Q&A: Fish and Human Health

• Is VHS a health risk to people?
• What are the symptoms of fish infected with VHS?
• Why do some infected fish die and others don't?
• Can we vaccinate fish to protect them from this disease?
• Does VHS threaten commercially harvested fish?
• Why are some trout vulnerable to VHS and others are not?

Is VHS a health risk to people?

No, VHS is not a health risk for people so anglers can continue to enjoy fishing and eating their catch. VHS has never been associated with human illness since first being discovered in European fish decades ago (DHFS, May 2007). Fish can be infected by VHS, but may not show signs of disease. Such fish are safe to eat so long as the fish is properly cooked. However, you should not eat fish you find dead, decomposing, or that appear sick, regardless of cause. Decomposing fish may attract other bacteria harmful to people.

While it is generally safe to handle fish, you should always wash your hands after handling fish especially if the fish is dead or appears diseased. Dead fish and fish with sores may be contaminated with bacteria and it is a good idea to wear protective gloves when handling dead fish.

What are the symptoms of a fish infected with VHS?

Like many fish diseases, the type of symptoms present in a fish change with the severity of the infection. Fish may display few to no symptoms, or as the infection worsens, signs may include bulging eyes, bloated abdomens, inactive or overactive behavior, bleeding in the eyes, skin, gills and at the base of the fins. Because many of these signs look like those caused by other fish diseases, testing is necessary to determine whether a fish is infected with VHS.

Thumbnails link to larger images.

The clinical signs of VHS include hemorrhaging in the muscle tissue and internal organs, pale organs, and bulging eyes.
© Dr Jim Winton (USGS, Seattle WA), Dr Mohamed Faisal (MSU, Lansing MI) and Dr Paul Bowser (Cornell, Ithaca NY)

Why do some VHS infected fish die and others don't?

The reasons for this 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. Additionally, 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 virus 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.

Can we vaccinate 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.

Does VHS threaten commercially harvested fish?

The Great Lakes strain of VHS was detected in Lake Whitefish from the Bay of Green Bay in May 2007 for the first time. 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.

Why are some trout vulnerable to VHS and others are not?

Evidence has shown that certain species of fish are more vulnerable to VHS than others. 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; it only means that VHS has yet to be documented in those fish.

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