Viral haemorrhagic septicaemia (VHS): Detection, distribution and combat

Viral haemorrhagic septicaemia (VHS) is considered to be one of the most economically important viral fish diseases. This review aims to summarize knowledge on the present distribution of VHS, advances in development of diagnostic methods and implications for resulting interventions against the disease. The disease was originally found as a problem in rainbow trout in European freshwater fish farms. However, the first detection of VHS virus (VHSV) in various marine species in the 1980s and 1990s initiated intensive investigation that have demonstrated the presence of the causative rhabdovirus in a broad range of marine and freshwater species of the Northern Hemisphere including the Atlantic and Pacific Coast of North America and the marine waters around Japan and South Korea. Recent mass mortalities in the Great Lake System of North America have also been attributed to VHSV. The number of known affected and susceptible fish species is steadily growing. At least three serotypes of the virus could be differentiated, which, however, do not allow discrimination between marine and freshwater isolates. Development of molecular methods allowed analysis of sequences of viral genes, particularly of the N, G and NV genes. These analyses revealed four major genotypes, which are divided into different sublineages. The genotypes appear to be more related to geographical than to species origin. The wide distribution of the virus, particularly in wild marine fish populations, requires new methods for the identification of genotypes, especially molecular tools and the investigation of the virulence of the different genotypes to potential aquacultural hosts in marine and freshwater. Further, it raises questions concerning actions to be taken against the disease and success of the initiated actions, particularly in the light of an apparently high adaptability of the virus to new environmental conditions and changes in virulence for certain fish species. These characteristics of the virus mean that taking different actions according to genotype is at least a questionable approach.