Sea Lice
Lepeophtheirus salmonis
Animal model: Atlantic salmon, Rainbow trout
Applications: Vaccines, functional feeds, repellents, physical barriers, environmental treatments (fresh water, temperature shock), immersion and in feed treatments, benchmarking commercial treatment efficies, genetic selection
Type of model: Infestation at copepodid or motile stages
Clinical Signs of Disease in our Model: Pre-adult male and female sea lice on fin of Atlantic salmon
Challenge conditions: Salt water (25-35 ppt); 6-15C
Other end points: Typical sampling and lice count regimen carried out
Starting Fish Size: 50 grams to 2kg
Summary:
The infestation of cultured Atlantic salmon (Salmo salar L.) with sea lice (Lepeophtheirus salmonis and Caligus spp.) is the most economically significant issue which impacts the culture of the species globally. The losses incurred can account for up to 10% of farm revenues and recent economic analyses have demonstrated that lice infestation and treatment accounts for as much as a third of the operational costs of growing out at sea. Numerous treatment options for sea lice are available and are used in conjunction with region-specific integrated pest management strategies. Drugs and pesticides have, and continue to, dominate these treatment regimens in many salmon farming regions. A recent increase in the use of alternative treatments (e.g. cleaner fish, plankton nets, deep water lighting, etc.), has emerged over the last decade, especially in Norway where alternative treatments have been more frequently used than drugs and pesticides since 2017. A major driver of the switch to alternative therapies has been the development of multi-drug resistant strains of sea lice, with few available replacement therapies. Thus, gaps in the market exist both for novel drugs/pesticides and alternative means of controlling sea lice.
At CAT, in vitro and in vivo models for testing treatments against L. salmonis have been developed to evaluate new and existing therapies of different types including vaccines, functional feeds, repellents, and environmental manipulations (temperature, etc.). Our models are tailored to the specific hypotheses that our clients wish to test and commonly include single or multiple lice stages at once. In addition, in vitro bioassays have been developed to screen lice for susceptibility to commercial and novel treatments. Bioassays are commonly incorporated into specific research studies and for routine monitoring of lice resistance/susceptibility on farms to best inform treatment decisions in the field.
In vitro studies with sea lice in the absence of hosts are considered in vitro bioassays and can be completed with free-swimming larvae (nauplii I and II), infectious copepodids, and motile stages (pre-adult I, II, and adult). In these studies, the solubility of the treatment in seawater is tested directly and for low-solubility compounds, a range of emulsifiers tolerated by lice and known to improve solubility are available. Lice are exposed to solubilized compounds for a designated duration followed by assessment of moribundity, mobility, and/or time to recovery. These assessments are often binary, producing percent survival at fixed concentrations or half-maximal effective concentrations using a log-based spectrum of doses. Alternatively, semi-quantitative scoring systems have been adapted or developed for all life stages to qualify the results of exposure of lice to the test compounds.
In vivo studies with sea lice infesting salmonids are used for testing of product safety and effectiveness. Products can be applied prophylactically or as a treatment for lice infestation, with the administration via a bath, injection, oral gavage, or in-feed. Infections with L. salmonis are completed by immersion with infectious copepodids followed by lice counts at predefined time points where the lice load of treated animals is compared to that of controls. Alternatively, mobile stages of lice can be transplanted onto hosts to create an adult-only population or a mixed infection in combination with copepodid exposures. Counts of lice are the most frequently used end-point measure to determine effectiveness; however, scoring of skin lesions and characterization of infection sites (e.g. skin, fins, gills) are also frequently documented as part of tank-based studies.
Copepodid stage of L. salmonis under microscope (10x).
Adult male and female L. salmonis displaying naturally occurring differences in pigmentation.
L. salmonis on the ventral side of an Atlantic salmon (S. salar L.).