Fish immune responses to parasitic copepod (namely sea lice) infection

• Differential host responses can be associated with different parasitic life stages.
• TH2-like responses associated with copepod infection have been identified in fish.
• Strong cellular and TH1-like responses are associated with reduced copepod burdens.
• Immune responses to copepod infection occur in presence of parasite modulation.
• Immunostimulation of fish can boost host responses to parasitic copepod infection.

Parasitic copepods, in particular sea lice, have considerable impacts upon global freshwater and marine fisheries, with major economic consequences recognized primarily in aquaculture. Sea lice have been a contentious issue with regards to interactions between farmed and wild populations of fish, in particular salmonids, and their potential for detrimental effects at a population level. The following discussion will pertain to aquatic parasitic copepod species for which we have significant information on the host-parasite interaction and host response to infection (Orders Cyclopoida, Poecilostomatoida and Siphonostomatoida). This review evaluates prior research in terms of contributions to understanding parasite stage specific responses by the host, and in many cases draws upon model organisms like Lepeophtheirus salmonis and Atlantic salmon to convey important concepts in fish responses to parasitic copepod infection. The article discusses TH1 and TH2-like host responses in light of parasite immunomodulation of the host, current methods of immunological stimulation and where the current and future work in this field is heading.