Largemouth Bass Virus (LMBV) and Its Effects on Largemouth Bass Resources in Florida

The largemouth bass virus, family Iridoviridae, is the only virus to have been associated with a lethal disease of largemouth bass Micropterus salmoides.  While LMBV has been isolated from a number of other species of warm-water fishes, the disease response has only been observed in largemouth bass.

Largemouth Bass Virus (LMBV) was first isolated from Lake Weir in 1991. The virus was first associated with mortality of largemouth bass in the wild during a fish kill in Santee Cooper Reservoir, South Carolina, in 1995 when at least 1,000 bass died.  Since 1995, LMBV has been implicated as a source of mortality in more than 25 fish kills in the United States, specifically throughout the Southeast and the Midwest.  Deaths of trophy-sized largemouth bass during many fish kills heightened concern about this possible pathogen among both anglers and fishery scientists.  Fortunately, evidence suggests that fish populations develop immunity following exposure to the virus.  Fish kills associated with LMBV have also declined over time, and to our knowledge, none have been observed over the past two years.

The disease associated with LMBV occurs with warmer water temperatures during the summer. Only bass larger than 300 millimeters (roughly 12 inches) are typically observed in fish kills.  Buoyancy or equilibrium problems have been associated with this disease. Swim bladder lesions were first described in LMBV-diseased bass collected from Santee-Cooper Reservoir, South Carolina, and later in bass collected from Sardis Reservoir, Mississippi.  However, this finding has been inconsistent.

Other diseased fish sometimes had slightly reddened, over-inflated swim bladders or appeared normal.  Healthy fish have also been observed with similar swim bladder lesions.  This lack of distinctive, easily recognized lesions makes the disease difficult to diagnose.  As a result, there has been some controversy and criticism among scientists that evidence linking LMBV to some, but not all, fish kills has been anecdotal.

Experimental exposure to LMBV has yielded variable results.  In one study, fish injected with a modest dose of the virus were still alive after 26 days. The virus has been lethal in other laboratory experiments when largemouth bass were exposed to higher levels of LMBV.  Temperature may also play a role.  One study found that experimentally infected juvenile largemouth bass experienced greater mortality and had higher viral loads at 30oC than at 25oC.  More concerns were raised among experts evaluating the impacts of LMBV when experiments indicated that virulence, or the ability of the virus to infect and cause disease, varied over five-fold between three genetically different strains of LMBV.

LMBV has been found only in the eastern United States.  Distribution of LMBV has been tracked by several fisheries agencies including the U.S. Fish and Wildlife Service's National Wild Fish Health Survey and the Southeastern Cooperative Fish Disease Project.  The origin of LMBV and the amount of time that it has been in the United States is unknown.  There is evidence that suggests that LMBV was recently introduced to the United States.  For example, bass-only fish kills similar to those caused by LMBV were not observed or reported prior to the 1990s.  Also, cell culture isolation techniques used to detect LMBV have been routinely used for diagnosis of fish diseases for several decades.  Therefore, the occurrence of LMBV would likely have been observed prior to the 1990s.  Conversely, the wide distribution of LMBV in populations throughout the Eastern United States and the occurrence of different strains of LMBV suggest that it has been in the United States for a considerable amount of time.  We do know that LMBV has been in Florida for at least 15 years.

Die-offs which may be representative of LMBV disease events have been infrequently reported in Florida during the past 10 years.  In the past three years, three fish kills in Florida were evaluated as potential LMBV fish kills.  A bass die-off in a private pond near Tampa was diagnosed as a fish kill caused by low oxygen levels.  A second disease event at Lake Butler in Orange County in 2003 was associated with an outbreak of the bacteria Aeromonus spp.  In 2004, a die-off of largemouth bass at Lake Hollingsworth in Polk County followed a lake-wide alum treatment by the county, and the results of the investigations were inconsistent with LMBV disease. To our knowledge, only a few other bass die-offs were reported during this period. As in the case of many fish kills, dying fish were not available for analysis. 

Scientists from the University of Florida's College of Veterinary Medicine, U.S. Fish and Wildlife Service (USFWS), Auburn University and the Florida Fish and Wildlife Conservation Commission (FWC) have collaborated to assess the significance of LMBV to Florida's black bass fisheries.  Disease and fish kills in Florida largemouth bass (Micropterus salmoides floridanus) have not been linked to LMBV in Florida.  However, buoyancy problems and swim bladder lesions, symptoms associated with the virus, and an antibody response to the virus were observed in Florida largemouth bass following a bass-only fish kill in Lake Harris in the early 1990s.  The largemouth bass virus was also isolated from largemouth bass that had been collected from Lakes Weir and Holly during a disease episode in this same period of the 1990s.  This was the first known case of an iridovirus, or the family of viruses to which LMBV belongs, being observed in wild largemouth bass.  The virus was not considered pathogenic (i.e., capable of causing disease) in these lakes, in part, because researchers found no relationship between the presence of viral antibodies and either body condition or anemia in a small number of fish that were tested from Lakes Weir, Holly, Newnans and Harris.  It was also reported that blood smears from antibody-positive fish did not have characteristics that would have been representative of an acute viral disease.

Tissue (kidney and spleen) and/or blood serum samples collected from black bass (largemouth bass and Suwannee bass Micropterus notius) in 45 water bodies since 1999 indicated that the virus, but not the disease, is widely distributed throughout Florida.  Seventy-two percent of these populations, ranging geographically from Seminole Reservoir at the Florida-Georgia border to Nine Mile Pond in Everglades National Park, had individuals that tested positive for the virus.   The frequency of LMBV-positive fish in our samples averaged eight percent and ranged from 0 to 40 percent (see Figure 1), but some of the negative water bodies may be "false negatives" due to low sample sizes and the relatively low prevalence of LMBV-positive individuals in most populations.  Prevalence of individuals positive for viral antibodies averaged 28 percent and ranged from 0 to 55 percent.

Results of laboratory studies strongly suggest that many largemouth bass become immune upon exposure to LMBV, and detectable blood serum antibody levels persist for long periods of time after tissue LMBV levels have fallen below detectable limits.  Also, minimum detection thresholds are unknown for both of the techniques used to detect LMBV and the viral antibodies, which may affect these results.  A distinction should always be made between fish that are infected with LMBV and fish that are diseased as a result of the virus.  Almost all of the populations sampled in Florida and included in our data set were not experiencing disease problems or fish kills.

Largemouth bass fingerlings that were produced at Florida's fish hatcheries have been tested for LMBV for several years now.  All samples of production fish were negative for the virus.