Food Safety Consortium researchers at Iowa State University found that weanling pigs, about four weeks old, are susceptible to West Nile virus infection from a relatively small level of virus delivered by mosquitoes.

The pigs’ susceptibility to the virus appears to decrease as the pigs grow older, but more research remains to be done to determine the risk level of older, market-age pigs.

Among the younger pigs studied, levels of the virus in the bloodstream can be quite high. “There’s enough virus in that blood that I wouldn’t want to cut myself with a knife that just went through there,” says Kenneth Platt, an ISU veterinary microbiology professor who conducted the study.

It’s also common among other animals and birds for the level of virus concentration to be much higher in the younger species than the older ones. If more extensive research into market-age 200-pound pigs shows the virus concentration at high levels for three or four days similar to what is found in the weanlings, “then I think there should be concern with individuals who are handling those carcasses and that fresh meat product,” says Platt.

The next step would be to examine how long the virus remains infective in fresh meat and frozen meat products.

Infected pigs can pose a problem not only for plant workers who come in contact with them, but can also cause uninfected mosquitoes to contract West Nile virus when they feed on pigs with the virus in their bloodstream. Even so, the pigs are still not a significant source of the virus for the mosquitoes, notes Platt.

Mosquitoes remain the most likely source to infect pigs when they are housed together on the farm or at an abattoir. “In our observations we’ve had infected and uninfected pigs in the same pen in intimate contact, and we saw no evidence of transmission from pig to pig,” says Platt. “The most logical conclusion is that pigs are going to become infected by the bite of mosquitoes.”

More research on the subject is necessary. For example, if a six-month-old market-age pig has a high level of the West Nile virus in the bloodstream, the threat to the processing plant employee could be exposure by cutting through infected meat or by incurring a scratch from a sharp bone.

“If you pick it up and handle it and you don’t have any scratches on your hand, I don’t think that would be a problem,” says Platt. “If you cut yourself on a bone or knife, it could be a problem. But we don’t really know yet.”