Epidemiology of Pythium

In areas where the disease is commonly observed, pythiosis occurs primarily during the summer months, especially after periods of high precipitation. It has been noted that horses allowed to graze for a period of time in stagnant water frequently develop pythiosis. However, cases of pythiosis in equines, dogs, and humans never exposed to stagnant water, indicate that the infection can also be acquired after contact with soil and grass containing P. insidiosum.

Due to its occurrence in wet environments and summer months the disease has been termed: swamp cancer, Florida horse leeches, summer sores, burusattee (“rain” in India).

Pythium insidiosum has been reported more frequently in tropical and subtropical regions of the world. However, cases in temperate areas of Japan and USA indicate that this organism can be found in cooler environments as well. Well documented cases of the disease have been reported in Australia, the Pacific islands, Asia, and the Americas. It would seem that the tropical climate of much of Africa would make it suitable for P. insidiosum. However, so far, no cases of pythiosis from that continent have been reported.

In the Americas, the disease is known to occur in North, Central and South America as well as the Caribbean islands. In the United States, the disease is more commonly reported in the states along the Gulf of Mexico: Alabama, Florida, Louisiana, Mississippi, and Texas. However, several cases in dogs, equines and humans have occurred in states such as Georgia, Illinois, Missouri, North, Oklahoma, and South Carolina, Tennessee (human), New Jersey, New York, and Wisconsin.

Cases in Central America have also been reported in Guatemala, Nicaragua, Costa Rica, and Panama. In South America, Argentina, Colombia, Venezuela, and Brazil have reported the disease.

LIFE CYCLE: Pythium insidiosum, like other Pythium spp, need wet environments to carry out their life cycle in nature although it can survive in wet soil with grass because of its ability to produce resistant spores. Several investigators have shown that this pathogen requires some plants to complete its life cycle in nature (lily and gramineae (grass). It is believed that the zoospores are the infecting units. Zoospores in water will become attached to the tissue in breaks in the host’s skin, intestinal tract and other sites. The zoospores will encyst and then produced hyphae that will, mechanically, penetrate the tissue and cause the disease. The majority of cases in equines occur through open wounds on the skin of the extremities and areas in contact with water or grass. In Dogs, the disease is observed in the skin and the intestinal tract. This is due to the fact that dogs may drink water and/or eat grass contaminated with P. insidiosum. 6/16/04