Abstract

Large Mouth Bass Virus (LMBV) was first isolated from moribund, adult largemouth bass (Micropterus salmoides) involved in a fish kill, which occurred in the Santee-Cooper Reservoir, South Carolina. Since then LMBV has been isolated from largemouth bass through out the southeast United States. Often times it is found in apparently healthy fish, suggesting that it is not always fatal and animals may act as carriers. LMBV is a Ranavirus in the family Iridoviridae. Several other Iridoviruses have been found in fish, and their effects range from minimal to severe pathogenicity. LMBV is the only virus known to infect largemouth bass. Moribund fish are often found at the surface having difficulty swimming. Upon examination the only signs of the disease are reddened fins and an enlarged swim bladder containing a yellowish exudate. However, these signs are not pathognomonic for the disease.

There is question about the origin of LMBV. A virus almost identical in nucleotide sequence to LMBV has been isolated from two species of ornamental fish, the guppy (Poecilia reticulata) and the "doctor fish" (whose identity is uncertain), which were imported from Southeast Asia in the early 1990' s. It is also possible that this Ranavirus has worldwide distribution and only now do researchers have the ability to isolate it. Certain factors such as unfavorable water chemistry or high stress conditions may weaken the fish's immune system, allowing the virus to become pathogenic. It has been reported that in the wild, only adult largemouth bass seem to be affected, but laboratory work shows that juveniles are susceptible when exposed to the virus either by injection or immersion.¹ The virus exhibits horizontal transmission when uninfected fish are introduced to LMBV positive fish. Wild fish sampling studies found LMBV in other species (smallmouth bass, spotted bass, suwanee bass, bluegill, redbreast sunfish, white crappie, black crappie), however the virus's effects on these species are unknown. Laboratory studies show that striped bass (Morone saxatilis) are susceptible to the disease. However, when compared to largemouth bass the striped bass are not as severely affected.¹

The impact of LMBV on largemouth bass populations is unknown. It has been found in a small number of hatcheries whose water source is within a watershed where LMBV is found in wild fish. In order to minimize the spread of this virus, hatcheries should closely monitor the health of their largemouth bass. Anglers and researchers alike are encouraged not to transfer water, fish, tissues or bait between different bodies of water and to properly clean boats and equipment.

The impact of LMBV on largemouth bass populations is unknown. It has been found in a small number of hatcheries whose water source is within a watershed where LMBV is found in wild fish. In order to minimize the spread of this virus, hatcheries should closely monitor the health of their largemouth bass. Anglers and researchers alike are encouraged not to transfer water, fish, tissues or bait between different bodies of water and to properly clean boats and equipment.

Reference

1.  Plumb, J.A. and D. Zilberg, 1999. The Lethal Dose of Largemouth Bass Virus in Juvenile Largemouth Bass and the Comparative Susceptibility of Striped Bass. Journal of Aquatic Animal Health 11:246-252.