Caligus rogercresseyi as a potential vector for transmission of Infectious Salmon Anaemia (ISA) virus in Chile

Abstract

The sea louse Caligus rogercresseyi is the most serious ectoparasite on farmed salmonids in Chile, mainly affecting Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss). In 2007 the highest infestation ever recorded for this sea louse struck the aquaculture industry. In the same year a serious epizootic due to the Infectious Salmon Anaemia (ISA) virus (Isavirus, family Orthomyxoviridae) also occurred.This study shows that C. rogercresseyi is capable of being a mechanical vector for ISA virus. The findings indicate that even after 48hours away from its host C. rogercresseyi can still transmit ISA virus to naïve Atlantic salmon, which was confirmed by sequencing of the virus. Both the amount and prevalence of ISA virus RNA were decreasing the longer C. rogercresseyi was away from its host. This indicates that ISA virus is not replicating in the lice and hence, it is not likely that C. rogercresseyi could be a biological vector.ISA virus positive Caligus sp. nauplius larvae or copepodids were neither detected inside nor outside the Atlantic salmon farming cage experiencing ISA outbreaks. Nor was it possible to detect ISA virus RNA in nauplius larvae or copepodids retained in the lab, even if they were the offspring of ISA virus positive C. rogercresseyi population. This indicates that ISA virus is not vertically transmitted from adult C. rogercresseyi to nauplii. Based on these results it is not very likely that Caligus sp. nauplius larvae or copepodids are vectors for ISA viruses.For the salmonid industry this means that a co-infection with ISA virus and C. rogercresseyi increases the risk of transmission of ISA virus within the site and also to other sites in the same hydrographical area.