Field and Laboratory Evaluation of the Microsporidian 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 fillet 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 1 year from three lakes in northern Minnesota with known infected populations to determine the importance of host demographic and environmental variables for influencing H. sutherlandae infection prevalence. Heterosporis sutherlandae was present during all sampling periods, ranging in prevalence from 1% to 11%. The prevalence of H. sutherlandae among Yellow Perch Perca flavescens varied significantly according to season, with winter having the lowest prevalence (1%) and summer having the highest prevalence (11%). For other fish species, the prevalence of H. sutherlandae also varied significantly with season: the lowest prevalence occurred during spring (1%) and the highest prevalence occurred in fall (9%). Rates of pathogen transmission were estimated by exposing Fathead Minnows Pimephales promelas in the laboratory. Transmission rates were 23% when naïve fish were fed infected tissues and only 2% when naïve fish were held in cohabitation with tissue-fed fish. Exposure method and exposure duration (d) 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 when the fish is exposed to spores present in the water column. The current rates of infection in wild fishes are 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.