Largemouth Iridovirus (LMBV) 2002 Update
By Ronald F. Dodson, Ph.D.
April 2002 -- In the summer of 1998 something went very wrong with the bass fishery in Sam Rayburn. A population of predominately larger bass began to surface and die during a period from late summer to mid-fall. The explanations from some biologists were that the lake was very low, the summer was very hot, and the fish were subjected to stress from both of these factors as well as from fishing pressure. Fisherman countered with the argument that some thing had been put in the lake or sprayed in the surrounding national forest, and ran into the lake.
In an article in this magazine, I explained that I did not buy either argument. If it was pollution then why did it kill only one species? Also there wasn't any rain during the summer so runoff was not a possibility. The stress issue was a bit more viable except that even if you bought into this argument you had to explain how bass were more "stressed" this summer than during the numerous years when the lake had been drawn down, and indeed the summers had also been hot. I suggested it had to be a species-specific killer, either a virus or bacteria as the first likely choices.
While the debate raged over the two popular explanations of what happened to Rayburn, another kill occurred. This time on Lake Fork in the summer of 1999. When it started the lake was several degrees cooler than the previous June and due to late spring rains was slightly over full. Pressure on the lake from tournaments was not a reasonable explanation since the slot limit made most tournament circuits avoid the lake. Furthermore some local groups had complained that the lake had been hurt by people taking illegal fish (slot fish) out of the fishery. The game wardens in response to this claim had saturated the lake in the spring.
Boats and livewells were checked with a vengeance. On some weekends the same wardens would check me up to three times. The results were that fishermen on the lake were found to not even be keeping legal (under-the-slot) bass. In fact some earlier lake surveys by biologists had shown such an abundance of fish below the slot that fishermen had been encouraged to keep some of these fish. Some of the dying or dead fish collected from Fork eventually were analyzed in labs and found to have a culprit called the Largemouth Bass Virus (LMBV).
There was little known about the disease except that this was the same virus, which had been associated with the 1995 die-off of bass in Santee-Cooper Reservoir in South Carolina. In fact most records attribute this as the first event where a die-off of bass was caused by this specific virus. However, later, we will see this link can be pushed back to the early 90's, but laboratory techniques had not shown the virus in fish from an earlier die-off until recently.
For an update on the Texas experiences I contacted Dave Terre, the Texas Parks and Wildlife regional fisheries director in Tyler. Terre is a member of a multi-state group of fisheries biologists from state agencies who have joined with researchers from academic institutions studying the virus and its effect on bass populations. Texas had additional bass kills associated with the viral infection during 1999 in Toledo Bend, on the Sabine River (down stream from Fork), and in Lake Conroe on the San Jacinto (just north of Houston). There haven't been any reported kills in Texas from the virus since 1999, but testing by the Parks and Wildlife has better defined the areas where the virus does exist in the bass fishery.
Terre stated that tests of lakes of the major river drainages indicated that the following tested positive for the virus: Red River, Cypress, Sabine, Neches, Trinity, Brazos, Colorado, Guadalupe, and San Jacinto. Major river systems (as represented by sampled lakes) where bass were negative for the virus included Arkansas (Canadian), San Antonio, Nueces, Rio Grande and the Sulphur. The last one is really an interesting finding since Lake Cooper (on the Sulphur) is only a few long casts north of Lake Fork. There is another interesting observation about the distribution of the virus based on these tests and that is if you draw a line approximately down interstate 35 you would note that the major lakes to the west were negative and those in east Texas were predominately positive.
The testing was a major undertaking and so a bit of qualifying is needed to appreciate the findings. To begin with relatively few bass were taken from each lake (60 per take). Of the 49 reservoirs that constituted the test group in 2000, 14 were considered to have fish that tested positive for the virus. Even this number needs to be clarified since not all the bass checked on the positive lake tested positive for the virus. The range was from a low of 1.7 percent (1 out of 60 fish in Tyler and Bridgeport) to a high of 13.3 percent of the sampled fish (8 out of 60 fish from Belton). Looking at this information in another way, the sampling by the department during the summer of 2000 resulted in a total of 899 bass being sampled with 45 (5%) being found positive.
Some of the first researchers involved in studying the effect of this virus in bass, as well as the effects of bass die-offs as related to this virus are folks in the Department of Fisheries and Allied Aquacultures of Aubum University. I had heard of their work in that their laboratories that provided confirmation of the viral induced die-off at Fork. One of the individuals who has been in the middle of trying to expand our knowledge about this pathogen is Dr. John Grizzle, a professor in this department. Dr. Grizzle provided some interesting new information about the virus and regretfully we could only share the frustration about the many things that we don't presently know about the virus.
To begin with the early tests to determine if the virus was there or not consisted of using ground up tissue from the fish which had been treated to reduce the bacterial component and then researchers observed the influence of the "soup" on cell cultures. He noted there are some concerns about using this method due to its level of sensitivity. The more accurate approach is to use modern molecular biology testing, which was available in only a limited number of the labs that were involved in doing the early testing so there may have been some false tests.
Nevertheless some things were consistent with all the kills across the country. To begin with it affects the larger (older) fish. If the females are those bass that grow to be larger, then my question was does the virus more selectively attack females? The answer is that no one knows, however it is known that male and females as well as northern, Florida, and cross strains can be infected.
