Atlantic Salmon Dying Of Cancer-Causing Virus
By Environmental News Network staff
(ENN) -- A virus believed responsible for cancerous tumors in Atlantic salmon has been identified by a team of federal and university researchers.
The virus, salmon swimbladder sarcoma, is not believed to be a human health hazard but could be devastating for Atlantic salmon populations, according to researchers at Cornell University College of Veterinary Medicine.
The virus is believed responsible for tumors that killed breeding-stock Atlantic salmon in the U.S. Fish and Wildlife Service hatchery in North Attleboro, Mass. 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.
"We don't yet know how this virus is actually transmitted from one fish to another," said James W. Casey, a Cornell professor of veterinary microbiology and immunology. "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."
The virus is not a health hazard to humans, said to Paul R. Bowser, the Cornell fish pathologist. "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 swim-bladders, said Bowser. 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.
Salmon swimbladder sarcoma is classified as a retrovirus, Casey says, 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 salmon swimbladder sarcoma 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," said Casey.
After their successful identification of the virus, the Cornell scientists hope to develop a rapid diagnostic test for the virus. 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 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," said Casey.
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