Porcine reproductive and respiratory syndrome (PRRS) gives pigs a fever and cough, but it costs American swine farmers over $600 million a year. Vaccines have been ineffective at fighting it, as has breeding pigs to be resistant. Last year, researchers in the Midwest used CRISPR-Cas9-based gene engineering to generate pigs that lack CD163, the protein that PRRS uses to infect its target cells in pigs. Now, the same team just demonstrated that the pigs lacking the receptor don't get sick when exposed to PRRS.

The three resistant pigs had deletions in both copies of their CD163 genes and thus made no CD163 protein. This lack did not seem to affect them in any adverse way. After weaning, these three and eight wild-type piglets were exposed to the strain of PRRS virus usually used in experimental infections, which happens to be a pretty virulent one.

The pigs were generated at the University of Missouri but infected at Kansas State University—the Kansans who did the infecting didn't know which pigs were which, making this what's called a blinded experiment. All of the pigs were kept in the same pens, so even if the initial infection didn't take, the pigs had plenty of opportunities to get sick from each other.

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