A first-of-its-kind University of Stirling study could better inform strategies to control salmon lice, after researchers uncovered major differences in the secretions the parasite produces as larvae.

Like other parasites, such as mosquitoes and ticks, salmon lice secrete substances from their glands which make it easier for them to feed or evade their host’s immune system.

The study, led by PhD researcher Alexander Dindial alongside colleagues Professor James Bron and Dr Sean Monaghan at Stirling’s world-renowned Institute of Aquaculture, in collaboration with Moredun Research Institute’s Kevin McLean, compared secretory proteins released by infectious young, larval stage, salmon lice (copepodids) with those found in adult lice.

They found considerable differences in proteins between the two life stages, which they believe could provide an important insight for successful early infection on susceptible hosts such as Atlantic salmon.

Salmon lice feed on the skin, mucus, and blood of the host, causing open wounds that can lead to infection – reducing their market value, and increasing the chances of secondary infections and susceptibility to other diseases.

Various treatments have been developed to tackle sea lice infestations in aquaculture – which costs the industry more than £1bn a year – but these can be expensive, unreliable, environmentally damaging and negatively impact animal welfare.

In total, 143 secretory proteins were found in copepodid secretions that are absent in adults, including many – such as serpins, previously identified in land‑based terrestrial ectoparasites, that have been shown to play a role in limiting the host’s immune response.

Lead researcher Alexander Dindial explained: “Because this is the very first stage of this parasite’s life cycle, it represents a vital linchpin in control strategies for this species. This work better helps us understand salmon louse biology and could play a vital role in informing future research into control of this parasite, such as through the identification of vaccine targets, which ultimately promote the sustainable production of healthy salmon and enhance global food security.”

The study saw researchers take a sample of larval salmon lice (over 100 copepodids per millilitre of liquid) and incubate them in either filtered seawater or a solution consisting of filtered seawater and isophorone - a chemical naturally found in Atlantic salmon mucus that is known to serve as an attractant for copepodids.

They then concentrated the secretion-containing solutions and analysed the good quality protein present.

A technique called liquid chromatography tandem mass spectrometry analysed the exact protein compositions of each of the samples. This involved the use of a state-of-the-art machine that separates the components of samples, breaks them into fragments, and weighs them - revealing the unique molecular make-up of each protein.

Researchers then filtered the data, identifying the proteins, determining which were secretory in origin, and comparing the compositions of each of the samples.

Dr Sean Monaghan, co-supervisor of the study, added: “This data provides key candidates for vaccines in the future. We are currently exploring the genes of these secreted proteins as part of a large BBSRC funded project, GeNoLice to determine if they are influenced by interactions with the host.”
Investigation of proteins identified in the secretory and excretory products (SEPs) of the infectious copepodid stage of the salmon louse Lepeophtheirus salmonis is published in the journal Veterinary Parasitology.

It was led by the University of Stirling, working in collaboration with the Moredun Research Institute and funded by EastBio.