Viral haemorrhagic septicaemia virus (VHSV) remains viable for several days but at low levels in the water flea Moina macrocopa

Viral haemorrhagic septicaemia virus (VHSV) Genotype IVb has been isolated from amphipods belonging to the genus Diporeia, but it has yet to be established whether crustacean zooplankton act as vectors of this virus for fish species. Therefore, we evaluated the viability of infectious VHSV in the water flea Moina macrocopa. VHSV was re-isolated from replicate groups of M. macrocopa that had been immersed with 10^8.0, 10^7.0, and 10^5.0 TCID₅₀ ml^-1 of VHSV (DK-3592B, Genotype Ia). Furthermore, 40 M. macrocopa that had been immersed with 10^8.0 TCID₅₀ ml^-1 of VHSV for 72 h had VHSV titers of 10^2.7-10^4.3 TCID50. Thus, VHSV was clearly taken up by M. macrocopa and remained viable in this crustacean for several days. However, no mortality was observed over a 28 d period in rainbow trout Oncorhynchus mykiss that were fed VHSV-contaminated M. macrocopa for 14 d, and we found that the virus titer significantly decreased after a 4 h incubation with pyloric caecal extracts from rainbow trout, indicating that passage through the gut is likely to result in a significant decrease in viral titer. This may explain why consumption of prey containing low levels of VHSV did not result in clinical VHS.