Speaking of infected, it appears that some other fish in the food chain such as crappie and bluegill can also carry the virus, but apparently there has never been a die-off of these species associated with the virus. This isn't a new concept in that a number of organisms are carried by one species that doesn't seem to cause it a problem, yet when the pathogen is transferred to another species can cause an illness. Dr. Grizzle did not know of any study that had tested shad to see if they can also carry the virus. He did state that all sizes of fish could have the virus. However the lowest percentage of fish with the virus is in that age group of fish representing those less than one year old. The percentage of positive fish increases within each older year's group in an affected lake until there appears to be a decrease in the percentage of fish in the more mature bass.
Looking at these findings you could explain the low percentage of positive bass in the less than one-year group as possibly telling us something about the link of food in spreading the virus. These little bass mainly feed on insects and other small creatures and have to grow before they feed on minnows and other small fish. This shift in food sources tends to occur after they have reached approximately one year of age. To make this an even more interesting issue is the fact that a graduate student in Dr. Grizzle's university infected guppies with the virus and when they were fed to bass, the bass became infected. But it ends up that the infected bass have also been found to have the virus on the mucus coating of their skin. So the spread could be through food source or from one bass to another.
How could you explain the reduced percentage of the total population in the more mature bass? Maybe the explanation is that by the time the more mature group is sampled those in their generation who were infected and susceptible to the virus had died. Dr. Grizzle noted that the experiences in Rayburn and Fork were unlike the die-offs that had occurred in many of the other lakes in that larger numbers of bass had died in the two Texas reservoirs.
For whatever reason, the virus can exist in a lake and apparently not result in a die-off. No one knows what causes the event to kick in and suddenly a die-off occurs. It was interesting to hear that Dr. Grizzle doesn't find a scientific reason to assume that the virus has been in the environment, or in the bass population, for years. He also had one possible explanation for the relationship of the die-offs occurring in warm water. It may not be stress in the bass as much as the fact that the virus replicates more efficiently in warmer conditions. So you could have a bass which has the virus and because of the relationship of the water temperature to that of the body temperature of bass, all of a sudden the environment warms up, the bass warms up and the virus begins to more rap idly replicate causing a sick fish.
What about the infected fish that does not appear to get ill and looks healthy? This is not a new concept. In fact if it were not for some humans having developed some resistance to the various infectious diseases that swept Europe in the middle ages most of us wouldn't have ancestors.
While there is only limited data to back it up, Dr. Grizzle said there is some indication that a population of fish that are positive and don't get the disease appear to have what could be called an immune response which, in animals, suggests resistance. No one knows if the virus can be passed on through the contamination of eggs during the spawn. However the fact that the first year fish have the lowest percentage of fish that are positive for the virus seems to speak against it.
I noted that most articles indicate that the first confirmed kill from the virus occurred in Santee-Cooper Reservoir in South Carolina in 1995. However Dr. Grizzle told me there were some reported bass kills in Florida in the 80's and early 90's. These were not of the magnitude of the Rayburn and Fork kills and kind of slipped through the crack as a historical footnote. Then Dr. Grizzle found some frozen bass from these kills and, using newer molecular biology tools, found that the tissue had a virus that was identical to that from Santee-Cooper fish.
So how does a virus get into a lake and spread all over it? We spent 15 minutes or so trying to figure out reasonable explanations, and none fit. I still thought the water turkey idea was good since I just don't like the things. But birds go from one lake to another although if you look at a map of states that have positive bass you are looking at the central and Atlantic flyway. As of now there are no positive lakes in states west of Texas. Is the virus carried by the wind or in rain? This seems to get shot down by the same logic that such a distribution would saturate wide areas and many lakes would be simultaneously affected. Yet in the Texas Parks and Wildlife samples there are a number of smaller lakes that appear to be negative whereas there are positive larger lakes a short distance away.
Is it possible that the hatchery themselves place "contaminated" fish in the lake when they stock them? It is reasonable this could be an explanation since several of the Texas hatcheries had positive fish and unlike west Texas, the lakes in east Texas get Florida strain to "improve the quality of the fishery." Unless these are the most nomadic fish on record, it is hard to envision they cover and contaminate all of a large lake like Fork - yet fish died in the most distant sections of the headwaters. A number of these bass were most likely natives (northerns) since the closest Floridas were stocked miles away. It is a lot of water and the stocked fish are released in a limited number of selected areas.
Another thing to confuse the issue further is that the virus has now been found in an increasing number of states. The release in April of 2001 from the Texas Parks and Wildlife indicated positive fish had been found in sample from 14 states. However during that summer you can add Illinois to the list. Just as in the southern states the fish may be positive and not show any sign of disease. Several of their hatcheries were also found to have positive fish, but the really interesting thing is that several of their lakes had positive fish that had never received hatchery fish. This means of course that something likely introduced the virus, unless it had been there all along.
So what is the take on the status of the effect of largemouth virus on bass populations? Well in some lakes none since nothing died. On Rayburn you can figure it out for yourself by reviewing the tournament weights since the die-off. Will the lakes come back and how soon? First you have a lot of variables not the least of which are the unknowns including whether the virus will surface as a cause of a new die-off. Fortunately this has not been reported in any of the lakes where a die-off had occurred. Add in the fact that these are aging lakes and the cover isn't what it was in a new lake and thus it is going to be harder to get the fry to survive and repopulate the lake.
On the bright side, as noted, there have been no new kills and maybe a population of bass has survived which has some inherent resistance to the virus. In any case gently release any fish you catch, but particularly it is critical in lakes where the fragile balance of a bass fishery has been threatened by the virus.