Trends in Parasitology
Volume 24, Issue 4, April 2008, Pages 176-183
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Review
Physiology and immunology of Lepeophtheirus salmonis infections of salmonids

https://doi.org/10.1016/j.pt.2007.12.010Get rights and content

‘Sea lice’ is a common name for a large number of species of marine ectoparasitic copepods, many of which are widespread and important disease-causing agents that infect both cultured and wild fish. Of these copepods, the salmon louse Lepeophtheirus salmonis is the most extensively studied because of its economic impact on the salmonid aquaculture industry and its possible impacts on wild salmonid populations. Different levels of infection by this parasite can affect the long-term survival and viability of its hosts. In this article, we review the nature of the interactions between L. salmonis and it hosts to identify crucial areas that warrant further research to aid understanding of the impact of infection with L. salmonis.

Section snippets

The biology and ecology of Lepeophtheirus salmonis

When present in high numbers [>0.5–0.75 adult (see Glossary) parasites g−1 fish], the ectoparasitic copepod Lepeophtheirus salmonis causes disease in both farmed and wild susceptible salmon species; without intervention, such infection can have devastating effects on host populations 1, 2, 3, 4, 5. Consequently, L. salmonis is the most widely studied of all of the parasitic copepods. Much of this interest stems from the view that L. salmonis derived from aquaculture sources is having a negative

Limitations of laboratory-based studies

The majority of information about the interactions between L. salmonis and its hosts has been obtained from laboratory studies. These studies have used different host species and/or strains, host and L. salmonis life-history stages, and levels of infection. Furthermore, there are large variations in the experimental conditions under which these studies were conducted and the tools used to study the interactions. These factors make it extremely difficult for comparisons between studies and

Host species differ in their susceptibility to infection

Initial work on the interactions between L. salmonis and its hosts used microscopy to observe host tissue responses to louse attachment and feeding (for reviews, see Refs 2, 3). Attachment and feeding can have varying effects depending on the species of host and the parasite life-history stages that are present (Box 3). Based on parasite loss and histological examination of the tissue responses, it was proposed that naïve Atlantic salmon are more susceptible to infection than are naïve

Immunological effects on the host

The observation of limited tissue responses to L. salmonis in Atlantic salmon led to the suggestion that salmon lice, as with other arthropod parasites, might secrete substances to aid feeding and to avoid host immune responses 20, 33. Trypsin has been identified in the secretions of L. salmonis and in the mucus of infected Atlantic salmon 20, 33. The midgut has been identified as the site of trypsin production 33, 34. Trypsin-like proteases are present in the secretions of other arthropod

Pathogenicity

It is well recognized that large numbers of mobile L. salmonis (>0.75 lice g−1 fish) can cause host morbidity and death 1, 2, 3. The stress of initial entry into seawater has recently been shown to exacerbate the physiological impact of salmon louse infection on sea trout (Salmo trutta) smolts, so that even lower levels of infection (13 lice per fish, ∼0.35 lice g−1 fish) can increase the chance of morbidity [43]. In most cases, morbidity can be attributed directly to physical damage caused by

Future perspectives

The dynamic parasite–host interaction between salmon lice and salmon is understood at a basic level. We have stressed some of the limitations of the data obtained from wild studies. In the future, controlled laboratory experiments or small-scale field experiments are needed to confirm observations made from natural infections. This is especially true with regard to the Pacific Ocean; knowledge is needed about host–parasite interactions in this area between L. salmonis and multiple salmonid

Acknowledgements

G.N.W. thanks R.S. McKinley, P.A. Bjørn and B. Finstad for their financial and technical support during his research involving salmon lice. M.D.F. thanks J.F. Burka and N.W. Ross for their insight and expertise in eliminating some of the knowledge gaps discussed in the present review.

Glossary

Adult
the final stage of the parasite life cycle, whereupon it becomes reproductively active. This life stage is also mobile and uses the suction of its cephalothorax to maintain its attachment to the host.
Chalimus
there are four chalimus stages in the salmon louse; these refer to the stages at which the parasite is attached to its host via the frontal filament.
Copepodid
mobile infective stage of the parasitic copepod. This life stage is the first obligate parasitic stage of the louse, and if it

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