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The effects of sea lice in modulating salmonid susceptibility to viruses



The salmon louse, Lepeophtheirus salmonis, is a naturally occurring marine parasite of salmonids and an important pest of farmed Atlantic salmon in both eastern and western Canada which can cause fish health issues resulting in economic impacts. Salmon lice are found on a number of wild host species in the marine environment and co-occur with endemic viruses, therefore co-infections of salmon lice and viruses are likely. L. salmonis-infected salmon may be more likely to develop severe infections with second pathogens, either because the louse serves as a vector, transmitting the secondary pathogens, or because the salmon louse infection compromises the host immune response. The latter possibility is supported by the observation of reduced expression of genes associated with anti-viral responses and adaptive immunity in several salmon species during laboratory infections with L. salmonis.

This research project focused on one viral pathogen, the Infectious Hematopoietic Necrosis Virus (IHNV). IHNV infects wild and cultured salmonids throughout the Pacific Northwest of North America. There is a need to better understand if sea lice parasitism influences virus transmission and susceptibility of salmon to infection. The goal of the study was to determine level of sea lice infestation at which intervention or pest management strategies may be needed to prevent further damage from viral infection. This research provides scientific information for management decisions regarding sea lice infestation thresholds for use in salmon aquaculture.


This research compared base-line physiological responses of Atlantic salmon and Sockeye salmon during L. salmonis infections. The results confirmed that the impacts of co-infections with salmon lice and IHNV are significantly greater than those of either pathogen alone. Specifically, osmoregulatory impacts of L. salmonis infections were more severe in co-infected sockeye salmon. Future research on other commercially valuable Pacific salmon species is required to better understand the mechanisms by which sea lice influence the host susceptibility to co-infections.


Long, A., Garver, K.M. and Jones, S.R.M. 2019. Differential effects of adult salmon lice Lepeophtheirus salmonis on physiological responses of sockeye salmon and Atlantic salmon. Journal of Aquatic Animal Health 31:75-87.

Long, A., Garver, K.M. and Jones, S.R.M. 2019. Synergistic osmoregulatory dysfunction during salmon lice (Lepeophtheirus salmonis) and infectious hematopoietic necrosis virus co-infection in sockeye salmon (Oncorhynchus nerka) smolts. Journal of Fish Diseases 42:869-882.

Program Name

Program for Aquaculture Regulatory Research (PARR)


2014 - 2017

Principal Investigator(s)

Simon Jones
Research Scientist, Fisheries and Oceans Canada, Pacific Biological Station, Pacific Region

Team Member(s)

Kyle Garver, Research Scientist, Fisheries and Oceans Canada, Pacific Biological Station, Pacific Region


Mark Fast, Atlantic Veterinary College

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