Q & A: About VHS

• What is VHS and where is it from?
• How did VHS get into our lakes?
• How does VHS spread to other fish and to new lakes?
• Why do fish biologists consider VHS a serious threat to Wisconsin fish?
• What is the long-term outlook for VHS?
• What are the chances we can stop this disease?
• Can VHS be spread by birds?
• What can provoke a VHS outbreak and will fishing restrictions be greater during these times?

What is VHS and where is it from?

Viral Hemorrhagic Septicemia, known as VHS, is an infectious disease of fish that was diagnosed for the first time ever in 2005 in fish in the Great Lakes, and was confirmed as the cause of fish kills in lakes Huron, St. Clair, Erie, and Ontario and the St. Lawrence River in 2005 and 2006. VHS was detected for the first time in 2007 in fish from Wisconsin waters of the Lake Winnebago System and Lake Michigan. Fish biologists believe the virus may soon be in fish from Lake Superior and the Mississippi River and their tributaries if it's not already there.

Historically, VHS was known as a very serious disease of farm raised rainbow trout in Europe. The Great Lakes strain of VHS is genetically different than the strains from Europe and the Pacific Northwest, in that the Great Lakes strain seems to affect a wider range of freshwater species over a broader range of water temperatures.

How did VHS get into our lakes?

VHS virus is considered an invasive species (not native to the Great Lakes), but scientists are not sure how the virus arrived. The virus may have come in with migrating fish from the Atlantic Coast. It may have hitch-hiked in ballast water from ships or it may have been brought in with frozen Pacific herring imported for use as bait. Fish may also have carried the virus to Lake Superior and ballast discharged from ships may have moved the virus to port cities there. A likely way the disease is spread is through moving live fish or water from one water body to another. The disease has been found in three inland lakes, one each in New York, Michigan and Wisconsin, and could have hitchhiked in a live well, bilge water, on a boat or in minnows or other live fish.

How does VHS spread to fish and to new lakes?

Infected fish shed the virus into a lake or river through their urine and reproductive fluids. The VHS virus is absorbed into the gills of healthy fish and can remain infective up to 14 days in water. Healthy fish can also be infected when they eat an infected fish. Infected fish and water can easily spread the virus if they are released into a new water body. That's why rules prohibit anglers, boaters and other water users from moving live fish and water from one waterbody to another.

Why do fish biologists consider VHS a serious threat to Wisconsin fish?

Fish biologists consider the virus a serious threat to Wisconsin fish for several reasons: it can spread easily between fish of all ages, it affects a broad range of our native game fish, panfish and bait fish as well as "rough" fish, and it often kills fish. The strain that has shown up in the Great Lakes is new and fish here have had no exposure to the virus, meaning their immune systems have no defense and are "highly susceptible". This is the first time a virus has affected so many different fish species from so many fish families in the Great Lakes.

What is the long-term outlook for VHS?

Fish that survive the infection will develop antibodies to the virus which will protect the fish against new VHS virus infections for some time. However, the concentration of antibodies in the fish will drop over time and the fish may start shedding the virus again, creating a cycle of fish kills that occurs on a regular basis. Nonetheless, experiences from other states indicate that fisheries can and have bounced back.

What are the chances we can stop this disease?

We stand a good chance of slowing the spread of VHS, and we must focus efforts on those pathways that present the greatest risk because they involve the virus at high enough concentrations to infect fish. While a little water left in a bilge, on fishing equipment or carried by a bird all carry a very low risk of transmission, moving infected fish or large amounts of water that contain the virus to new locations are high risk activities. That's why DNR's rules require draining all water from boats and do not allow live fish to leave boat landings. In the Great Lakes, efforts need to be directed to the movement of large amounts of ballast water by cargo ships, especially in cases where ballast water is taken on board when active VHS outbreaks are occurring.

Can VHS be spread by birds?

VHS cannot be transmitted through the feces of birds that eat infected fish. The virus is inactivated in the birds' gastrointestinal track and does not survive the birds' high internal body temperature. The European strain of VHS can survive on the feathers or feet of birds feeding on a pile of infected fish or sitting in water containing the virus, so theoretically, a bird could move the virus to nearby waters but practically, it would take repeated trips of birds from an infected lake to another waterbody to transfer enough virus to create the concentrations of the virus needed in the water to infect fish.

What can provoke a VHS outbreak and will fishing restrictions be greater during these times?

There are two important factors that can influence the severity of a VHS outbreak: water temperature and stress. The European strain of the virus grows best in fish when water temperatures are 37°-54°F and most infected fish will die when water temperatures are between 37°-41°F. We do not yet know the temperature ranges for the Great Lakes strain of VHS. Freshwater drum and walleye have died when water temperature ranged from 66°-70°F.

Any stressors, including poor water quality or lack of food, release the stress hormone, cortisol, which suppresses the fish immune system. Additionally, other hormones related to spawning can also suppress the immune system. This may be why so many of the fish kills in the Great Lakes have occurred just before, during, or right after the spawning period. If VHS is detected in a particular waterbody and a fish population appears to be in jeopardy, fisheries management actions may be taken as appropriate to protect the fish populations.

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