Questions and Answers on Viral Hemorrhagic Septicemia (VHS)

Important Questions and Answers on Viral Hemorrhagic Septicemia (VHS) in Great LakesFish.

During the past two years, the fish kills on Great Lakes have involved thousands of fish infected by the Viral Hemorrhagic Septicemia Virus (VHSV); this virus was not previously known to exist in the freshwater environment of North America.Here is some information on VHS compiled by NY Sea Grant.

What is VHS? What is the significance of the recent discovery of VHS in the Great Lakes?
Viral Hemorrhagic Septicemia is an infectious viral disease of fish that has been found in fish from the AtlanticCoast of Europe and Atlantic and PacificCoasts of North America. Historically, VHS was known as a very serious disease of freshwater-reared rainbow trout in Europe. At least four different genetic strains or forms of the virus are known to exist. The North American marine strain has a relatively low infection rate compared to that of the European freshwater strain.Until 2005, VHS was only found in the marine environment in North America. Several fish kills in the Lower Great Lakes since 2005 have been associated with VHS. To date, VHS has been confirmed from wild fish in the Bay of Quinte Lake Ontario, Lake St. Clair, Lake Erie and the St. Lawrence River. Scientists believe that this appearance may represent an invasion of the freshwater strain in North America.

What North American freshwater fish species are affected?
In the Great Lakes, VHS has been found in smallmouth bass, yellow perch, crappie, muskellunge, northern pike, bluegill, walleye, round gobies, sheepshead, and some sucker species. Scientists are concerned; however, that VHS could also strike native salmonids such as trout, salmon and whitefish in the wild, and salmonids in hatcheries and net pen operations.

How did VHS get here?
It is unclear how the virus spread to the Great Lakes; it is possible the marine virus may have been introduced to the Great Lakes some time ago and it simply evolved to live in freshwater.The VHS virus is a strain that undergoes rapid mutations (spontaneous genetic changes) and may have adapted to freshwater environments in North America. Recently, VHS was found in stored fish samples that were collected in the Great Lakes during 2003, suggesting the virus has been present in the Great Lakes for some time.

How does VHS spread?
It is unclear exactly how the disease is spread but it appears that the virus can be shed by infected fish into the water through metabolic waste materials, particularly by fish that survive the disease and become carriers. It also appears that carrier fish or offspring of carriers become more resistant to the disease. The virus can infect fish of all ages. It may enter a host fish through the gills or food or contact with some contaminated object. It does appear that stressed fish more vulnerable to viral infection. Typical fish stressors include sudden water temperature changes, crowded hatchery conditions and, spawning activity.The timing of the recent fish die-off in the Great Lakes coincided with the spawning by some of the fish species, such as muskellunge.

What does it do to fish? What are the symptoms of a fish with VHSV infection?Like many fish diseases, the type of symptoms present in a fish change with the severity of the infection.At low infection intensity fish may display few to no symptoms as is the case in most wild disease outbreaks.Hatchery or pen-reared fish are much more susceptible because of the confined conditions. As the infection severity increases, fish become darker and the eyes bulge with some bleeding around the eye and base of the fins. The gills are usually quite pale with some pin point bleeding.Mortalities appear at this point because hemorrhaging reduces the oxygen carrying ability of the blood. Dark red patches may appear on the front and sides of the head.
If the fish is opened up, bleeding on the surfaces of the intestine, liver, swim bladder can be seen. Fluid also builds up in the body cavity giving the fish a swollen belly. Later, if infection increases, the body continues to darken and the eyes really stick out of the head. At this point, the gills look gray or even white and the fish may swim in a corkscrew pattern. Most fish kills from VHS occur in water temperatures from 40 to 60 F (3-12 C) and few occur at temperatures above 62 F (15 C).NOTE:The detection of a VHS infection can only be made from sophisticated laboratory testing.A diagnosis cannot be made based solely on the observation of visible signs because many different diseases of fish have very similar signs of disease.

What is the long-term outlook for VHVS in the Great Lakes?
Diseases like VHS run their course just as they do in human populations. At first mortalities may appear to be large, but many biologists believe that most fish can survive the disease if they are not otherwise stressed because mortalities generally occur in weaker, stressed fish. The remainder will build up a natural immunity to the virus and the numbers of fish killed by the virus will decline.

Is it a health risk to people?
There is no apparent health risk for people contracting VHS.Because it takes a long time to identify the causes of fish kills in lab studies, people should be cautioned against handling or eating any fish that does not act or appear to be healthy because of the risk of contracting avian botulism a bacterial disease that does pose a human health threat.

What is being done to control it or prevent it?
One of the best management options is containment.Efforts should be made to eliminate or reduce the potential spread of the virus from the locations where it is known to exist.Actions such as not moving fish from the endemic area to areas outside the Great Lakes may be important.Many of the same preventative actions that are taken to reduce the spread of invasive species may also be appropriate.These include cleaning of boats before moving them between different bodies of water; cleaning, draining and drying of live wells and not moving bait minnows or other live bait from an endemic area to a non-infected area.To be fully effective, these containment efforts will need to be practiced by all users of the aquatic resource.

In a hatchery, the best means of controlling the disease is to prevent the contact of the virus and fish. This can be done by hatchery disinfection, egg treatment with anti-viral agents, and using ultra-violet light treatment of hatchery water. It is important to stock disease free fish and to monitor freshwater populations for signs of further spread.Information on diseased wild fish is difficult to obtain because they often die undetected and fish can decompose rapidly making disease diagnoses very difficult.New research is being developed that would allow more rapid detection of the disease.

What should I do if I see a fish kill?
If you observe a fish kill on the Great Lakes, please contact the Wisconsin Department of Natural Resources at (608) 266-8782 or your local DNR office. If you see fish with any of the outward signs of VHS as described above please mention this as well.This will help biologists keep track of where the disease may be appearing.

Source