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Herpes kills thousands of Wisconsin river fish

There is no cure for koi herpes in wild carp.
There is no cure for koi herpes in wild carp.Jim Hagerty

A strain of herpes is believed to be the cause of a massive Wisconsin fish kill that has seen tens of thousands of dead carp in Wisconsin, wildlife officials announced Tuesday, Aug. 5.

Koi herpes, a strain of the virus that only affects koi and carp, was recently discovered after two weeks of mysterious fish deaths that began in the Horicon Marsh and Lake Sinissippi, near Horicon, Wisconsin. The kill is believed to have started around July 17, The Rock River Times reported July 30. That's when hundreds of dead carp began appearing along the shores of the marsh and the banks of the Rock River.

Department of Natural Resource (DNR) officials said the virus is not harmful to humans or other fish. Citizens are warned, however, that thousands of decomposing fish can be susceptible to an array of secondary bacteria that can make people sick.

“The public is not at risk from the koi herpes virus and from what we have seen in other states, sport fish and forage fish such as shiners have not been harmed,” said Laura Stremick-Thompson, a fisheries biologist.“However, DNR encourages use of protective clothing such as gloves in removing the dead carp due to other bacteria the fish may be hosting.”

Bacteriology testing began July 21, showng the presence of columnaris, a common bacteria that has killed fish in Wisconsin in the past. However, it didn't appear to be the main culprit, as columaris is not known to kill carp in large numbers, if at all. An ensuing investigation led biologists downstream to Silver Creek, a tributary to the Rock River in Watertown, Wisconsin, approximately 42 miles from the Hustisford dam. That's where the herpes virus was found, officials said Tuesday.

Tissue samples were sent to Michigan State University after they tested positive for koi herpes July 31. The U.S. Fish and Wildlife Service La Crosse Fish Health Center also observed the samples. Damaged gills, sunken eyes and enlarged spleens in the fish collected from the Horicon die-off helped officials identify the herpes virus.

“We identified koi herpes virus as a possible cause of the Horicon event after finding clinical signs consistent with the disease and noting adequate dissolved oxygen levels in the water,” Stremick-Thompson said. Fish from the Silver Creek event are being sent to the La Crosse center for necropsy and testing.

Although koi herpes has killed carp in New York in 2005, Michigan in 2011 and in Ontario, Canada in 2007 and 2008, this marks the first time the virus has been discovered in Wisconsin. The disease often sets in when water temperatures climb rapidly. During the first die-off round July 17, water temperatures in the marsh, lake and Rock River climbed to around 73 degrees Fahrenheit. Officials say that koi herpes, if not addressed, could be spread to ornamental carp and domesticated koi. Infected fish that survive the outbreak could also become carriers.

Koi herpes is spread by contact with skin and gills. It can live in the water for about a week before attaching to carp. There is no way to stop the virus from spreading in wild fish. For that reason, officials are expecting more carp to die until water temperatures drop and kill the strain.