Erik Slinde has spent 40 years developing vaccines. Not for humans; for salmon. “Back in the 1980s, it was looked upon as a joke,” says Dr Slinde, the former director of aquacultural research at the Institute of Marine Research in Norway.
Nobody is laughing now. Sea lice—rice-grain-sized crustaceans that have evolved to parasitise salmon fish and live inside them, munching away at their skin, protective mucus and blood—have been ravaging salmon aquaculture worldwide. The burden is heavy—infestations cost the British aquaculture industry alone more than £20m ($25.6m) each year—and, for now, there are no good, long-term solutions. The development of a successful louse-preventing vaccine could change that. Some teams think they might be close.
Sea lice come in many varieties, but all share a common modus operandi. By eating salmon alive they weaken them and compromise their immune systems, leaving entire farm populations susceptible to new infections acquired through their open wounds. As they enjoy warm waters dense with fish, aquaculture and climate change have boosted their numbers.
To lose the lice but save the salmon, some farms apply potent, broad-spectrum chemical pesticides on all of their fish. These achieve decent results, but are heavily polluting. Years of overexposure have also rendered many lice resistant. Mechanical delousing systems (brushing, bathing and the like) require expensive machinery and a lot of personnel, as well as stunting the fish’s growth, raising their stress levels and making them yet more vulnerable to disease.
A vaccine capable of immunising salmon against the parasite would be the ideal solution: practical, safe and eco-friendly. And the idea is not as mad as it sounds— young farmed fish regularly get an abdominal prick to protect them against pathogens. But sea lice are a trickier foe. “Parasites are really hard to get vaccines against,” says Ian Bricknell at the University of Maine. “They have evolved so closely with their hosts, they develop mechanisms to overcome the immune defences.”
Traditional vaccines work by injecting neutralised pathogens into a host to help its immune system recognise them in future. This works for viruses and bacteria, which are simple organisms that can be inactivated in a lab. But sea lice are more complex, have different life stages and come in different species, all of which makes inactivation impossible.
This is why scientists have mostly been trying to make peptide-based vaccines instead. Rather than training the host’s immune system to recognise a whole organism, vaccines of this kind inject the host with specific peptides, chains of amino acids found on proteins that are crucial for a parasite’s functioning. This induces the host to make antibodies to neutralise those proteins, and, thereby, the parasite.
Making a splash
In early 2021 Dr Slinde’s team identified one protein crucial to the lice’s digestion system, and copied a chain of 13 amino acids found on its surface. This chain was then synthesised and injected into the salmon. In small-scale initial trials, salmon saw a 70% reduction in European sea lice, and a 92% reduction in Chilean sea lice. Larger trials have been less promising, but Dr Slinde hopes to run more tests soon.
Others are taking a similar approach. A team from the Moredun Research Institute in Scotland and the University of Stirling’s Institute of Aquaculture are using artificial intelligence to identify all proteins in European sea lice that are essential to their survival and recognisable by a salmon’s immune system. They hope to design a jab that can strike several of these at once.
Even if such experiments lead to an effective vaccine, though, it may not be that all farmers can use it. Making proteins from scratch requires cutting-edge tech, meaning that a single dose could cost between $0.80 and $1. Millions of tonnes of salmon are farmed worldwide each year, and some vaccines need to be administered multiple times. Such expenses would raise costs for consumers.
It might not prove affordable, according to Cristian Gallardo Escárate, deputy director of Chile’s Interdisciplinary Centre for Aquaculture Research. His team is returning to an older idea. While analysing the entire genome of the Chilean louse, they saw genes characteristic of bacteria. This led them to discover more than 90 bacteria within the louse gut that are integral to their digestion and, therefore, survival. Since these nested symbiotes are simple unicellular organisms, they can easily be cultured and inactivated in a lab—just as traditional vaccines require.
Dr Gallardo Escárate and his team have already formulated such vaccines. In their trials, 90 to 95% of some 2,000 vaccinated salmon counted as lice-free in the three months after injection, compared with none in the control group. The jab can also be mass-produced at an estimated cost of between $0.10 and $0.20 per dose. It is currently under local regulatory evaluation and is to be tested on more than 1m fish in Chile by the end of 2024.
This is real groundbreaking work, says Dr Bricknell. But as sea lice in other parts of the world have different microbiomes, this might limit the Chilean vaccine’s scope. Ultimately, say researchers, vaccines will become one tool among many. “Will we have a vaccine as good as, say, a measles vaccine, which is 98% effective,” asks Dr Bricknell. “Probably not.”
Source: The Economist.
