Tackling a tiny terror of the sea

A UCD team has had some promising results in the search for a vaccine against sea louse, a parasite that has plagued Irish salmon farms and is threatening wild stocks

THE ANIMAL glowing blue in the picture below right might look pretty, yet its effects are anything but. It’s a sea louse that attaches to salmon and causes damage to skin and scales, along with a general decline in the fish’s health.

The not-so-pretty parasite is a major problem for fish farmers and could potentially spread to wild populations, which is why researchers at University College Dublin are looking for an effective vaccine to help fish fend off the unwelcome hitchhikers.

Salmon farming confines large numbers of fish at close quarters, a practice that has provided the parasitic louse with something of a sitting target. So despite its tiny size, the sea louse costs Ireland an estimated €670,000 each year, with losses of around €200 million annually worldwide, according to Grace Mulcahy, professor of veterinary microbiology and parasitology at UCD.

“[The sea louse] burrows in and causes little erosions or bleeding to appear, and that progressively affects the wellbeing of the fish – they don’t put on weight as quickly, they don’t feed so much, there are unsightly changes to their skins, and if you see them in the supermarket, you wouldn’t be as inclined to go for them,” she says.

“There are also concerns that lice populations from farmed fish can also spread over to the environment and affect wild populations as well.”

At present, infestation levels are monitored nationally and chemicals are used to treat the affected fish, but this also raises some environmental concerns, says Mulcahy.

“Some of the drugs we use are very broad-spectrum, so they kill sea-lice ectoparasites but also other crustaceans in the environment. That has spurred the chase for alternative methods of control,” she says.

An alternative approach would be a vaccine to stop the parasite settling into the animal in the first place, but finding a way to block a sea louse is more complicated than it sounds.

“We are all used to thinking of vaccines against influenza and certain bacterial diseases, but parasites, whether they be worms or sea lice, are very much more complicated organisms,” explains Mulcahy.

“If a virus is a single Lego brick, the parasite is like the most complicated Lego toy you can make. It’s a much harder target to attack. If you develop a vaccine against one protein, sure as anything the parasite will have another way around it to avoid the vaccine.”

So to look for the most suitable vaccine candidates, Mulcahy and her team at UCD first combed through information from the fish genome in search of potentially important targets. They based their search on their understanding of other parasites, such as liver fluke in cattle and sheep.

The next step in the project, in which UCD collaborates with the Marine Institute and McGill University in Canada, was to choose the top five most likely candidates and put them to the test.

Working with Dr David Jackson at the Marine Institute, the scientists administered selected protein antigens to the salmon and then looked at how the fish handled infestation. So far, one of the proteins has shown promising results, notes senior researcher Dr Elaine McCarthy.

“Vaccination with this antigen resulted in lower attachment rates and establishment of the parasite on the host when compared to control groups . . . and we are optimistic that this antigen may lead to a vaccine against sea lice in the future,” she says.

Using a vaccine to prevent the parasites from taking hold in the first place could offer a more environmentally and consumer friendly route than using chemicals to treat the fish after the parasites have climbed on board, says Mulcahy.

“If you are vaccinating then you give a protein on a once-off or maybe twice-off basis. The protein is long gone by the time that animal will come into the food chain,” she says.

“All that has changed is the animal has been enabled to make an immune response against the parasite.”