Pythium is a genus of parasitic oomycetes. They are commonly called water moulds. The genus Pythium consists of about 200 species and are common pathogens causing disease in plants and fish. Pythium insidiosum is the only species which causes infection in animals.
Pythium insidiosum, the etiological agent of pythiosis insidiosii, causes life-threatening infections in animals. The disease most commonly infects horses and dogs, but can also infect cats, cattle, equines, captive polar bears and humans.
Pythiosis has frequently been reported in tropical and subtropical regions of the world. In the United States, the disease is more common in states along the Gulf of Mexico and East coastal areas, but can also occur in cooler and dryer areas of the U.S. as well.
Pythium insidiosum, like other Pythium spp, need wet environments to carry out their life cycle in nature. Pythiosis occurs primarily in the fall and early winter after warm summer months, especially after periods of high precipitation. Animals exposed to warm, standing water water are more likely to encounter the infectious zoospores and may have an increased risk for the disease; however, the infection can be acquired after contacting moist soil and grass.
It is suspected that the invading zoospores enter an animal through open wounds, either in the skin or in the gastrointestinal tract. Water lilies and other aquatic plants and submerged grasses, including rice plants, are thought to be normal hosts. The zoospores have a strong attraction for hair, water-lily, and grass leaves.
Young dogs are most often affected, with several breeds such as the Labrador Retriever, German Shepherd and the Cavalier King Charles being seen most often.