NFHS responds to an emerging conservation challenge.

A fish disease known as Viral Hemorrhagic Septicemia (VHS) is an emerging issue with important implications for people, commerce, recreation, and conservation. As is frequently the case, the National Fish Hatchery System's (NFHS) aquatic animal health experts are among the Nation's first responders to provide effective surveillance, diagnostic, and management capabilities when disease outbreaks occur.

The disease made big news in the summer of 2006 when it was implicated as the cause of massive fish kills in the Great Lakes. Scores of dead fish --sport fish and those with commercial value--fouled the lake shores in Ohio, Michigan, New York, and Ontario. It is caused by an aquatic rhabdovirus, of which four strains have been identified. Three strains occur mainly in Europe and Japan, while the fourth has been found only in fish in North America, Japan, and Korea. First reported in the United States in 1988 in the Pacific Northwest, the virus was subsequently found in both wild and hatchery-raised salmon, Pacific herring, and Pacific cod populations off the coast of Alaska, Canada, and Washington. A sub-type of the North American virus has also been isolated from Atlantic herring and Greenland halibut in the Atlantic Ocean.

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The ability of the VHS virus to cause disease varies by strain and by affected fish species. Our understanding of the virus changed drastically in 2005. Freshwater dram, previously thought not to be susceptible to the virus, suffered a massive die-off on Canada's side of the St Lawrence River. Later that year, the virus was isolated from a large muskellunge die-off in Lake Ontario. In December 2005, a VHS outbreak occurred on the U.S. side of the Great Lakes when round goby, a non-native fish species, died en masse. Scientists, suspicious of VHS, then analyzed stored tissue samples of muskellunge that perished in 2003 in Lake St. Clair and found that they died from the disease, which marks the first known VHS outbreak in Great Lakes waters. Fourteen of the fish species that died in the Great Lakes in 2006 were previously not known to be susceptible to the VHS virus, and it appears the North American sub-strain of the virus is the cause.

How VHS arrived in the Great Lakes is not known, but it appears to be a recent arrival. It may have come from ballast water purged by shipping vessels, or it could have been carried by fish species that migrate to and from the ocean. Birds may also play a role in spreading the virus, as could anglers, recreational boaters, and even biologists if they fail to properly disinfect boats and gear moved between waters. Another potential vector is the movement of commercially caught baitfish. Emerald shiner, the most popular baitfish harvested from the Great Lakes, are susceptible to the VHS virus.

The manner in which massive numbers of multiple fish species in the Great Lakes have died from a virus formerly thought to affect solely marine species speaks to how populations of animals react in their first encounter with a new disease-causing pathogen. It also indicates that the virus has mutated in some manner. We do not know whether this mutation occurred before or after the virus was introduced into the Great Lakes watershed.

Currently, at least 40 freshwater and marine species are susceptible to the North American strain of the VHS virus. They include salmon, trout, pike, muskellunge, black basses, perch, walleye, drum, herring, cod, smelt, flatfishes, and others. Preliminary studies looking at Chinook salmon, steelhead, and lake trout in the Great Lakes show them to be susceptible to this new sub-strain of the virus, demonstrating significant levels of mortality. While the impact to sport fish has been graphically evident in many large-scale fish kills, the potential impacts on threatened and endangered species is unknown. But the broad host range of the virus, coupled with the large kills, suggests strong actions are needed to prevent this virus from moving into other populations.

Fish that survive VHS infections can be lifelong carriers, capable of spreading the virus. Inoculating fish in the wild is impossible; control methods for VHS currently rely on fish health surveillance programs and measures such as eradication and fallowing culture facilities (removing fish and water, then letting the facilities dry for a time). The virus could move to new' species and new waters outside the Great Lakes drainages. The Division of the NFHS employs Hazard Analysis and Critical Control Points planning to prevent the spread of the virus.

The issue of VHS is a serious one, and with experience as a guide, the NFHS will address it as it did the emergence of Largemouth Bass Virus and Spring Viremia of Carp Virus. We are already working on another front of animal health with the emergence of Batracochytrium dendrobatidis, commonly called chytrid fungus. This fungus has caused worldwide declines and extinctions of amphibian species. In the U.S., the Chiricahua leopard frog, mountain yellow-legged frog, California red-legged frog, and Wyoming toad have each had populations devastated by the fungus.

The Division of the NFHS works with its partners, the Department of the Interior, and the Congress to find creative ways to deal with these emerging conservation issues in a rapid, efficient, and effective manner.

Dr. Robert Bakal, DVM, is the Aquatic Animal Health Coordinator, Division of the National Fish Hatchery System. He can be reached at [e-mail]