Field and laboratory evaluation of the microsporidia parasite, Heterosporis sutherlandae Prevalence, severity, and transmission.

Heterosporis sutherlandae is an invasive microsporidian parasite in the Great Lakes region of North America that infects the skeletal muscle of numerous fish species, rendering the filet unfit for human consumption. Although H. sutherlandae has been identified as a pathogen of concern by state management agencies, there is little information to inform regulation and intervention. We sampled fishes over one year from three lakes in northern Minnesota with known infected populations to determine the importance of host demographic and environmental variables to H. sutherlandae infection prevalence. Heterosporis sutherlandae was present during all sampling periods, ranging in prevalence from 1-11%. The prevalence of H. sutherlandae varied significantly according to season, with winter having the lowest prevalence among Yellow Perch Perca flavescens (1%), and summer having the highest prevalence (11%). For fishes of other species, the prevalence of H. sutherlandae also varied significantly with season, where the lowest prevalence occurred during the spring (1%) and the highest in fall (9%). Rates of pathogen transmission were estimated by exposing Fathead Minnows Pimephales promelas in the laboratory. Transmission rates were 23% when naive fish were fed infected tissues, and only 2% when naive fish were cohabitated with tissue-fed fish. Exposure method and time exposed increased the probability that a fish was infected with H. sutherlandae. These findings suggest that H. sutherlandae transmission is greater when a susceptible host consumes infected tissue than exposure to spores present in the water column. The current rates of infection in wild fishes is in stark contrast to the prevalence documented in 2004 (28%), suggesting a reduction in H. sutherlandae prevalence within at least one Yellow Perch population in the Laurentian Great Lakes region since the early 2000s.