Thursday, June 07, 2007

Whirling disease parasite in Rainbow trout at Alaska

The first evidence of the whirling disease parasite's presence in the state of Alaska was recently discovered. Using a very sensitive molecular test, DNA of the parasite _Myxobolus cerebralis_ was detected in rainbow trout from Elmendorf State Fish Hatchery in Anchorage. The discovery occurred during a risk assessment, designed by researchers at Oregon State University, to determine the risk of the parasite's introduction into the state. There are no previous reports of _M. cerebralis_ in Alaska.During 2005 and 2006, the risk assessment research team from Oregon State University collected 180 rainbow trout from Elmendorf and Fort Richardson state fish hatcheries. These were tested using quantitative polymerase chain reaction (qPCR), a very sensitive test, to detect DNA of the whirling disease parasite. A total of 14 fish from Elmendorf tested positive for the parasite's DNA. A nested PCR assay was used to confirm the detection. To rule out false positive results, the 18S rRNA gene was sequenced and compared with laboratory isolates from other locations. The qPCR test estimates the amount of DNA present in a sample and suggests that the parasitic infection among Elmendorf State Fish Hatchery rainbow trout is very slight.These samples contained an estimated 10 to 1000 parasites per fish. No external signs of whirling disease have been observed and no stages of the parasite have been detected during microscopic examination.This is the 1st evidence of the whirling disease parasite in the state of Alaska. It is unknown how and when the parasite may have been introduced or how widespread its distribution may be within the state. The closest location to Alaska where the parasite is known to occur is the Snake River basin in north east Oregon, south east Washington, and Idaho. The parasite has also been found in fish from the Sakhalin Islands of Russia, across the Bering Sea. The introduction could have occurred with the transfer of infected fish or fish parts, movement of infected water or sediment, movement of the parasite by wildlife, or other pathways.The American Fisheries Society describes procedures to detect _M. cerebralis_ during fish health inspections. These procedures are to conduct pepsin-trypsin digest (PTD) for initial detection of the parasite. This is then followed by confirmation using PCR or histology, microscopic observation of fish tissues by a pathologist. The US Fish and Wildlife Service National Wild Fish Health Survey has tested more than 2000 fish from 26 locations in Alaska using PTD, and have not detected _M. cerebralis_. In addition, public and private hatchery fish in Alaska are not routinely monitored for the parasite; however, trout culture occurs at only 3 state hatcheries (Elmendorf, Fort Richardson, and Fairbanks) while all other facilities rear various Pacific salmon species.Elmendorf State Fish Hatchery is located on Ship Creek in Anchorage, AK, in the Cook Inlet basin. The hatchery relied on surface water from Ship Creek for its water supply, the most likely source of the parasite. Alaska Department of Fish and Game personnel will be conducting further tests to determine whether the parasite has become established in Ship Creek. The Department is also planning to investigate rainbow trout populations in watersheds considered high risk for the whirling disease parasite.There are 3 state-owned fish hatcheries in Alaska: Elmendorf State Fish Hatchery, Fort Richardson State Fish Hatchery, and an experimental hatchery in Fairbanks. The Oregon State University research team sampled rainbow trout from Fort Richardson State Fish Hatchery and did not detect _M. cerebralis_ using qPCR from any of those fish. Fort Richardson is also located on Ship Creek and is the original source of the trout that later tested positive at Elmendorf Hatchery.Alaska Department of Fish and Game policy would require that all fish at Elmendorf State Fish Hatchery be destroyed and disposed of if there had been evidence of clinical whirling disease or if the parasite was visually observed. The parasite's presence was only confirmed genetically, so the fish will not be destroyed; however, the Department will take precautionary measures to reduce the risk of spreading the parasite. No fish from Elmendorf will be transferred to other facilities and, besides one exception, fish will only be stocked into lakes with no inlet or outlet and have no reproducing salmonid populations. The only exception is that Chinook salmon from Elmendorf which will be released into Ship Creek to maintain broodstock. More than 94 000 hatchery fish that would have otherwise been destined for open watersheds will be transplanted into closed systems in 2007.

No comments: