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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Buy VHS-negative bait fish and eggs from licensed dealers. © Greg Matthews
Buy VHS-negative bait fish and eggs from licensed dealers.

© Greg Matthews

August 2007

Q&A about VHS

Quick answers to common questions about an emerging fish disease.

Alisa Lopez


What is VHS and where is it from?
Viral hemorrhagic septicemia, known as VHS, is an infectious disease of fish. The viral strain of concern was first diagnosed in Great Lakes fish in 2005, 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 in Wisconsin waters for the first time this spring in fish from the Lake Winnebago System and Lake Michigan. Biologists have sampled fish for signs of the virus in Lake Superior, the Mississippi River and their tributaries.

VHS has been known as a disease of farm-raised rainbow trout in Europe for decades. The Great Lakes strain of VHS is genetically different from strains found in Europe and the Pacific Northwest, and seems to affect a wider range of freshwater species over a broader range of water temperatures.

Is VHS a health risk to people?
No. Anglers can continue to enjoy fishing and eating their catch. VHS has never been associated with human illness. Fish can be infected, carry and shed VHS without showing signs of disease. Such fish are safe to eat as long as the fish are fresh, properly handled and cooked. Never eat fish you find dead, decomposing, or that appear sick, regardless of cause. Decomposing fish host other bacteria harmful to people.

While it is generally safe to handle fish, always wash your hands after handling fish especially if they appear diseased or are dead. Dead fish and fish with visible sores may be contaminated with bacteria and it is a good idea to wear disposable, protective gloves when handling such fish.

Why do fish biologists consider VHS a serious threat to Wisconsin fish?
VHS can spread readily among fish of all ages. It infects a broad range of our native game fish, panfish and bait fish as well as "rough" fish, and it often kills fish. The strain identified 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 considered highly susceptible to disease. This is the first time a virus has affected so many different fish species from so many fish families in the Great Lakes.

How did VHS get into our lakes?
VHS is considered invasive and scientists are not sure how it arrived in Midwestern waters. The virus may have come in with migrating fish from the Atlantic Coast. It may have hitchhiked in ballast water from ships or it may have been brought in with frozen Pacific herring imported for use as bait. A likely way the disease spreads is through moving live fish or water from one body of water 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 in a fish population 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 for up to 14 days in water. Healthy fish can also be infected when they eat diseased fish. Infected fish and water can easily spread the virus if they are released into a new lake or river. That's why emergency rules prohibit anglers, boaters and other water users from moving live fish and water from one body of water to another.

Can birds spread the virus?
We don't know yet whether the VHS strain found in the Great Lakes can be spread by birds, but the European strain cannot be transmitted through the feces of birds that eat infected fish – the virus is inactivated in the gastrointestinal tracts of birds. The European virus does appear to be transmitted on the feathers or feet of birds that are feeding on a pile of infected fish or sitting in water containing the virus.

What are the symptoms of a fish infected with VHS?
As with many fish diseases, symptoms vary with the severity of the infection. Fish may initially display few or no symptoms. As the infection worsens, signs may include bulging eyes, bloated abdomens, inactive or overactive behavior, or bleeding from the eyes, skin, gills and at the base of fins. Because many of these signs resemble those caused by other fish diseases, testing is necessary to determine whether a fish is infected with VHS.

What can provoke a VHS outbreak and will fishing restrictions be greater during these times?
Two important factors influence the severity of a VHS outbreak: water temperature and stress. The European strain of the virus grows best in fish when water temperatures range between 37-54F and most infected fish die when water temperatures are between 37- 41F. We do not yet know the critical 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's 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 biologists will act to protect the fish populations.

Why do some VHS infected fish die and others don't?
The reasons are complex and unpredictable. In general, some species may be naturally more tolerant of VHS infection than others. If a population is already stressed from the factors listed above, more fish will die than if the population was not stressed. If fish are exposed to the virus when water temperatures are rising out of the range that allows the virus to reproduce, the number of virus particles in the fish will be lower. Therefore, the fish's immune system will produce antibodies that capture the virus and prevent it from damaging tissue.

Once a fish has produced VHS antibodies, it will be protected from future infection by the same virus strain for some time. This means that after the first disease outbreak occurs in a lake, the older, surviving fish will be protected. Younger fish will not have antibodies to the virus, so they will likely die at a higher rate when the next disease outbreak occurs.

