By purging the halothane or “stress” gene from the genetic pool in the 1990s, U.S. pork industry participants thought they would eliminate the main cause of porcine stress syndrome and the resulting pale, soft and exudative pork.

But, Michigan State University research shows that some pigs known to PSS-free are abnormally sensitive to halothane, and may produce inferior quality pork when subjected to rigorous handling. Now the pork industry faces a new challenge.

A defect in the halothane gene has been linked to a pig’s increased susceptibility to become stressed and to produce PSE pork. Even with the help of the HAL-1843 DNA test and removal of the mutant gene through genetic selection, the incidence of PSE pork has increased. Specifically, it has increased from 10.2 percent in 1993 to 15.5 percent in 2003, according to the National Pork Benchmarking Audit. At the current level, PSE is estimated to cost the industry $90 million a year.

Suspecting there had to be other genetic reasons for PSE pork, Matt Doumit, Michigan State animal scientist, graduate student Chuck Allison, and collaborator Roger Johnson of Farmland Foods, studied the effects of halothane sensitivity and animal-handling methods on several swine genetic lines.

They found that pigs free of the HAL-1843 defect still exhibited abnormal sensitivity to halothane anesthesia. These halothane-sensitive pigs also appear to be more prone to becoming non-ambulatory and producing inferior quality pork when handled rigorously.

“This research shows that it’s more complex than just a single gene defect,” says Doumit. “We saw quite a range of responses from the pigs exposed to halothane — from a normal reaction (sleeping) to limbs becoming rigid and skin blotching.”

Allison was surprised to see the HAL-1843-free pigs react to halothane in a negative way. “All pigs will react to halothane gas because it’s an anesthetic, but we never
anticipated the level of adverse responses. We’re trying to understand the biological reasons involved.”

Using a scoring system, the researchers rated pigs in the trial according to halothane response. Once classified, pigs were raised under commercial conditions, and some were put through rigorous handling prior to slaughter.

Pigs that were handled gently, regardless of their halothane score, showed little difference in meat quality. But halothane-sensitive pigs that were handled rigorously tended to produce inferior quality pork.

The researchers say the pork industry’s work on these issues is far from over, but they remain hopeful. “We are fortunate that the HAL-1843 defect was essentially eliminated in the 1990s,” says Doumit. “But now we need to focus on identifying other genetic abnormalities and eliminate them.”

Citing the more than 30 known genetic abnormalities associated with a related gene in humans, Doumit says the likelihood of finding more genetic oddities in pigs is great. “It’s likely there are other genetic causes for the increase in PSE pork and the occurrence of non-ambulatory pigs in the industry. That’s why we’re committed identifying the causes and finding ways to solve them.”