In the process of completing a class project, I recently came upon the topic of viral hemorrhagic septicemia, or VHS, and I decided to do some research. VHS is a viral fish disease that can cause large fish kills. It is a rhabdovirus, a group of viruses that includes other disease-causing agents in fish. “Rhabdo” means rod and refers to the shape of the virus. There are a number of different types of the VHS virus. The type found in the Great Lakes is nearly identical to the type isolated from the Maritime Region of Canada.
VHS is not a human pathogen and dies quickly at human body temperature. However, the virus can cause large fish kills and change the dynamics of fisheries. In Denmark, the disease caused deaths in rainbow trout farms leading to losses near $60 million US dollars annually in the early 1990s.
Large fish kills have also been seen in the Great Lakes region. In 2006, fish mortalities were seen in several lakes throughout the region. In 2008, round goby fish kills were seen in western Lake Michigan, and earlier this year a fish kill of thousands of gizzard shad was documented in the Milwaukee Harbor ship canals.
Once fish have been infected, deaths can occur days to weeks later. It is important to note that some fish are able to fight off the disease. Fish that are affected show hemorrhaging in the skin or near the eyes creating red patches. Inside the fish, organs such as the liver, spleen and intestines are often filled with hemorrhages. The ultimate cause of death is usually organ failure.
VHS is transmitted between fish by exposure to bodily fluids or eating infected prey. It also may enter the fish’s body through the gills or through open wounds. VHS can also live outside of a fish in the water if conditions are right. The most likely ways in which VHS is spread throughout waters are through the movement of fish (natural or by humans) and the movement of infected water in ballasts of shipping vessels or live wells of fishing boats. Because the virus does not survive in birds and mammals, animals that eat infected fish are not likely to spread the disease.
It is not well known how or when the virus arrived in the Great Lakes region. A likely explanation is that ballast water discharge from shipping vessels brought the virus to the lakes. There are currently no effective treatments to stop fish to fish transmission of the disease or to treat infected fish. Therefore, it is important that fishing and boating industries as well as recreational boaters and fishermen take precautions to avoid further spread of the virus. With continued research and prevention steps, the virus will hopefully be stopped, fish kills will decrease, and we can all enjoy the beauty and the fish of the Great Lakes region for years to come.
For more information about VHS, visit the Wisconsin DNR website or other DNR sites.