Emerging fish diseases threaten already at-risk wild fish populations, sport fishing, and aquaculture in Minnesota. Heterosporis is an important disease that is caused by the microsporidian parasite Heterosporis sutherlandae. It was recently identified as a high-priority invasive microbe by the 2014 MAISRC Research Needs Assessment. Although scientists at the University of Minnesota have been working on this disease for several years and have developed new tools and information, much is left to be done. To inform effective management efforts, we must understand how this disease is transmitted, how it impacts the host fish, and what this means for recreational, commercial, and aquaculture fish production in Minnesota.

MAISRC researchers are working to better understand this disease so they can inform science-based management decisions. Click here to download a factsheet about pathogens and harmful microbes research at MAISRC.

About Heterosporis

What it is

Heterosporosis is caused by the parasite Heterosporis sutherlandae. This disease was first reported in 1990 in Leech Lake (Cass County, MN) and Catfish Lake (Vilas County, WI). Heterosporis was recently identified as a priority species by the Minnesota Aquatic Invasive Species Research Center and a pathogen of concern by the Great Lakes Fish Health Committee.

Life cycle

The small, microscopic spores of this parasite live inside of the muscle cells of their fish host. The parasite replicates, matures, and ultimately ruptures the cell and liquefies the muscle. The parasite is transmitted from one fish to another through the consumption of free-floating spores or an infected fish. Unlike many other parasites of fish, no intermediate hosts are needed to complete the life cycle.

What it affects 

Heterosporis sutherlandae is known to infect 15 species of freshwater fish, including ecologically and economically important species such as walleye, yellow perch, and fathead minnows. Humans and other animals are not known to be susceptible; however, it is advised to remove infected pieces of the fillet and cook thoroughly prior to consumption.

Where it's found

The disease was first confirmed in 1990 and has since been reported in 26 lakes in Minnesota, 16 lakes in Wisconsin, two lakes in Michigan, and one lake in Ontario. The origins of Heterosporis sutherlandae are unknown; however, related species are found in marine fish of the West Pacific and Arabian Sea. Click here for the full list of lakes where Heterosporis has been found.

How it spreads

Heterosporosis can spread by moving through infected fish or contaminated water. You can do your part to prevent the spread of this disease by cleaning your boat and equipment between lakes, not releasing bait or aquarium fish into new areas, draining the water from your boat, and not disposing of infected fish carcasses in our waterbodies.

What it looks like 

Because the parasite essentially liquefies muscle tissue, infected muscle has a freezer-burned or cooked appearance and is soft when touched. In severe infections, the fish will appear to curve inward due to muscle loss. The disease is most often observed once the fish has been filleted.

Heterosporis research at MAISRC


Completed efforts

  • Formally named and described the appearance, genetics, and pathology of Heterosporis sutherlandae (Phelps et al. 2015. PlosOne. DOI:10-1371).
  • Validated a highly sensitive and specific molecular diagnostic assay.
  • Developed an infection model to understand the disease over time in live fish.
  • Conducted surveys of Minnesota fish populations to estimate current distribution and species affected.

Current efforts

An interdisciplinary team of MAISRC researchers, in collaboration with fish health and invasive species managers, is working to better understand this disease so they can inform science-based management decisions. Field and laboratory research is underway to estimate the prevalence of the disease in important fish populations, the effect that it has on the endurance and fitness of the host, and potential long-term and seasonal infection variability. The data that are being generated by this research will be used in a disease ecology and population model to estimate the impact of Heterosporis on Minnesota wild fish populations.

Current projects

Completed projects

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