Largemouth bass virus (LMBv) is a disease that impacts several fish species but only appears to cause death in some largemouth bass. In fact it is the only known virus to cause mortality in largemouth bass. LMBv was first discovered in Lake Weir, Florida in 1991 and the first reported fish kill occurred four years later at Santee Cooper Reservoir, South Carolina. As it spread throughout the southern United States, LMBv was responsible for other largemouth bass die-offs in the late 1990’s. However, in many reservoirs LMBv did not result in significant die-offs but only led to a decrease in survival and growth rates. When such decreases in survival and reduced growth rates occur, anglers catch fewer quality-size largemouth bass – bass greater than three pounds. The good news is that these impacts from the virus outbreak are normally short lived and largemouth bass fisheries recover in about 3-6 years as individuals build up a resistance to the disease.
LMBv has been classified in the family Iridoviridae of the genus Ranavirus. The only noticeable behaviors that might be expressed by infected bass are a loss of equilibrium and fish floating on the surface of the water unable to submerge due to an over inflation of the swim bladder. No other external cues are obvious in fish infected with the virus. Disease outbreaks are most common in August through October.
The Virginia Department of Wildlife Resources (DWR) first tested several water bodies in 2001 with most either having no occurrence of LMBv or very low infection rates (see table). Subsequent virus testing coordinated by DWR in the summer of 2010 revealed that about 40% of largemouth bass had been exposed to LMBv at John H. Kerr Reservoir/Buggs Island Lake and the disease was likely responsible for the recent decline in the bass fishery on this reservoir. Largemouth bass sampled from Briery Creek Lake and Sandy River Reservoir, the only other two lakes examined in 2010, also tested positive for the virus. A small largemouth bass mortality event which occurred at Briery Creek Lake in late June 2010 was also likely the result of LMBv. No apparent impacts to the fishery have been detected at Briery Creek Lake or Sandy River Reservoir.
During August 2011, the DWR tested sixteen bodies of water statewide including large impoundment, small impoundments, and major rivers to test for presence of the virus. LMBv was found in all of the tested waterbodies except the tidal James River. Despite the spread and discovery of LMBv across many waterbodies in Virginia, there are reasons for optimism. Reservoirs like Lake Chesdin, Smith Mountain Lake, and Lake Anna, where LMBv has been found, maintain excellent bass populations and anglers are enjoying improving success at Kerr Reservoir. Bass do build up immunity to the disease and reservoirs in the southern states hit hard by the virus in the late 1990’s have recovered to pre-exposure angler success rates.
Due to the popularity of the largemouth bass fishery in Virginia, anglers have expressed concerns about LMBv spreading to other area reservoirs. While LMBv has obviously spread since 2001, it is still very important for anglers to use every precaution available to help prevent the spread to those waters that may not be infected – even private waters. Anglers who remove bass to stock other water bodies are encouraged to halt this practice due to the high likelihood of spreading the virus. Responsible care and handling, which is a major goal in the Department’s effort to promote proper natural resource stewardship, of all largemouth bass is vital to maintaining healthy populations regardless of the presence of disease. When LMBv is present, one of the best things that anglers can do for the fishery is to limit largemouth bass tournaments during the warmest months. Stress on largemouth bass is greatest when water temperatures are high and with the additional stress of disease, mortality rates can be very high during summer fishing trips. Tournament organizers tend to do a good job about avoiding tournaments during the heat of summer but we can always do better.
The following are several concerns that anglers have expressed to us and other agencies concerning the impacts of LMBv. These are some of the most commonly asked questions that we encounter.
Can we cure the disease? No, the virus will have to run its course and hopefully the fish will build up immunity to LMBv. So far, lakes affected by the disease in the southern U.S. have not experienced additional large LMBv outbreaks since the initial ones in the late 1990’s.
Are there any risks to humans from the virus? No, fish are safe to eat and the water is safe for drinking water supply and recreation. This virus cannot be passed to humans.
What causes an outbreak of the virus? It is not fully understood what causes an outbreak of LMBV. It is likely that stressful conditions such as low reservoir levels, high water temperatures, or increased handling time make bass more susceptible to LMBv.
How can you tell if a largemouth bass that you’ve caught has the disease? There are very few external cues that the bass might have the disease and most show no signs of the disease. Fish that are very sick from the virus may appear bloated and swim erratically due to the impacts of the virus on the swim bladder.
How does the disease spread? Fish that come in close contact (like in a livewell) can easily infect one another. Transmission through the water and eating infected prey are also ways that the disease is spread.
How does LMBv impact largemouth bass populations? Anglers may notice LMBv impacts in the form of reduced catch of larger fish. The virus can increase mortality and reduce growth in the short term which does not noticeably impact the population as a whole but when these two factors are combined, the results are often decreased number of older and larger fish in the population.
Table. Largemouth bass virus (LMBv) testing history in Virginia. N = number of largemouth bass sampled for the virus. Not all tests produced an exposure rate (%) due to pooled sampling on some water bodies to test for presence or absence of the virus.
|Date||Site||N||LMBv present?||Exposure rate (%)|
|8/20/2001||South Holston Reservoir||48||Yes||1/48 = 2.1%|
|8/21/2001||Briery Creek Lake||60||No|
|8/21/2001||Buggs Island Lake/Kerr Reservoir||59||Yes||1/59 = 1.7%|
|8/20/2001||Flannagan Reservoir||57||Yes||1/57 = 1.8%|
|8/21/2001||Smith Mountain Lake||55||No|
|8/21/2001||Claytor Lake||29||Yes||4/29 = 14%|
|8/29/2001||Occoquan Reservoir||60||Yes||1/60 = 1.7%|
|8/30/2010||Buggs Island Lake/Kerr Reservoir||51||Yes||21/51 = 41%|
|8/30/2010||Briery Creek Lake||20||Yes||6/20 = 30%|
|8/30/2010||Sandy River Reservoir||20||Yes||3/20 = 15%|
|8/11/2011||Lee Hall Reservoir||32||Yes||5/32 = 16%|
|8/11/2011||James River (tidal)||25||No|
|8/11/2011||Chickahominy River (tidal)||50||Yes||pooled|
|8/11/2011||Beaverdam Swamp Reservoir||30||Yes||pooled|
|8/17/2011||South Fork Shenandoah River||30||Yes||pooled|
|8/17/2011||South Holston Lake||30||Yes||pooled|
|8/23/2011||Briery Creek Lake||30||Yes||9/30 = 30%|
|8/23/2011||Buggs Island Lake/Kerr Reservoir||60||Yes||14/60 = 23%|
|8/24/2011||Smith Mountain Lake||60||Yes||35/60 = 58%|
|9/1/2011||Lake Anna||59||Yes||19/59 = 32%|
If you have questions or concerns about any fishery in Virginia please feel free to contact any of the DWR offices located throughout the state.
View a video produced by the Concerned Bass Anglers of Virginia (CBAV) on proper care and handling practices of largemouth bass and for more information on LMBv.
For more information on proper care and handling practices to increase bass survival visit the BASS website or your organization’s conservation page for similar information.