What is LMBV and what happens when fish become infected? Is it a
concern in Florida? This article chronicles what we currently know
about LMBV and its effects on Florida's largemouth bass
What is the largemouth bass virus (LMBV)?
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.
Researchers are shown bleeding wild largemouth bass for detection
of LMBV antibodiesin blood serum using laboratory
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.
What are the symptoms of the disease caused by
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
What affects the severity of the disease caused by
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
Necropsy of wild Suwannee bass. The kidney and spleen were
removed to check for the presence of LMBV using laboratory
techniques. U. S. Fish and Wildlife Service (USFWS) scientists
shown in the photo are members of the National Wild Fish Health
Survey program. Photo credit: USFWS
Where has LMBV been found?
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.
Have any fish kills in Florida been the result of
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.
What research on LMBV has been done in
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.
Figure 1. Frequency of largemouth
bass and Suwannee bass samples collected from Florida water bodies
that tested positive for Largemouth Bass Virus (LMBV) from 1999 to
2005. The frequency of LMBV-positive fish in samples averaged
eight percent and ranged from 0 to 40 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.
What about fish raised in Florida's fish hatcheries -
have they been tested for LMBV?
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.
Note: This article made reference to previously
published research. For a listing of these studies,
contact FWC researcher Wes Porak at the Eustis Fisheries Research
Lab at 352-732-1225