What is the long-term outlook for VHS in the Great Lakes and state fish populations?
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.

Can we vaccinate baitfish and stocked fish to protect them from this disease?
Although research is being done on vaccines to protect fish from VHS, there are no acceptable methods of vaccinations at this time.

Why are some of our local trout and catfish species considered vulnerable to VHS and others like brook trout and flathead cats are not?
Certain fish species are more vulnerable to VHS infection. The fact that some species are not on the list of susceptible fish species doesn't guarantee that they are not susceptible to the virus, only that VHS hasn't be detected in those fish.

Does VHS threaten commercially caught fish like lake whitefish or chubs?
The Great Lakes strain of VHS was detected for the first time n lake whitefish from the Bay of Green Bay in May 2007. Currently, only a small percentage of fish are affected by the virus and it is too soon to tell what the effects will be on the lake whitefish population.

The Pacific Northwest strain of VHS is distinct from the one found in the Great Lakes. Pacific herring in Prince William Sound, Alaska first experienced disease outbreaks due to VHS in 1989. Since then, the population of Pacific herring has decreased in size and the commercial fishery for herring has closed in those areas.

Who bears the cost of testing for VHS?
Costs of VHS testing, fish collection, sample preparation, the purchase of radio and TV public service announcements, signs at boat landings, and brochures are paid by revenues from fishing license sales.

What are DNR researchers doing to locate infected waters and why can't all waters be checked immediately?
Based on what is known about disease distribution, the vast majority of waters are not considered to be infected. DNR biologists are testing suspicious fish from reported fish kills. A substantial number of inland waters throughout the state already have been tested and the virus has not shown up yet, other than in the Lake Winnebago System.

The testing period is confined to spring and fall because the virus typically does not replicate once water temperatures rise above 60 F. Testing conducted in the summer would therefore be inconclusive.

A long-term surveillance plan has been developed to provide information on the presence of VHS in Wisconsin waters. The Department of Natural Resources will test fish from what are classified as high risk waters. These waters are based on their proximity to the Winnebago system and whether they contain stocked fish originating from the Winnebago system. Since VHS is likely transported via boating activities, the high risk waters have a greater chance of being infected with VHS, based on the observed spread of zebra mussels which are transported by similar modes.

What is being done to prevent the spread of VHS through bait fish from dealers?
Wisconsin's Department of Agriculture, Trade and Consumer Protection regulates farm-raised bait fish more stringently than any other state. Any live fish on the APHIS (Animal and Plant Health Inspection Service) list of susceptible species moving out of Great Lakes states must test negative for VHS before they can be moved. All fish and eggs, wild harvested or farm-raised, and entering Wisconsin from states where VHS has been found, must test negative for VHS. If for any reason, inspectors suspect that farm-raised fish have been exposed to VHS, the farm will be quarantined and tested.

The Department of Natural Resources also now requires that anyone who harvests and sells minnows from the wild carry a free bait harvest permit and keep records of their bait collection and sale. This information will help fisheries officials trace new outbreaks of VHS.

How do I recognize that I am buying bait from a registered licensed bait dealer and why is that important?
Licensed bait dealers must keep their licenses available for inspectors at all times. Registered licensed bait dealers are required to keep records of all transactions in producing, buying and selling bait. This information can be an important tool for fisheries officials tracking down new cases of VHS.

Should I be as concerned about live bait like leeches, worms, grubs and hellgrammites as I am about bait fish?
No. Unlike bait fish, leeches, worms, grubs and hellgrammites cannot be infected with VHS. Therefore, there is less of a chance that VHS will be spread when using live bait other than bait fish. However, the bait listed above could carry and transmit the virus if it has been in contact with infected waters or fish. As a result, it's necessary that you empty your live well or bucket, and not move live bait to any new waters to help prevent the spread of VHS.

Are there any rules related to VHS that I need to be aware of as an angler or boater?
Yes. The state Natural Resources Board has adopted emergency rules that prohibit anglers, boaters and other recreational users from moving water, live fish, including bait minnows, from the Lake Winnebago watershed, Great Lakes, Mississippi River, and those waters' tributaries up to the first barrier impassable by fish. The rules also require that people fishing in those waters use minnows purchased only from Wisconsin licensed dealers, or, if harvesting their own minnows, that the bait is used only on the water in which it was caught. Dead fish, eggs, crayfish and frogs may only be used on the waters where they were captured.

