The lethal dose of largemouth bass virus in juvenile largemouth bass and the comparative susceptibility of striped bass

Largemouth bass virus (LMBV) is an iridovirus that was isolated from wild a dult largemouth bass Micropterus salmoides in the southeastern United State s in 1994. Although originally isolated from moribund wild fish, its virule nce to juvenile largemouth bass is uncertain. To help clarify this point, t wo LMBV titrations were made in juvenile largemouth bass. Titers of LMBV in fathead minnow cells were 10(4.8) and 10(5.8) tissue culture infectious do ses- 50% cytopathic endpoint (TCID50) per milliliter, respectively. Tenfold serial dilutions of LMBV employed in each cell culture titration, injected intraperitoneally (0.1 mL/fish) into largemouth bass produced calculated l ethal dose-50% mortality endpoints (LD50s) of 282 (10(2.45)) and 288 (10(2. 46)) infectious doses in two consecutive infectivity: trials, Virus yield o f assayed infected fish averaged 10(8.5) TCID50/g and 10(7.7) TCID50/g in v iscera of moribund and dead fish in the two trials and 10(6.5) TCID50/g in surviving exposed fish 14 d after infection. In a second experiment, largem outh bass had 100% mortality 5 d after injection while virus immersed fish had a significantly (P less than or equal to 0.005) lower mortality of 17% at 14 d. Similarly treated juvenile striped bass Morone saxatilis suffered 63% mortality after injection and significantly (P less than or equal to 0. 005) lower mortality of 10% after immersion. In a third study of 25 d, 100% of injected largemouth bass died by 5 d after injection, and all of them w ere virus-positive. Injected striped boss had a significantly (P less than or equal to 0.005) lower mortality of 24% all three fish were virus-positiv e initially, two fish were virus-positive at is d, and none were positive a t 25 d. Juvenile largemouth bass were highly susceptible to LMBV injection and striped bass were moderately susceptible, but both species were only mi ldly susceptible when exposed by immersion.