Department of Microbiology and Immunology

Researchers identify virus causing Atlantic salmon cancerous tumors

salmonProfessor Paul Bowser, left, and Associate Professor James Casey examine a swimbladder tumor taken from a 3-year-old Atlantic salmon in their research lab in the Veterinary Medical Center. Charles Harrington/University Photography

By Roger Segelken
Researchers in Cornell's College of Veterinary Medicine, working with federal fisheries personnel, have made the first identification of a virus believed responsible for cancerous tumors in Atlantic salmon in the New England region.

The name given to the newly characterized virus is salmon swimbladder sarcoma virus (SSSV). It is not seen as a human health hazard.

Researchers used PCR (polymerase chain reaction) cloning and gene-sequencing techniques developed at Cornell to make the first positive identification of the virus, believed responsible for tumors that killed breeding-stock Atlantic salmon in the U.S. Fish and Wildlife Service hatchery in North Attleboro, Mass. The infected fish originally had been collected from the Pleasant River in Maine and were part of a breeding program to restock Atlantic salmon in the Pleasant River.

In the past the disease was found in commercial aquaculture where salmon were grown in off-shore pens. A cancer with similar symptoms was reported in some farm-raised Atlantic salmon in Scotland in 1978, before modern genetic testing methods were available. At the time, Scottish scientists reported seeing "virus-like particles" during microscopic examination of the salmon tumors. To date this disease has not been seen in any commercially raised Atlantic salmon in the United States.

The virus from the North Attleboro hatchery fish was identified at Cornell by James W. Casey, associate professor of veterinary microbiology and immunology and a specialist in molecular virology; Paul R. Bowser, professor of veterinary microbiology and immunology and a specialist in fish pathology; and Sandra L. Quackenbush, a molecular virologist working in Casey's laboratory, now an assistant professor at the University of Kansas. The Cornell scientists worked in cooperation with Larry Lofton and John Coll of the U.S. Fish and Wildlife Service.

"We don't yet know how this virus is actually transmitted from one fish to another," Casey said. "This could occur through so-called vertical transmission in reproductive products, the eggs and sperm of breeding salmon; or perhaps through horizontal transmission, with the virus entering the bodies through the skin or gills, in water they ingest or on food they eat. Our concern is that the virus is in the environment, and the levels of infection need to be determined.

SSSV is not a health hazard to humans, according to Bowser, the Cornell fish pathologist. "These viruses are generally species-specific, meaning that they only infect and cause disease in a single species. There is absolutely no evidence that this virus is a human pathogen. But infection is a real problem for the Atlantic salmon. Although tumors are fatal, it appears that viral infection alone reduces their vitality and reproductive success."

The virus causes a slow-growing cancer that can take two years to produce a noticeable tumor on swimbladders, Bowser notes. By the time the swimbladder tumor is detectable, from a bulging in the side of the fish, the salmon are near death. An earlier sign of swimbladder sarcoma might be blood hemorrhaging from the fins.

The salmon tumors are similar to viral tumors seen on the skin of a freshwater fish, the walleye, Casey said. His laboratory has identified five different retroviruses that are associated with tumors in walleye and perch in lakes in upstate New York. The walleye tumors usually do not kill the fish, but rather appear to disappear spontaneously. Such regression of tumors is of interest to Casey, Bowser and other cancer researchers because an explanation for the phenomenon could lead to better treatment or prevention of human cancers.

SSSV is classified as a retrovirus, Casey said, because it contains hallmark sequence similarities to other members of this group. There are many well-studied retroviruses that cause cancers in chickens, mice, cats and humans, but SSSV is the first retrovirus of salmon that has been identified. "As natural habitats decline and investigators more intensively monitor aquatic animal health, additional tumors like these will be found," Casey said.

After their successful identification of the virus, the Cornell scientists hope to develop a rapid diagnostic test for SSSV. If routinely utilized at the salmon breeding facilities, such a test could help eliminate infected fish before the disease is transmitted to others, the Cornell scientists predict.

In the next step of the SSSV study, Bowser, Casey and Quackenbush plan to investigate virus transmission in a cooperative effort with Rocco Cipriano of the National Fish Health Laboratory of the U. S. Geological Survey. They also plan to examine various wild populations of Atlantic salmon in Maine with Michael Opitz of the Cooperative Extension Service of the University of Maine and fish health specialists from the Lamar Fish Health Unit of the U. S. Fish and Wildlife Service.

"In the long term, this fish disease will be an excellent model to study the process of tumorogenesis and provide information relevant to the prevention of all cancers, whether human or animal," Casey said.

December 17, 1998