In Northern Europe and Canada, the salmon louse, Lepeophtheirus salmonis (Krøyer), seriously affects the marine phase of salmon production. Although the problem is long-standing, the development of sustainable methods of pest management has been unable to keep pace with the intensification of production, leading to large-scale reliance on very few chemotherapeutants. This runs the risk of selecting for genetically determined resistance in target organisms. There are many examples of similar evolutionary adaptations in arthropod pests of arable crops, livestock and human health. Several hundred pest species are now documented as being resistant to one or more chemical classes of insecticides and acaricides. Many of these compounds are identical or closely related to ones currently employed against salmon lice. It is, therefore, opportune to consider what lessons have been learnt from contending with resistance in terrestrial organisms, the implications for sustainable use of chemotherapeutants in aquaculture, and the potential for developing effective resistance management strategies. An EU-funded project named SEARCH (QLK2-CT-2000-00809) has been initiated to explore in more detail the diagnosis, incidence, dynamics and management of resistance to chemotherapeutants in L salmonis.

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This paper describes a rapid, standardised method for testing the susceptibility to bluetongue virus (BTV) of northern Palaearctic Culicoides species midges that can be used to assess the competence of both field-caught and laboratory-infected midges. The method has been used to show that Culicoides scoticus can replicate btv serotype 8 and BTV serotype 9 strains to more than 3 log(10) TCID50/midge, the first evidence of the potential of this species to transmit BTV.

Lepeophtheirus salmonis is a specific parasite of salmonids that occurs in the Atlantic and Pacific Oceans. When infestations are heavy fish mortality can occur although the factors that are responsible for causing epizootics, especially in wild salmonid populations are still largely unknown. Over the past 20 years this parasite has caused significant economic losses in farmed salmon production and possibly in wild salmonid populations locally. Understanding the connectivity between populations is crucial to an understanding of the epidemiology of infections and for management of infections in aquaculture. Data from genetics, pesticide resistance, larval dispersal models and spatial and temporal patterns of infestation in wild and farmed hosts suggests a spatially highly structured metapopulation the components of which have different levels of connectivity, probabilities of extinction and influence on the development of local infestations. The population structure is defined mainly by the dispersal dynamics of the planktonic stages and the behaviour of the host. Until recently virtually nothing was known about the relationship between the parasite and the host, or how the host may influence lice at local or population level. Typically, impacts on the host have usually been reported in terms of pathological lesions caused by attachment and feeding of the adult stages, as well as localised mild epithelial responses to juvenile attachment. However many studies report pathology associated with severe infestation. Recent new studies on the host-parasite interactions of L. salmonis have shown that this parasite induces stress-related responses systemically in the host skin and gills and that the stress response and immune systems are modulated. In the second part of this review, these new studies are presented, together with results from other host-parasite model systems where data for caligid sea lice are missing. One of the most revealing methods reported recently is the application of a net confinement stressor to examine modulation of the stress response and immune system of the host fish. This approach has shown that although until now, infective stages of L. salmonis were not thought to affect the host, they do induce systematic effects in the host that result in a stress response and modulated immune system. Host-parasite interactions affecting these stress responses and the immune system may be key factors in facilitating epizootics by reducing the host's ability to reject the parasites, as well as reducing disease resistance under some environmental conditions. The host-parasite interaction therefore needs to be incorporated into any model of population structure and dynamics.

Infection o f postsmolt Atlantic salmon (Salmo solar) with three, six, or 10 preadult and adult sea lice (Lepeophtheirus salmonis) per fish resulted in changes to epithelial structure and at sites in the skin and gill, distant from lice attachment and feeding. In the skin, increased apoptosis and necrosis occurred in the superficial epithelial cells and numbers o f mucous cells decreased. In the gill, where no lice were found, uplifting o f the epithelium, intercellular swelling, and infiltration by leukocytes occurred in filaments and lamellae. High cell turnover o f chloride cells was associated with significantly elevated gill Na+/K+-ATPase activities. Serum chloride levels were elevated in the 3 and 6 lice/fish groups, and the serum Na to Cl ratio was lower in all parasitized groups at 5 days. The results indicate that infection with low numbers o f the preadult and adult parasite induced changes characteristic o f a stress response. In the low-and medium-infested groups, homeostatic recoveiy had occurred by 10 days, but recovery was incomplete in the highly infected group. Thus, 10 lice per fish, which is a low infestation level in nature, is stressful and creates a long period during which the overall condition o f the skin and gill epithelia may render the fish susceptible to secondary infections.