In areas where the disease is commonly observed, pythiosis occurs during the summer months, specially after periods of high pluvial precipitation. It was noted that horses that had grazed for a period of time in stagnant water frequently developed pythiosis. However, cases of pythiosis in equines, dogs, and humans never exposed to stagnant water, indicated 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 have been termed: swamp cancer, Florida horse leeches, summer sores, burusattee (means 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 also reported the disease.

LIFE CYCLE. Pythium insidiosum, like other Pythium spp, need wet environments to carried out their life cycle in nature. It also can grow in wet soil with grass because 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. After exposure to water containing zoospores they will be attracted by small wounds 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.