Virus Distribution and Signs of Disease after Immersion Exposure to Largemouth Bass Virus

Abstract Largemouth bass virus (LMBV) can cause a fatal disease in largemouth bass Micropterus salmoides . In previous laboratory studies, largemouth bass have been infected with LMBV by injection, gavage, cohabitation with infected fish, or immersion in water containing the virus. To determine which organs are infected by LMBV and the changes in distribution among organs over time, we immersed largemouth bass (mean weight, 79 g; mean total length, 181 mm) for 1 h in water containing LMBV (1.7 × 10 3 plaque‐forming units/mL) and then necropsied fish daily for 27 d. Cell culture was used to detect LMBV in gill, liver, spleen, trunk kidney, head kidney, swim bladder, heart, brain, gonad, intestinal cecum, and intestine. Of the 225 fish exposed to LMBV, 2 fish died of undetermined causes and 5 moribund fish were necropsied. All moribund fish were observed at the water surface, had an overinflated swim bladder, were infected with LMBV, and had no other known cause of the morbidity. No signs of disease were observed in an additional 187 fish that we necropsied, although 31% of these fish were found to be infected with LMBV. Largemouth bass virus was detected in internal organs at the end of the 1‐h exposure and for 24 d postexposure. The percentage of infected fish was highest 4 and 5 d after exposure, when LMBV was isolated from all fish tested. The organs that were infected varied among fish, but the swim bladder, gill, and trunk kidney were most frequently infected. Moribund fish had an average of 4.6 (range, 2–7) organs positive for LMBV, which was significantly greater ( P < 0.05) than the mean of 2.3 (range, 1–7) positive organs in subclinically infected fish. Our study demonstrated that LMBV can rapidly enter largemouth bass and then decrease to below detectable levels within a month.