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Viral Hemorrhagic Septicemia: Are Our Fish Doomed?

Two separate discoveries announced in 2010 documented the arrival of viral hemorrhagic septicemia virus (VHSV) in Lake Superior.

Cornell University researchers Paul Bowser, Jim Casey and others using a highly sensitive technique referred to as qRT-PCR found VHSV in fish collected from three locations in Lake Superior. VHSV-positive fish were found in eastern Lake Superior near Paradise, Michigan, in central Lake Superior near Skanee, Michigan, and at the westernmost end of the lake in the Duluth-Superior Harbor. The fish were collected in spring 2009 and analyzed later in the year. Identification of VHSV in samples was then confirmed by the USGS Western Fisheries Research Center, Seattle, Washington, using a similar qRT-PCR technique.

Mohamed Faisal, a Michigan State University researcher, also detected VHSV in ciscoes (formerly lake herring) that were collected from the Apostle Islands area of Lake Superior in December 2009. This was the first time VHSV was detected in cisco and the first time the virus was isolated by using tissue culture for a Lake Superior fish. Tissue culture is the USDA APHIS standard for confirming VHSV in fish.

Many anglers and biologists wonder how damaging VHSV might be to Minnesota's fisheries and how likely is it to spread throughout the inland waters of the state.

Background

VHSV is a highly contagious disease that infects numerous species of marine and freshwater fish. The disease caused by VHSV was first described in European freshwater fish in 1931, but the virus wasn’t linked to the disease until 1963. Researchers found the disease in European saltwater fish in 1979. VHSV has since been detected in fish from Japan, North Korea, and both coasts of North America where it has been found primarily in saltwater fish. Beginning as early as 1988, Chinook and coho salmon, and steelhead returning to hatcheries in Washington were found to be carrying the virus.

A large freshwater drum die-off in the Bay of Quinte, Lake Ontario, heralded the arrival of VHSV into the Great Lakes system in May 2005. But the virus was infecting fish in the system at least two years prior to that; upon reexamination, muskies collected and preserved from Lake St. Clair in 2003 were found to be infected with VHSV. It is unclear how VHSV entered the Great Lakes system.

There are four primary strains of the virus. The strain in the Great Lakes appears to most closely match that found in marine and freshwater fish from New Brunswick and Nova Scotia beginning in 2000. Prior to the new millennium, this strain was found only in Pacific Ocean fish. As of January 2009, VHSV has been found in all of the Great Lakes except Lake Superior. It has also infected fish in a few inland locations in New York, Wisconsin, Michigan, and Ohio.

How VHSV spreads

Although our understanding of VHSV is incomplete, we know the virus can spread when infected live 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 the fish's gills or wounds, or if the fish eats an infected fish. The movement of water carrying viable viruses in live wells and bait containers is a risk, but experts have suggested this risk is small considering the dilution factor. Draining water as recommended to prevent the spread of other invasive species reduces the risk of spreading VHSV to near zero.

VHSV tends to fare best in cold water. Mortalities are highest at temperatures from 37–54oF. This virus cannot survive the body heat of warm-blooded animals, so it does not pose a threat to humans. This temperature sensitivity is also why VHSV cannot be spread long distances through the feces of birds that eat infected fish.

Are fish populations doomed?

The short answer is – "Nobody knows for sure." Looking at the history of the disease, where it has infected wild fish around the world over the last 45 years, and what has occurred in the Great Lakes over the last six years, may help to put the threats in perspective.

The primary concern with this disease worldwide and the reason it has gained such notoriety has been 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. When fish are confined and stressed, as they often are in an aquaculture production setting, the disease can be devastating.

Examples of VHSV decimating wild fish populations are more difficult to find. Even though VHSV has been detected in many marine and freshwater species, records of significant wild fish die-offs in Europe as a result of VHSV are notably missing. VHSV reportedly damaged populations of Pacific herring in Prince William Sound, Alaska and Puget Sound, Wash.; however the die-offs were also correlated with poor body condition prior to spawning in the spring. Poor body condition was associated with limited food resources in Prince William Sound. Other VHSV outbreaks have occurred (Pacific herring, Pacific hake, and walleye pollock died in Alaskan waters and large numbers of sardines died off the coast British Columbia), but evidence that these die-offs resulted in significant population level declines is lacking.

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 in significant ways.

Sensational stories about several large mortality events in the Great Lakes traced to VHSV are concerning, but 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.

The researchers suggest that the rapid and visual accumulation of dead fish may have caused an initial over-estimation of the die-off’s effect. Frequently, the size of a fish die-off gets magnified in news reports. Additionally, inappropriately equating VHSV to the Ebola virus (which has an extremely high fatality rate) tends to increase the scare and shock factor. 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. It is also encouraging that repeat mortalities in years following an outbreak are rare and often it is difficult to even find the virus in fish from those areas.

The die-off of muskies in Lake St. Clair created great concern. Muskies appear especially susceptible and it is understandably distressing to see large and trophy size muskies dead and dying. The Michigan Department of Natural Resources reports that several thousand muskies died in Lake St. Clair in 2006. They also indicate that this was a relatively small component of the population (estimated 2-4%). Muskies continue to provide a strong sport fishery in the area, although it is not yet clear if there have been population level effects. No similar musky mortality events have occurred there in subsequent years.

