One of the worst things that can happen to a pig farmer is a pen infected with porcine reproductive and respiratory syndrome virus (PRRSV). It emerged in the 1980s, and the syndrome now afflicts these hoofed animals worldwide, causing illness, death and miscarriage. In fact, it has been designated the most economically significant disease for swine, costing livestock producers in North America $600 million annually from deaths and medical treatments. Vaccinations have mostly failed to prevent the syndrome's spread, but a new approach by biologists at the University of Missouri may mark a turning point. They are one of the first teams to develop a commercial agricultural application for the revolutionary CRISPR/Cas9 gene-editing method—to breed pigs resistant to infection.

CRISPR/Cas9 is a gene-manipulation tool that allows scientists to make changes to DNA with razor-sharp accuracy. The tool has generated excitement in the research community because it allows rapid modification of gene function, replacing older and less efficient methods. For porcine reproductive and respiratory syndrome, Missouri's Randall Prather, Kristen Whitworth and Kevin Wells turned to the technique to breed three piglets that lacked a protein on cells that acts as a doorway for the virus. The edited piglets were grouped together in a pen with seven normal piglets, and then they all were inoculated with PRRSV.

About five days later the normal pigs grew feverish and ill, but the genetically edited pigs did not. Despite sharing close quarters with their sick pen mates, they remained in top health throughout the 35-day study period. Blood testing also revealed that the edited animals did not produce antibodies against the virus—further evidence that they evaded infection entirely. “I expected the pigs would get the virus but not get as sick,” Prather says. “But it is just night and day. The pigs are running around with the other pigs coughing on them, but they are just fine.” The study's results were published in the journal Nature Biotechnology.

This work and other recent experiments demonstrate the promise of CRISPR/Cas9 for the care of domestic animals. Late last year geneticists at the University of California, Davis, employed the new technique to breed dairy cows that do not grow horns. The outcome is a boon: cows are routinely dehorned to protect farmers and other cattle from being injured, but the process can be brutally painful and dangerous for the bovines.

More livestock will likely be produced in such a way, says Alison Van Eenennaam, a geneticist who worked on the development of the hornless cows. “This is analogous to breeding,” she notes. “It's just precision breeding.”

Click here to learn more about the role of CRISPR technology in agriculture.