Largemouth bass virus

A smallmouth bass with necrotic lesions caused by a coinfection of bacteria and LMBVLargemouth bass virus (LMBV) is a recently discovered virus that primarily infects sunfish and is only known to cause severe disease in black bass species (i.e., large and smallmouth bass). The virus is likely widespread in Minnesota but generally does not kill fish. High water temperatures and angling pressure are known to increase the potential for mortality events of black bass caused by LMBV. It is important for anglers to understand that fish are more susceptible to disease when they are stressed and proper fish handling and quick release can help reduce the risk of LMBV disease outbreaks.

Cause of disease

Largemouth bass virus is a virus of the family Iridoviridae that was first identified in 1995 in mass mortality event of ~1,000 adult largemouth bass in South Carolina.

What does disease caused by LMBV look like

The most common sign of disease in infected fish is behavioral changes such as difficulty maintaining buoyancy or equilibrium in the water or fish may appear lethargic or be found floating at the water’s surface. Fish infected with LMBV may not have any obvious lesions (red spots, bumps or wounds) on the surface but secondary infections with bacteria (bacterial infections that result from a weakened immune system) may cause necrotic red lesions to appear on the face and body of fish. Internally infected fish may have reddening around the swim bladder or fibrous tissue around the organs.

Fish species affected by LMBV

While the virus has been isolated from several sunfish, temperate bass, and esocid species (i.e., bluegill, black crappie, spotted bass, striped bass, and chain pickerel) the virus has only been associated with largescale mortality of largemouth bass, and more recently smallmouth bass. LMBV was recently isolated from a mortality events of smallmouth bass in Green Lake in Kandiyohi County.

Where has the LMBV been found

Though mortality events of LMBV have been documented in other states in North America, there has only been a single mortality event associated with LMBV in Minnesota (Green Lake in Kandiyohi County in 2021). Several small surveys have identified LMBV in healthy populations of largemouth bass in Minnesota, indicating that the virus is likely widespread. However, due to the lack of disease associated with virus detections, LMBV is not considered to be a threat to populations of bass in Minnesota.

Are fish infected with LMBV safe to eat

The virus does not have any impact on humans through direct contact or via fish consumption.

How can I prevent impacts of LMBV to Minnesota fish

Largemouth bass virus associated with mortality of bass is most common during warmer temperatures over 77⁰F (25⁰C) and disease is more severe at 86⁰F (30⁰C). You can do your part by treating bass with special care when angling in waters at these temperatures. This may include; avoid fighting fish to exhaustion (use of tackle appropriate for quickly landing fish), releasing fish quickly after catching, and maintaining adequate aeration in live wells when they are used. Largemouth bass virus detection is common in fish sampled from bass tournaments nationwide and research has demonstrated that LMBV may increase post-release mortality of bass from the average post release mortality rate of 25-26% to over 75%. Scheduling tournaments during cooler conditions and maintaining cool and well oxygenated live well conditions during transport and weigh-in may reduce the disease impacts of LMBV.

If you observe a fish kill/die off

Report a killThe DNR is interested in documenting mortality events of any species. Largemouth bass virus is of particular interest since this virus is still being studied to determine the extent of its impacts on wild populations of black bass.

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