OREGON STATE UNIVERSITY

Risk Assessments May Help Control Spread of Whirling Disease

12/01/2008

CORVALLIS, Ore. – Improved techniques have been developed by microbiologists from Oregon State University to assess the threat from whirling disease, a deadly parasitic infection of rainbow trout and other salmonids, and new reports are outlining some of the most likely areas to which it could spread.

In the Willamette Valley of Oregon, the Clackamas River and Santiam River basins appear to be at higher risk for the parasite to cause problems. In Alaska, Ship Creek and some other areas of south-central Alaska are more at risk, researchers have concluded.

In other parts of the United States, particularly Colorado and Montana, fish populations have already been decimated by this invasive disease that first came to the U.S. in 1958. Rainbow trout suffered localized extinction in some areas, and recovery efforts have been expensive and slow to show effects.

Because of the severity of the concern, scientists in recent years have learned much more about using molecular DNA techniques to track the disease, understand the exact conditions needed for it to thrive such as water temperatures or overlap of hosts, and even outline which of the various sub-lineages of “Tubifex tubifex” worms are the most conducive to allowing the parasite to complete its life cycle.

There are no effective ways to control the disease once it’s introduced. This makes it important, researchers say, to identify the most vulnerable river systems and take all the necessary steps, including public education efforts, to prevent it from becoming established.

“Some river systems in Colorado and Montana have lost 90 percent of their rainbow trout, and because of that fishery managers take this issue very seriously,” said Jerri Bartholomew, an OSU associate professor of microbiology.

“We’ve had some success in preventing its spread by fairly stringent regulations on movement of fish by humans,” she said. “And now we’re learning a lot more about which areas are most at risk, and where we need to focus our efforts.”

Whirling disease is caused by the parasite Myxobolus cerebralis, which infects juvenile fish and causes them to “whirl” in an awkward pattern instead of swimming normally. This makes them more vulnerable to starvation and predation, and can cause high levels of mortality. The parasite spends part of its life cycle in a worm, Tubifex tubifex, that’s quite hardy and common in trout streams. Whirling disease is now found in 26 countries and 25 U.S. states.

In Oregon, the only areas where the disease is already endemic are in the northeastern part of the state, including the Imnaha and Wallowa River. Researchers are concerned, however, that it could become more prevalent in the Deschutes and Willamette River basins, and have studied those areas intensively in recent years.

Another study was also just published in the Journal of Aquatic Animal Health, which confirms that conditions are ripe for this disease in south-central Alaska, where the parasite has been detected and conditions are appropriate for its establishment.

The primary known routes by which the parasite in introduced, both in Alaska and Oregon, include movement of infected fish by humans, natural dispersal via stray salmonids and some migratory birds, and recreational activities. Human movement of fish appears to pose the greatest risk, researchers concluded, but it is also the route of potential spread we have the most control over.

Several areas of the Willamette River basin were recently studied, all of which have high concentrations of susceptible fish hosts, high angler traffic, hatcheries that rear susceptible fish species, private fish ponds and organic loads that increase habitat for the Tubifex tubifex worms.

The studies concluded that shipments of infected fish have already been made within the Willamette River basin, and noted that historically, many fish pathogens and non-native species introductions have been a result of human movement of organisms.

Researchers also found that the areas facing the highest risks were the Clackamas and Santiam River sub-basins. The Clackamas River has already experienced an introduction of the parasite, has the largest concentration of hatcheries, a popular sport fishery and is the closest major tributary to the large bird populations on the Columbia River estuary. The Santiam River basin has a popular sport fishery, received the highest number of stray fish in the whole Willamette River basin, has the second largest concentration of hatcheries, and has higher levels of the particular lineage of the Tubifex tubifex worms most likely to support the parasite.

“The risk of establishment of M. cerebralis in certain locations in the Willamette River Basin is high,” the researchers concluded in one study to be published soon. “If introduced, conditions are appropriate for the parasite life cycle to proliferate.”

The researchers recommended that rules and regulations controlling the transfer of live salmonids should be strictly enforced and articulated more clearly to the general public, particularly anglers and other recreational users.

Other steps to consider, they said, might include identification and removal of stray adult salmon, and prohibiting use of fish heads as bait in south-central Alaska.