A long-dormant disease blamed for killing fish in October of 2018 has spread to two additional Michigan lakes.
Largemouth bass virus, which was previously found in Cedar Lake in Iosco County, has now been confirmed in Alpena County’s Beaver Lake and Montmorency County’s Avalon Lake.
Prior to its detection in Cedar Lake, the virus hadn’t been detected in Michigan for more than 15 years. It previously affected Michigan largemouth bass in different lakes in the early 2000s.
These latest discoveries indicate the virus is spreading northward in Michigan, according to the Department of Natural Resources.
“The largemouth bass virus likely compromised the immune system of smallmouth bass in Beaver Lake, causing secondary bacterial infections to become more lethal and allowed the virus to be a direct factor in the fish kill,” said Gary Whelan, the DNR Fisheries Division’s research manager.
“Because these latest detections are at the northern edge of where LMBV has been found, we may see different responses than what was documented in southern Michigan.”
In the early 2000s, the pathogen killed 10 to 20 percent of the larger adult largemouth bass in southern Michigan lakes at first exposure, with populations recovering in a few years.
LMBV is one of more than 100 naturally occurring viruses that affect fish and is closely related to viruses found in frogs and other amphibians.
Its origin and how it is spread are unknown, but anglers are considered a likely path for transmitting the virus through the movement of live, infected fish from one water to another, or by using contaminated and uncleaned gear or boats in uninfected waters.
LMBV is not known to infect humans, and infected fish are safe to eat if the fish is thoroughly cooked. LMBV cannot be eradicated from lakes, nor can infected fish be treated.
LMBV usually causes fish kills during periods when fish are most stressed, possibly due to very hot weather, intensive recreational fishing and aquatic weed or other treatments made during hot weather.
Affected fish usually appear normal, although they may be lethargic, swim slowly and be less responsive to activity around them.
Dying fish often are seen near the surface and have difficulty remaining upright. Infected fish usually have bloated and yellowish swim bladders.