Mass die-off of freshwater mussels in Wisconsin leads to discovery of new parasite

Researchers discovered a new parasite in hundreds of freshwater mussels that died along the Emberlus River in Shawano County six years ago.

of Investigation result The study was recently published in the international peer-reviewed journal Parasitologia. Researchers from the U.S. Fish and Wildlife Service, the U.S. Geological Survey, and the Wisconsin Department of Natural Resources were among the agencies that collaborated to study the 2018 die-off. Biologists observed diseased mussels, which are normally buried in the river bottom, lying on the river surface with their shells still open.

The researchers tested 29 mussels from six different species in the river, then cut them in half and examined the organs of some samples to see if the mussels were healthy or diseased.

Eric Reis, a fish biologist with the La Crosse Fish Health Center, said samples found the new parasitic bacteria in females of three species of clams: the moquette, fat moquette and pocketbook clam.

Reiss said the parasites, which the researchers named “microsporidia,” Hirsutnosema embarrassiihad infected developing eggs within the ovaries of those mussels.

“The eggs are basically being replaced by these microsporidia, and that has an impact on whether these eggs survive or not,” Reis said.

While it’s unlikely that parasites caused the die-off, parasitic bacteria could affect the mussels’ reproduction and survival, Reis said.

“This is significant given that many mussel species are in decline,” Reis said.

North America is home to about 300 of the world’s 1,200 species of freshwater bivalve mollusks, many of which are at risk of extinction. Nearly two-thirds of North American freshwater bivalve mollusks are threatened or vulnerable. Jordan Richard is a biologist with the U.S. Fish and Wildlife Service’s Endangered Species Program.

“Ten percent of North American species have become extinct over the past 100 years or so, mostly as a result of human activities, habitat destruction and pollution,” Richard said.

Still, Richard said little is known about the mussels’ health or why they’re dying. He said efforts are underway to understand the cause of the mussel decline and to keep infected mussels from being introduced to farms that are working to restore populations.

As part of their research, they used genetic sequencing, known as fingerprinting, to identify the microsporidia parasites, Richard said. Reiss noted that such infections are usually associated with hosts whose immune systems are weakened or unable to fight off the disease.

“We don’t know if there are stressors in the environment or what is stressing the mussels enough to cause these severe infections,” Reis said.

Richard said mysterious mass deaths are often caused by illness, but there tends to be a confluence of factors at work, which he likened to the COVID-19 virus, which has claimed more than 1,000 lives. 7 million Globally, 140 million people have been infected with COVID-19 since the pandemic began in 2020. While many lives have been lost, most people have survived, he said, adding that existing medical conditions often make people more susceptible to catching the virus.

“The same thing is happening with the mussels. There’s a lot going on there: pollutants, sediment, water quality, declining populations, parasites, viruses, bacteria,” Richard says. “They’re all kind of working together.”

This is important, the researchers say, because mussels are so important to healthy ecosystem functioning and clean water. He noted that mussels are filter feeders that live on the bottom of waterways, calling them the livers of rivers. That means they filter out bacteria, viruses, pollutants and more to purify the water, Reis said.

“They’re coming into contact with everything, from what we put down the drains to what we put on the fields, so they’re coming into contact with a lot of stressors,” Reis said.

The researchers say the discovery of the parasite is significant because it now allows them to develop tools to test for the parasite and find disease-free mussels that can be used in restoration efforts. They plan to expose mussels to the microsporidia parasite in the lab to see what the course of infection is and whether it ultimately kills the mussels.

Reis said the bigger goal is to grow healthy mussels that can be released into rivers where numbers are dwindling.

Richard added that it’s important to know such parasites exist so people don’t introduce infected mussels to fish farms or spread them to other rivers.

“It takes a lot of work to make sure that when you go out on the river there are healthy fish and mussels and birds and everything else around the river,” Richard said. “This is what we need to do to keep it going.”