogranulomatous disease manifested by severe segmental transmural thickening of one or more areas of the stomach or intestine.
- In the skin, pythiosis typically results in the development of non-healing wounds and invasive masses that contain ulcerated nodules and draining tracts.
Systems Affected
- GI and cutaneous forms of the disease are encountered with equal frequency in the dog. In cats, which are infrequently infected, the cutaneous form is more common. With the exception of occasional dissemination to regional lymph nodes, pythiosis usually affects only one body system in each patient.
- GI pythiosis most often affects the gastric outflow region, proximal small intestine, ileocolic junction, or colon. The surrounding mesentery is often involved. Rarely, the esophagus may be affected.
- In dogs with GI disease, P. insidiosum-induced local thromboembolic events or vascular invasion may lead to bowel wall ischemia and GI perforation or hemoabdomen.
- Dogs with cutaneous pythiosis most often are presented for solitary or multiple cutaneous or subcutaneous lesions involving the extremities, tailhead, ventral neck, or perineum.
- In cats, cutaneous lesions or subcutaneous masses involving retrobulbar, periorbital, or nasopharyngeal regions, the tailhead, or the footpads have been observed. GI lesions are rare in cats.
- Multisystemic involvement is rare.
Genetics
Although large-breed dogs are most often affected, no genetic predisposition has been documented.
Incidence/Prevalence
- Dependent on geographic distribution.
- Affected animals more often are presented for signs of disease in the fall or early winter months.
Geographic Distribution
- Disease caused by P. insidiosum occurs primarily in tropical and subtropical areas of the world.
- In the United States, pythiosis occurs most often in states bordering the Gulf of Mexico; however, it has also been documented in Oklahoma, Arkansas, Missouri, Kentucky, Tennessee, North and South Carolina, Virginia, southern Indiana, New Jersey, Arizona, Wisconsin, and California.
- Outside the United States, pythiosis has been reported in Australia, Brazil, Burma, Colombia, Costa Rica, Indonesia, Japan, New Guinea, and Thailand.
Signalment
Species
Dog and less commonly cat
Breed Predilections
- Large-breed dogs, especially those used in hunting or field trial work near water.
- Labrador retrievers are overrepresented.
- German shepherds may be predisposed to cutaneous pythiosis.
Mean Age and Range
Animals <6 years old are most likely to be infected.
Predominant Sex
Males are affected more often than females, possibly because of increased exposure.
Signs
General Comments
Affected dogs are not usually severely ill until late in the course of disease.
Historical Findings
- Chronic weight loss and intermittent vomiting are the most common signs.
- Diarrhea may be evident if the colon or a large segment of the small intestine is affected.
- Regurgitation is noted with esophageal disease.
- Cutaneous disease is characterized by non-healing wounds with nodules that ulcerate and drain.
Physical Examination Findings
GI Pythiosis
- Emaciation is common.
- An abdominal mass is often palpable.
- Despite severe weight loss, affected dogs are usually bright and alert.
- Fever is usually absent.
- Systemic signs and abdominal pain may occur with intestinal obstruction, infarction, or perforation.
Cutaneous Pythiosis
- Cutaneous or subcutaneous lesions appear as non-healing wounds; boggy, edematous regions; or poorly defined nodules that become ulcerated.
- Multiple tracts draining a serosanguineous or purulent exudate often are present.
Causes
P. insidiosum
Risk Factors
- Environmental exposure to swampy areas, bayous, ponds, or lakes containing infective zoospores.
- Outdoor activities such as hunting.
