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VHSv-Free, So Far

Photo by Chris J. Benson

Viral hemorrhagic septicemia virus (VHSv) has not been found in Lake Superior or in Minnesota waters as of March 2009. However, the highly contagious fish disease has been infecting a variety of species in all other Great Lakes and in a few inland locations in other Great Lakes states.

The virus can spread when infected fish, fish parts, or water with sufficient virus concentrations are moved to new locations. Infected fish shed the virus into the water with urine and ovarian fluids. The virus can enter a fish through its gills or wounds, or if the fish eats an infected fish. Moving water that carries viable viruses in live wells and bait containers is a risk, but experts have suggested this risk is small considering the dilution factor. This virus cannot survive the body heat of warm-blooded animals, so it does not pose a threat to humans and cannot spread long distances through the feces of birds.

VHSv tends to fare best in cold water. Fish mortalities are highest at temperatures from 37–54º F (3–12º C). This disease has gained notoriety because of its destructive impacts on aquaculture. Protecting the U.S. aquaculture industry has been a motivating force behind the USDA Animal Plant Health Inspection Service’s response to the disease outbreak in the Great Lakes.

VHSv Symptoms

It is impossible to determine for certain if fish have VHSv by merely looking at them. Moreover, an infected fish might not exhibit all symptoms. That being said, VHSv causes:

  • Bulging eyes
  • Pale gills
  • Darkened color
  • Distended (fluid-filled) belly
  • Bleeding around eyes, base of fins, sides, and head
  • "Corkscrew" swimming behavior
  • Death

Report suspicious fish kills to the Minnesota Department of Natural Resource's State Duty Office, (651-649-5451 or 1-800-422-0798).

Except possibly in Prince William and Puget Sounds where other factors likely played a role, VHSv does not seem to have negatively affected wild fish populations.

Sensational stories about several large mortality events in the Great Lakes traced to VHSv warrant discussion. The largest die-offs have been in populations of freshwater drum, round gobies, gizzard shad, and muskies. Researchers with the Ontario Ministry of Natural Resources examined the impact of the first large die-off of freshwater drum in Lake Ontario in 2005. Their conclusion:

The abundance of drum age-1 yr and older did not decline after the die-off. Either the number of drum that died was insignificant relative to the total population or deaths caused by the disease replaced deaths that would have occurred due to other causes. Paradoxically, there was a significant increase in young-of-year drum abundance after the disease outbreak.

Many of the gamefish and commercially harvested fish species that have been identified as susceptible species in the Great Lakes have not experienced a significant die-off; they have mainly been found to carry the disease, not succumb to it.

The 2006 die-off of muskies in Lake St. Clair created great concern. Apparently, muskies are especially susceptible to the virus and it is understandably distressing to see large and trophy size muskies dead and dying. The Michigan Department of Natural Resources reports indicate that this was a relatively small component of the population (estimated 2-4 percent). Muskies continue to provide a strong sport fishery in the area.

VHSv has undergone changes as it entered the Great Lakes and infected nearly 30 species of freshwater fish. Since the virus will likely continue to mutate and could become more virile in some wild fish populations, it would be foolhardy to dismiss its lethal potential. Furthermore, we should do what is needed to prevent its spread.

It is possible, but certainly not inevitable (as some contend), that the virus will spread through Minnesota's inland waters.

Regulations barring the transport of bait, live fish, and water from potentially VHSv-infested waters are in place and will be important in preventing an inland leap for VHSv. It is illegal to import baitfish into Minnesota and baitfish production in Minnesota is currently operating under beefed-up regulations that will help ensure bait is VHSv-free. Similarly, regulations are helping to ensure that any fish stocked in Minnesota are VHSv-free.

VHSv can enter Minnesota water via connected waterways (Lake Superior and the Mississippi River), but because of barriers to fish passage, most of Minnesota's 10,000 lakes will be protected. The likeliest routes that VHSv will take into Minnesota are through the illegal movement of baitfish by anglers and tournament fishermen, or through illegal stocking activities. Educational programs are helping reduce these risks as well.

By Jeff Gunderson
April 2009

Return to April 2009 Seiche

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