2/17/99
MEDIA CONTACT:
Kristin Weidenbach, (650) 723-0272 or (650) 723 6911, [email protected]
FOR COMMENT: Peter Parham, PhD, [email protected], fax (650) 723-8464
GRAPHICS: For a graphic of whirling disease affected rainbow trout, please contact Walter Hangad (650) 723 7897 or (650) 723 6911, [email protected]
FOR IMMEDIATE RELEASE
Study to target whirling disease and its devastation of trout populations
STANFORD -- Researchers at Stanford University and the University of California, Davis, have received funds to study whirling disease, a parasite-borne disease that is devastating native trout populations in the Northeast and Pacific Northwest. Young rainbow trout are particularly susceptible to the disease that causes them to swim in erratic circles, known as tail-chasing behavior. Debilitated fish have difficulty feeding and eventually starve or succumb to predators before they are old enough to reproduce.
"The problem is extensive and it's getting worse," said Irving Weissman, MD, Stanford professor of cancer biology, pathology and developmental biology and a member of the Whirling Disease Foundation's scientific advisory board. Weissman is also an avid fly-fisherman. "It has utterly devastated the fishing in the Madison River," he said, referring to a river in Montana. "The rainbow trout used to be several thousand per mile, and they are now down to less than fifty per mile."
The depletion of native trout populations is raising alarm among environmentalists and fly-fishing enthusiasts alike. According to the Whirling Disease Foundation, the parasite is not transmissible to humans, but researchers hope to figure out why rainbow trout, cutthroat trout and chinook salmon fall victim to the disease while close relatives such as coho salmon and brown trout become infected with the parasite but rarely show any signs of clinical disease.
Peter Parham, PhD, Stanford professor of structural biology and microbiology and immunology, and his collaborator, Ronald Hedrick, professor of veterinary medicine and epidemiology at UC Davis, have been awarded $75,000 each from the Whirling Disease Foundation for their collaborative studies.
"What we can do at Stanford is look at the genetics to see if there's a resistant allele," said Benny Shum, research assistant in Parham's lab and lead investigator of the whirling disease project. "We want to understand the diversity of these fish and see if some of them have genetic resistance to the disease."
Whirling disease is caused by a parasite (Myxobolus cerebralis) that invades young fish through the skin and then rapidly multiplies within the head and spinal cartilage. The ensuing pressure on nerves in the brainstem and spinal cord causes the fish to adopt the characteristic whirling that lends the disease its name. The parasite, a European native, was introduced into North American waters in the late 1950s and has since spread to 22 states.
When a diseased fish dies, thousands of parasite spores are released into the water. The spores are highly resilient and can survive for up to 30 years in an aquatic environment. In the water, the spores are ingested by the tubifex worm, the alternate host of the parasite. Inside the worm, the spores hatch into the parasitic form that can once again infect young trout. Fish can also become infected by eating other diseased fish.
Parham and Shum believe that genes in the fishes' immune system may be the key to why some fish are susceptible to the disease while others remain resistant. They are focusing on cell surface molecules encoded by a family of genes belonging to the major histocompatibility complex (MHC). These molecules bind foreign antigens such as small fragments of viruses, parasites or bacteria, and display them on the outside of the cell - alerting other immune system cells that the host cell has been invaded by a foreign entity. The MHC molecules are highly variable so that they can bind a plethora of unwelcome cellular guests.
In humans, different varieties of these MHC molecules are associated with resistance to certain infectious diseases and Parham and Shum suspect that the same may be true in fish. They plan to study the MHC genes of a range of fish in an effort to correlate immune system genes with symptoms of whirling disease. Fish to be investigated will include those that have been experimentally infected with known doses of the whirling disease parasite, natural populations of disease-resistant fish from different rivers and a random sample of fish from hatcheries.
Parham and Shum hope to identify genes or genetic combinations that are associated with inhibition or elimination of the parasite in disease-resistant fish. Breeding programs will benefit from increased knowledge of genetic diversity in the fish stock, and this knowledge will be critical if overwhelming parasitic infestation necessitates re-introducing fish into barren waters. Shum acknowledged that repopulation of lakes and rivers is a controversial issue. "But if we get to a stage where we have to repopulate, then we'd want to know the best fish to re-stock with," he said.
Weissman believes that a simple breeding program could be instituted if a resistance factor linked to the MHC was found. Wild trout could be bred with trout that have the resistant gene or genes and be reared in a parasite-free environment. The resistant fish could then be released back into their native regions. "Peter Parham is the world's leading researcher in the [MHC] field," said Weissman. "This approach is our one best hope."
Hedrick's lab at UC Davis will collect the samples for the joint project and conduct a pathology study to examine the interactions between the parasite and different fish cells.
The Whirling Disease Foundation is a non-profit organization based in Bozeman, Mont. The Foundation's fifth annual symposium on the disease will be held in Missoula, Mont., February 18-20, 1999.
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