By Samantha Mathewson

Oct 25, 2015 09:54 PM EDT

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VHSv is a deadly fish disease that has wiped out hundreds of small drum living in Lake Winnebago. Walleye (pictured here) are a popular game fish and drum are their primary food source. Researchers explain the importance of conserving populations to prevent future die-offs. (Photo : Flickr: Jerry “Woody”)

What causes freshwater fish to develop red splotches and swollen, bulging eyes? Researchers from the University of Wisconsin-Madison discovered that a deadly disease known as viral hemorrhagic septicemia virus, or VHSv, is to blame. These findings follow the mass death of hundreds of freshwater drum, also known as sheepshead, in Lake Winnebago and nearby Little Lake Butte des Morts in Wisconsin during May 2007.

The Wisconsin Department of Natural Resources (DNR) worked alongside researchers from theWisconsin Veterinary Diagnostic Laboratory (WVDL), to analyze the disease more closely. They now are able to provide anglers and boaters with ways to help slow the rate at which VHSv may spread to other lakes in Wisconsin, according to a news release.

“It’s still possible to transmit the virus to fish in other lakes,” Kathy Toohey-Kurth, one of the researchers from the WVDL, explained in the release. “It shows the virus is still transmitting and people still have to be careful to follow all the guidelines from the DNR, like not carrying buckets of bait between waters.”

VHSv was first detected in the U.S. among freshwater fish in 2005. During this time, populations of muskellunge, perch and walleye experienced mass fish die-offs. The recent diagnosis test has led to far fewer fish dying and washing up on shore. However, those fish that survive the infection could still potentially spread the disease to others. When infected, VHSv essentially cause fish to bleed to death.

While humans are not vulnerable to this disease, over 28 species of fish are. Species such as walleye and sauger are popular game fish, and drum are their primary food source.

For their study, researchers examined the internal organs of infected fish to isolate the virus. They also required small blood samples from the fish, but those individuals were released back into the wild after sampling, according to the release.

Using the blood samples, researchers looked for evidence suggesting the fish had once been infected. For example, a specific antibody would be present in the blood, in response to the infection. In total, researchers collected blood from nearly 600 drum from Lake Winnebago, the release noted.

Researchers found that only one single, large, older female drum was harboring the active virus. This provided “proof that the virus is still present in the lake,” Tony Goldberg, leader of the study and a professor of epidemiology and pathobiological sciences at the University of Wisconsin-Madison School of Veterinary Medicine (SVM), said in a statement. “Fish are still being exposed.”

So why have die-offs steadily declined? Researchers believe that enough drum have been infected and survived, that they have developed a resistance to being re-infected. This is what they call “herd immunity.” Therefore, if enough individuals are “vaccinated,” less become infected, researchers explained in their study.

However, as new fish are born, the ratio of protected individuals decreases and the virus could spread more readily. This could cause another wave of VHSv-induced deaths.

“This test will continue to be useful to monitor VHSv transmission,” Toohey-Kurth added, “and with further refinements we will be able to better assist our state partners with management of the disease.”

Their study, recently published in the Journal of Clinical Microbiology, provides insight on how VHSv spreads and what measures can be taken to prevent future infections. This research will also benefit stocking efforts.