When covid-19 began to race around the world, countries closed businesses and told people to stay home. Many thought that would be enough to stop the coronavirus. If we had paid more attention to pigs, we might have known better. When it comes to controlling airborne viruses, says Bill Christianson, “I think we fool ourselves on how effective we can be.”
Christianson is an epidemiologist and veterinarian who heads the Pig Improvement Company, in Hendersonville, Tennessee. The company sells elite breeding swine to the pork industry, which for the last 34 years has been fighting a viral disease called porcine reproductive and respiratory syndrome PRRS.
The pathogen causes an illness known as blue ear, for one of its more visible symptoms; when it first emerged, in the 1980s, it was simply called “mystery swine disease.” Once infected with PRRS (pronounced “purrs”), a sow is liable to miscarry or give birth to dead, shriveled piglets.
“And I’m going to say yes, it’s worse for pigs than covid is for us,” says Christianson.
To stop PRRS, as well as other diseases, pig farmers employ measures familiar to anyone who has been avoiding covid-19. Before you enter a secure pig barn, you get your temperature taken, shower, and change clothes. Lunch boxes get bathed in UV light, and supplies are fogged with disinfectant. Then there’s the questionnaire about your “last pig contact”—seen any swine on your day off? Been to a country fair? (Answering yes means a two-week quarantine away from work.)
Despite the precautions, the virus can slip in. Once inside, it quickly spreads in the close quarters. Swift “depopulation”—i.e., culling—of the animals is the most effective way to get rid of it. In bad years, American pig farmers lose $600 million to PRRS.
Now Christianson’s company, which is a division of the British animal genetics firm Genus, is trying something different. Instead of trying to seal animals off from the environment, it’s changing the pigs themselves. At an experimental facility in the central US (the location kept secret for security reasons), the company has a swine IVF center and a lab where pig eggs are being genetically edited using CRISPR, the revolutionary gene scissors.
During a virtual tour, a worker carried a smartphone through the editing lab into the gestation area, where sows spend nine months until giving birth—“farrowing” is the farmer’s term. Then he led the way to a concrete room where gene-edited piglets grunted and peered at the camera. According to the company, these young pigs are immune to PRRS because their bodies no longer contain the molecular receptor the virus docks with.
Every virus attacks cells by fusing with them and injecting its genetic cargo. With covid-19, the virus attaches to a receptor called ACE-2, which is common on airway and lung cells—the reason the disease causes problems with breathing. With PRRS, it’s CD163, a receptor on white blood cells. These experimental pigs don’t have a complete CD163 gene because part of it was snipped away with gene editing. No receptor, no infection.