What can I do to help prevent the spread of VHS?
The DNR is asking the public to take precautions similar to those used in stopping the spread of invasive species:

  • Put your catch on ice and do not move live fish (including unused bait minnows) away from the landing or shore.

  • Drain all water from bilges, bait buckets, live wells and other containers when leaving the landing or shore.

  • Use live minnows purchased only from registered bait dealers in Wisconsin or catch them yourself in the same water you fish.

  • Before launching and reloading watercraft for the day, inspect and clean all watercraft for visible plants and animals. This advice applies equally to sailors, jet-skiers and rowers as for anglers.

How do I disinfect my boat and equipment?
The Department of Natural Resources recommends that if you are spending time in VHS infected waters, disinfect personal protective gear, small equipment and the inside and outside of larger equipment such as boats, trailers, live wells, bilges and pumps before entering another body of water. A solution of one-third cup chlorine bleach per five gallons of water can be used to properly disinfect these items. Follow these steps to help stop the spread of VHS:

  • Remove all aquatic plants, animals and mud from your boat, trailer and equipment.

  • Drain all water from your motor, live well, bilge, etc.

  • Dispose of unwanted bait in the trash.

  • Scrub or spray equipment with the disinfection solution and leave it wet for 10 minutes of contact time.

  • Rinse all treated surfaces with clean water, or if you are going to a different lake on the same day, you can rinse the boat with water from the new lake before you launch. This is best done over gravel and away from the lake. Disinfection can also be done at home. It does not have to be done at the boat landing unless you plan to go to another lake immediately.

As a general practice, organize your schedule so that your time spent in infested waters is always your last stop before disinfecting your rig/gear and heading home.

Scrub down and disinfect your boat and trailer to stop disease spread. © Greg Matthews
Scrub down and disinfect your boat and trailer to stop disease spread.

© Greg Matthews

What should I stock in my boat or vehicle to reduce the risk of VHS?
To properly disinfect your boat and equipment, it's recommended that you carry bleach, clean water if none is available on site, a sponge for scrubbing, bucket, gloves, eye protection and rain gear. DNR staff has also found it helpful to use a backpack sprayer filled with the prepared disinfection solution, which might be a safer and easier method than transporting and mixing disinfectants on site. don't use sprayers formerly used to apply pesticides for this purpose. As a reminder, disinfection should be done away from the lake and is best done on gravel to prevent the solution from running into the lake, river or stream.

What should I do if I see a fish kill or diseased fish?

  • Note the waterbody, date, fish species, and approximate number of dead/dying fish.

  • If you caught a suspicious looking fish, place the fish in a plastic bag and then in a cooler on ice.

  • Contact either your local fisheries biologist or call the DNR TIP line at 1-800-TIP-WDNR (1-800-847-9367).

  • Please call first and please don't bring suspect fish into a DNR office or hatchery unless instructed to do so. DNR hatcheries operate under strict rules to prevent any disease from entering the hatchery.

How can I dispose of dead fish?
If you are a landowner with a few dead fish, the best action is to bury the fish under at least eight inches of dirt away from the water, or put the fish in a heavy duty plastic bag and dispose of it with trash that is headed to a landfill. Your municipality may have other requirements, so check with your waste hauler for specific instructions.

In the case of a lot of dead fish, locally available disposal options should be investigated and a plan developed. Dead infected fish may carry diseases that pose a risk to live fish, may host bacteria, attract vermin and promote other disease conditions. Land filling at a licensed municipal solid waste landfill may be the preferred option in many cases, in part because it is expeditious and sanitary. For further information, to find a contact person to ask about solid waste disposal regulations in your county, call (608) 266-2111 or visit the Waste Management Program.

Where can I get updates on where VHS has been found?
You can visit VHS and You: Keeping Wisconsin Waters Healthy, contact DNR Service Centers or call the toll-free call center at 1-800-282-0367 or 1-608-266-2621 for information and updates.


Alisa Lopez is a communications specialist working with DNR's fisheries management program.