Koi herpes killing Rock River carp

MADISON — A virus new to Wisconsin waters that affects common carp and koi likely is to blame for two Rock River carp die-offs in Dodge County.

State Department of Natural Resources officials say the koi herpes virus is not harmful to humans or other fish. However, caution is urged in handling affected carp because as they decompose, they become susceptible to multiple secondary bacterial infections that can cause illness in humans.

Laura Stremick-Thompson, a fisheries biologist serving Dodge and Jefferson counties, said that the DNR began investigating the first die-off on July 21 in the state and federal waters of Horicon Marsh and Lake Sinnissippi.

In addition, an investigation now is under way following reports from July 31 of dead and dying carp in Silver Creek, a tributary to the Rock River in Watertown approximately 42 miles downstream from the Hustisford dam.

Since that time, Stremick-Thompson said she has also received reports of dead fish in smaller numbers in Fort Atkinson.

“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 Stremick-Thompson. “However, the DNR encourages use of protective clothing such as gloves in removing the dead carp due to other bacteria the fish may be hosting.”

Tissue samples from the first die-off tested positive for the koi herpes virus using a technique called polymerase chain reaction that amplifies portions of the pathogen’s unique genetic code. Department fish health specialists sent the tissues samples to Michigan State University and the U.S. Fish and Wildlife Service La Crosse Fish Health Center after observing damaged gills, sunken eyes and enlarged spleens in the fish collected from the Horicon die-off.

“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 were being sent to the La Crosse center for necropsy and testing.

Stremick-Thompson said the virus is transmitted through waterborne contact with the gills and skin and it can survive outside of fish for up to seven days.

She said that the onset of the virus appears to occur as water temperatures climb; water in the July die-off registered at about 73 degrees Fahrenheit.

Koi herpes virus caused significant carp die-offs in New York in 2005, Michigan in 2011 and in Ontario, Canada, in 2007 and 2008.

The presence of the virus in other states has been linked to the release of ornamental fish such as domesticated koi. Wild fish that do survive an initial outbreak can become carriers and may not show signs of the disease. The virus might cause disease and mortality periodically in the future, depending on water temperatures and fish stress.

Given the dense carp population in the Rock River system, DNR biologists said that additional carp die-offs might take place until water temperatures begin to cool with the onset of fall.

The mortality rates of carp infected with koi herpes virus are greatest when water temperatures are between 71 and 79 degrees Fahrenheit. There is no treatment available to control the virus in wild fish.

“As long as temperatures stay in that window, it’s going to continue to move down stream,” Stremick-Thompson added. “It has to work it’s way through the system until the fish can build up some immunities.”

Stremick-Thompson noted that the die-offs this summer are assumed to be the initial outbreak of the virus in Wisconsin, causing the high number of deaths.

Once the virus is in the system, it will always be in the system; however, Stremick-Thompson said, the fish eventually will build immunities, resulting in fewer deaths.

She said she expects that if the virus does not reach Lake Koshkonong this summer, it will by next summer.

Typical of fish infected with koi herpes virus, fish from the original die-off also were infected with bacterial pathogens. The bacteria are present naturally in surface water and when fish are stressed or their immune system is suppressed, the bacteria can cause systemic infections and disease.

Many dead carp have been seen in the Rock River where it meets Lake Koshkonong.

Citizens who observe dead or dying carp are asked to contact Laura Stremick-Thompson by email via Laura.Stremickthompson@wisconsin.gov, or by phone at (920) 387-7876.

Koi herpes virus is a federally reportable disease and DNR officials have completed the initial notification.

Koi herpes killing Rock River carp