Geographic distribution of Pythium insidiosum infections in the United States

OBJECTIVE To describe the geographic distribution of infections caused by Pythium insidiosum in dogs, horses, and other animal species in the US. ANIMALS For the last 20 years, we have collected data from cases of pythiosis in 1,150 horses, 467 dogs, and other species (59) from various geographic locations in the US. PROCEDURES Due to lost data (from 2006 to 2016), the selected cases include years 2000 to 2005 and 2016 to 2020. The selection of cases was based on infected host clinical features, serum samples demonstrating strong positive anti-P insidiosum IgG titers in serologic assays, and positive results on 1 of the following diagnostic modalities: microbial culture on 2% Sabouraud dextrose agar, histologic evaluation, PCR assay, and wet mount cytologic evaluation (with potassium hydroxide). RESULTS Most confirmed P insidiosum infections were found in horses and dogs in the southeastern US. Interestingly, in Texas, no cases were found west of longitude 100 degrees W. Few pythiosis cases were diagnosed in west-coast states. Equine cases were more often diagnosed during summer and fall months, but canine cases were more often diagnosed between September and February. Cases in other species were discovered in the same geographic areas as those in dogs and horses.