Scientists Find Bird and Human E. coli in Wild Fish

July 1, 2008

Scientists at the University of Minnesota have found that some of the potentially harmful bacteria in the Duluth-Superior Harbor come from an unlikely source: the fishes. It's not the fishes' fault, though. They are just carrying around bacteria that are already in their environment.

University of Minnesota researchers Dennis Hansen, John Clark, Satoshi Ishii, Michael Sadowsky, and Randall Hicks are the first to discover the sources of E. coli (Escherichia coli) in several species of wild fish. They collected carp, brown bullheads, Eurasian ruffe, round gobies, white perch, and rock bass from the Duluth-Superior Harbor as part of a Minnesota Sea Grant-funded study to determine the sources of bacteria that result in local beach closures.

In a peer-reviewed paper recently published in the Journal of Great Lakes Research, the scientists describe that most of the E. coli were found in bottom-dwelling fishes (brown bullheads, ruffe, carp, and round gobies) and the genetic matches were most similar to E. coli found in bottom sediments, Canada geese, mallard ducks, and human wastewater. The researchers didn't test the bacteria for pathogenicity.

"We didn't find the bacteria in the fish meat -- it's carried in their intestine," said Randall Hicks, biology professor at the University of Minnesota Duluth. "Anglers shouldn't worry about using the fish as food. They should just be careful not to cut open a fish's intestine."

If an angler happens to cut open fish intestines during cleaning, Jeff Gunderson, associate director with Minnesota Sea Grant, suggests they thoroughly wash the fish with clean water and cook it fully.

E. coli is an indicator of potential pollution. Levels of it are used to determine whether local beaches should be posted with "no water contact" advisories. There are a variety of types of E. coli. The most worrisome for humans is usually the E. coli from other humans (often from sewage overflows). While many strains are harmless, some cause gastrointestinal illnesses. Symptoms include vomiting, diarrhea, or other more serious conditions people would not want as a reminder of a fun day at the beach.

"Fish probably acquire E. coli when they eat food contaminated with feces," said Hicks. Researchers don't expect E. coli to flourish in cold-blooded fish, since the bacterium is more common in warm-blooded animals. "However, it is possible that fish may reintroduce E. coli bacteria into waterways when they excrete their own waste," Hicks said.

"Currently, it's probably more appropriate to consider fish as carriers of E. coli from other sources, rather than a new source of contamination in our waterways," Hicks added.

Until 1966, E. coli was thought to survive only in warm-blooded animals such as birds and mammals but it has since been discovered in the intestines of wild fish. The source of the bacteria in these cold-blooded animals was thought to be from polluted water and food, but researchers did not attempt to trace it.

Subsequently, E. coli was discovered in the intestines of farm-raised tilapia and rainbow trout. The fish were not the source for the E. coli, rather, the suspect was their food, which had been contaminated by pigeon droppings.

For more information on this project, order the free journal article: Sources and Sinks of Escherichia coli in Benthic and Pelagic Fish, from Minnesota Sea Grant by visiting www.seagrant.umn.edu/publications/JR544 or calling (218) 726-6191. Ask for JR 544.


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