Largemouth Bass virus

From Fish
A largemouth bass infected with Largemouth Bass Virus.(La Crosse Fish Health Center)

Largemouth bass virus (LMBV) is a Ranavirus (Iridovirus) that was isolated from moribund largemouth bass in South Carolina in 1995. LMBV is in the family Iridoviridae. There are four genus level groups in the Iridoviridae family, Iridovirus, Chloriridovirus, Ranavirus and Lymphocystisvirus. It is not known what genus this specific virus belongs to[1].

The origin of LMBV is unknown. It was first discovered in the United States in Florida. It has since been detected in 18 other states including Alabama, Arkansas, Georgia, Illinois, Indiana, Kentucky, Louisiana, Michigan, Missouri, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, Virginia, Vermont and Wisconsin.

LMBV does not only infect largemouth bass, it also has been found in guppies, smallmouth bass, spotted bass, Suwanee bass, bluegill, redbreasted sunfish, white crappie and black crappie. This virus usually only causes death in largemouth bass.

Clinical signs

The disease is not well understood because the virus is commonly isolated from tissues of clinically normal fish. In the 1995 fish kill, ~1,000 fish died over a 2-3 mo period in an area that encompasses >66,000 hectares. It is thought that stress may contribute to clinical emergence of the disease, along with hot weather, stress factors might include poor water quality caused by pollution.

The signs of a fish infected with LMBV are sometimes hard to recognize. Some carriers of the virus will seem completely normal. If the virus has triggered disease in a fish that fish will be near the surface, having trouble staying upright, and having difficulty swimming. LMBV seems to infect the swim bladder of fish. Some bladders will have a thick yellow or brown exude, or it could only be slightly red and over inflated, and sometimes the swim bladder will look normal. For precise diagnosis a DNA based test must be preformed.


Diagnosis is based on isolation of the Iridovirus using Fat-head minnow (FHM) cell line[2].


There is nothing that can be done to eradicate LMBV in the wild. Preventative measures are critical to minimise spread, including the proper disposal of all unused bait in the trash or on land, never into the water, avoiding the transfer of live catch from one body of water to another and not discarding fish entrails or skeletal parts in a body of water[3].


  1. Merck Veterinary Manual
  2. [ Merck Veterinary Manual]
  3. B.A.S.S. Inc. Largemouth Bass Virus Fact Sheet. 16 March 2004. Texas Parks and Wildlife. 11 Aug 2004. <>.