CRISPR pigs are ready for breeding, but Danish farmers are not interested
Every year, millions of pigs cough and snort in barns after being infected with the infectious disease PRRS (porcine reproductive and respiratory syndrome).
In addition to discomfort for the pigs, it also costs the global industry billions of DKK annually, so there is a strong desire to keep the virus out of the barns.
As the disease’s name suggests, it is a disease that affects not only the lungs, but also the reproductive system of pigs. If pregnant sows become infected, they risk, at best, that the piglets will be born with a low weight, and at worst, that they will be stillborn.
They are vaccinated and isolated, but Genus, a British biotech company, has for a number of years been working to get rid of the disease in a different way, namely by destroying the receptor in the pigs’ cells that the specific PRRS virus attaches itself to.
It is the so-called CD163 receptor that Genus has exploited by sending the CRISPR scissors into the cells of early pig embryos in the laboratory, before they were then put into a sow for breeding.
Danish farmers choose a different way
According to the journal Science, the company already has some of the first lines of pigs ready for commercial breeding and is hoping for FDA approval this year, but Denmark is not interested in CRISPR-edited pigs for the time being.
This is emphasised both by the research community and the industry.
“It’s a very widespread virus, with which a third of the Danish pig herds become infected, but we believe that there are easier and better ways to avoid disease,” says Kristian Møller, director of Veterinary and Quality Services at the Danish Agriculture & Food Council.
As far as PRRS is concerned, farmers with at least 10 sows or 100 pigs in the barns have since October 2023 been tasked with reporting whether they have infected pigs. This is part of a national programme backed by an ordinance from the Danish Ministry of Food, Agriculture and Fisheries.
If there are any pigs with PRRS, they must be regularly visited by a veterinarian, and the price of slaughter pigs needs to be lower. The herds can be rehabilitated by not bringing in new animals, vaccinating, and keeping the pigs as far away from the neighbouring farms as possible.
“It should serve as an incentive to get rid of the virus, and the programme will run until we’ve hopefully eliminated it. Zealand and the surrounding islands are almost free of the virus today,” he says.
Not easy to tinker with genes
Kristian Møller emphasises that this is a virus that requires a national programme with regional efforts as it can be carried so far by the wind that a neighbouring farm can be infected by a sick herd. And although effective vaccines have been created, they do not keep the infection completely contained.
But is it not a quite obvious to eradicate the virus by making the pigs immune to it?
He does not think so, however problematic the virus may be during an outbreak.
“We have read about the method and also have experience with what happens when you tamper with genes in breeding, and it’s not that easy,” Kristian Møller says.
“We tried to breed pigs that lacked a gene that E. coli bacteria attached to, which gave the piglets weaning diarrhoea, but over time, the bacteria just found a different place to attach to,” he says.
He therefore puts more trust in the more common Danish strategy: To take advantage of the fact that we are surrounded by water and thus have some protection from the outside, that we are a small country with a manageable number of herds, and that we do not import pigs from abroad.
He takes an example from the 1980s, when Aujeszky’s disease caused by a herpes virus was eradicated by a similar clean-up tactic.
“But I can imagine that the method may be relevant in other countries that don’t have the same advantages as us, but for the time being, I’m sure that vaccines and our program will make a big difference,” Kristian Møller says.
Previous attempts failed
At Aarhus University’s Department of Animal and Veterinary Sciences, Professor Søren Alexandersen is not that keen on the genetically modified pigs either.
“We’ve been talking about the possibility for as long as we have known about PRRS in pigs, because there will often be a population of an animal that is less susceptible to a given disease, and we can then choose to breed it,” he says.
“The problem is that in the cases we’ve seen, it often affects productivity, and since we have some of the world’s best pigs, we don’t want to mess with them too much,” Søren Alexandersen says.
He mentions a project that tried to get rid of the tumour-causing Marek’s disease in chickens by breeding a line that had proved highly resistant to the virus that caused the disease.
But these chickens were, on the other hand, far weaker in a number of aspects in terms of production when it came to the big picture.
“And then they had to go back to vaccinating instead,” Søren Alexandersen says.
“It does seem that the CRISPR pig, apart from the small piece of DNA that has been cut out, is identical to the pigs that have not had the PRRS gene cut out. But we have some special pigs in Denmark, into which it would then have to be bred, and it would probably take five to ten years before it’s bred in, and then it may be that the virus has already mutated, or that there’s a completely different virus that causes problems,” he says.
