GENETIC COMPARISON OF VIRAL HEMORRHAGIC SEPTICEMIA VIRUS ISOLATES FROM NORTH-AMERICA AND EUROPE

Viral hemorrhagic septicemia virus (VHSV) is the causative agent of a serious rhabdoviral disease of rainbow trout Oncorhynchus mykiss in Eu rope. The first isolation of the virus in North America occurred in th e fall of 1988 when it was recovered from adult chinook O. tshawytscha and coho O. kisutch salmon returning to 2 hatcheries in the state of Washington, USA. The following year, VHSV was isolated from adult coho salmon at 2 other hatcheries in northwestern Washington. In 1990 and 1991, VHSV was recovered from Pacific cod Gadus macrocephalus caught i n Prince William Sound, Alaska. Genetic variation among the 4 isolates from salmon and the 1990 isolate from Pacific cod was determined usin g T1 ribonuclease fingerprinting. In addition, 4 diverse isolates from Europe were included for comparison. The North American isolates of V HSV formed a single fingerprint group in which the 4 isolates from sal monids were highly similar to each other and the isolate from Pacific cod was related but less similar. The 4 European isolates, which inclu ded an isolate from Atlantic cod G. morhua, formed a second fingerprin t group. The genetic diversity among the isolates within each fingerpr int group was estimated to be less than 5 % while the North American a nd European strains of the virus were judged to differ by more than 5 %. The results indicate that the North American isolates of VHSV are n ot of European origin and that the virus may be enzootic within the ma rine environment.