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. VHSV will lurk as a background infection and then manifest when fish are stressed, at which time it could damage fish populations. Therefore, preventing its spread to new waters should be a serious undertaking. It is possible, but certainly not inevitable (as some contend), that the virus will spread through Minnesota's inland waters.

Is the spread of VHSV through Minnesota inevitable?

True, the virus will likely spread beyond the Great Lakes. Experience with zebra mussels demonstrates the likely spread of VHSV through connected waters. The Great Lakes are connected to the Mississippi River drainage through the Chicago Sanitary and Ship Canal and the Illinois River. There is an electrical invasive species barrier in the canal; however, it won’t stop the downstream spread of infected dead and dying fish and viable viruses in the water. It seems logical that the virus will spread throughout the Mississippi River drainage wherever temperatures are suitable. This includes, eventually, moving up the Mississippi River into Minnesota. The spread up the Mississippi, however, will likely be much slower than we saw for the barge-assisted zebra mussel expansion. Fish carrying the disease will have to swim upriver past 17 locks and dams just to enter Minnesota waters. They will have to traverse another 11 locks and dams to get past the Twin Cities.

How VHSV found its way into Lake Superior fish will likely remain a mystery since it was found simultaneously at the eastern end (near Paradise, MI), in the central region just east of the Keweenaw Peninsula (Skanee, MI), and in the western end of the Lake in the St. Louis River (Superior, WI). However, it is not surprising that it found its way into Lake Superior which is connected to the infected waters of the other Great Lakes. While navigational locks are generally poor fish passage devices, it is clear that fish like smelt, lamprey, alewives, carp, and American eels have moved into Lake Superior through the Soo Locks. Therefore, it is likely that fish could move the virus into Lake Superior. In addition, approximately 3 billion gallons of water from the lower lakes arrives in Lake Superior each year as ballast water in lakers (ships moving cargo within the Great Lakes). While it has been suggested that ballast water is not an effective vector for the spread of the virus, ballast water certainly has the potential to move infected small fish and viable virus particles. The Great Lakes shipping industry has designed and adopted ballasting strategies that have helped to minimize this risk.

The question is then – Now that VHSV has been found in fish in the Lake Superior drainage or if it reaches Minnesota waters via the Mississippi River in the future, will it rapidly spread through all the waters of the state?

The Lake Superior watershed comprises less than 7% of Minnesota and barriers on most streams and rivers block upstream fish movement. Therefore, the area potentially accessible to infected fish is much smaller. The virus is most likely to be moved in live fish and in Minnesota we have an enviable track record of not moving fish like ruffe, round and tubenose gobies, three- and fourspined sticklebacks, alewives, sea lamprey, white perch, and others from Lake Superior into inland waters. Zebra mussels, spiny waterfleas, and Eurasian watermilfoil tend to hitchhike on anglers' gear and boats; not so with fish (and their pathogens). Through education, incentives, and regulations it is possible to prevent the movement of infected fish into inland waters.

The Mississippi River watershed comprises a much greater portion of the state (56%). Yet with dams and barriers that prevent fish passage, the watershed area where VHSV might spread may be closer to 30% of the state. Even if fish infected with VHSV move upriver into Minnesota, barriers on connecting channels will keep most of our lakes, rivers, and streams safe. It will take the movement of live fish, fish parts, or infected water overland or over fish barriers to spread VHSV. The state is well positioned to prevent this from happening.

Stopping the spread

There are several aquatic invasive species programs and regulations with a successful history that will help prevent the spread of VHSV.

An example of an effective education program is the Stop Aquatic Hitchhikers campaign. This campaign has reached tens of millions of people in our region with the “Remove, Drain, Dispose of Bait” message through billboards, news releases, highway radio messages, signs at water accesses, stickers, windshield flyers, regulation booklets, watercraft inspectors, lawn banners, and displays at rest areas, stores, and airports. Evaluations show that the advertising blitz makes a difference in boater and angler awareness and behavior and helping prevent the spread of invasive species. The same behaviors will also be effective in preventing the spread of VHSV.

Regulations barring the transport of bait, live fish, and water from potentially VHSV-infested waters such as Lake Superior and the Mississippi River are in place and will be important in preventing an inland leap for VHSV. It is illegal to import baitfish into Minnesota. While most baitfish businesses around the country are changing operations to reduce the risk of spreading VHSV, this Minnesota-specific regulation will help prevent the spread of the virus into the state. Baitfish production in Minnesota is currently operating under beefed-up regulations that will help ensure our Minnesota bait is VHSV-free. Similar regulations are helping to ensure that any fish stocked in Minnesota are VHSV-free. Because of the proactive approach taken in Minnesota to prevent the spread of aquatic invasive species through targeted regulations, the risk of VHSV spread is greatly reduced.

VHSV might spread to inland waters, but it is certainly not inevitable that it will move to all our inland waters. Aside from spreading through connected waters, 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.

VSHV will continue to spread via connected waters, but we can help block its progress by adopting appropriate behaviors. If VHSV does find its way into the waters of our state, don't panic … evidence to-date suggests that VHSV won't doom the fisheries we know and love.

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Keywords: viral hemorrhagic septicemia, VHS, VHSV, VHS virus, fish virus

By Jeff Gunderson


This page last modified on May 04, 2016     © 1996 – 2017 Regents of the University of Minnesota     The University of Minnesota is an equal opportunity educator and employer.
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