Environmental sampling reveals that Pythium insidiosum is ubiquitous and genetically diverse in North Central Florida

Pythiosis is a deadly disease of horses, dogs, and other mammals, including humans, in tropical and subtropical regions. In the United States, the disease has been reported in the Southeast as well as in the temperate North and the dry Southwest. The causal agent of pythiosis is Pythium insidiosum, one of few mammalian pathogens in the fungus-like Oomycetes. P. insidiosum has not been studied in the environment in the United States. Given anecdotal reports of pythiosis in Gainesville, Florida dogs, we hypothesized that warm standing water in lakes and ponds in North Central Florida is suitable habitat for P. insidiosum. We sampled 19 lakes or ponds to examine the environmental distribution of P. insidiosum and to determine which of the three previously described genetic clusters of P. insidiosum are present. We found P. insidiosum in 11 of the sampled lakes and ponds. Sequencing of the ITS region separated isolates into three genetic clusters, including a distinct group previously represented by a single isolate from South Carolina. AFLP genotyping of isolates showed genetic variation in Cluster I, which is the group associated with the majority of characterized clinical isolates from the Americas. Our results indicate that animal exposure to P. insidiosum in North Central Florida is common. This study provides the first evidence that P. insidiosum may be more widely distributed in freshwater lakes and ponds in the Southeastern United States than previously appreciated.