Animal-nutritionist John Goihl knows Minnesota farmers who feed the remains of dead baby pigs to hogs used for breeding in attempts to ward off infections of a deadly virus in offspring.

In Oklahoma, farm workers are mixing manure from swine sick with the disease, known as Porcine Epidemic Diarrhea virus (PEDv), into the food of healthy animals to build their immunity.

In Kansas, farmers are spraying a mixture of hog manure containing the virus and water on the noses of pigs to create a "natural vaccine."

Across the Farm Belt, U.S. pork producers are doing whatever they can to shore up their herds' defenses against the virus that killed up to 8 million pigs, a tenth of the nation's herd, two years ago, and that farmers fear could return this winter.

The virus, which causes severe diarrhea that kills baby pigs, thrives in cold weather, and declining immunity in the U.S. herd has raised the risk of another outbreak to the highest level since 2013, when pork prices soared to record highs, veterinarians said. A resurgence could drive up prices again and hurt profits for processors including Tyson Foods and JBS USA [JBS.UL], which have benefited from low hog prices.

Farmers are better prepared to fight the disease than they were two years ago after implementing procedures to prevent the spread of the virus via farm vehicles, workers' shoes and animal feed. Veterinarians said that attempts to deliberately expose hogs to the virus also help reduce the risk of an outbreak as big as the one that began in 2013, although it is not clear how many farmers are taking such precautions.

Michael Blackwell, chief veterinarian officer for the Humane Society of the United States, said feeding baby pigs to other hogs "seems to be pretty barbaric," but that he understands why farmers are doing it. "It is not as inhumane as having millions of piglets killed in an outbreak," he said.

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VACCINES AND "FEEDBACK"

Veterinarians said commercial vaccines available from Zoetis Inc and Merck & Co-owned Harrisvaccines offer limited help preventing outbreaks. They do not specifically target the gut and are mainly effective on hogs that have already been exposed to the virus.

Harrisvaccines said that vaccines are not a "silver bullet," and Zoetis said there was not enough data that prove that vaccines effectively protect herds that have not previously been infected.

Such limitations, along with concerns that immunity levels have waned, make farmers turn to methods such as "feedback," where intestines of piglets killed by the virus are fed to female pigs used for breeding. Immunity has declined because a growing number of hogs have never been infected, meaning they lack natural immunity they could pass on to their babies, veterinarians say. Also, immunity wears off over time in hogs that were previously infected.

Feedback allows female hogs to become infected and pass on immunity to piglets, which are more likely to die from the disease than older hogs. Those fed infected food or otherwise exposed to the virus usually become sick for a few days, but then get well again.

Purposefully exposing hogs to the virus is "really important because that's one way we can have local establishment and local building of immunity," said Lisa Becton, director of swine health information and research for the National Pork Board, an industry group.

Matt Ackerman, a prominent hog veterinarian based in Indiana, estimates that more than a million pigs could die between June 1, 2015 and May 31, 2016, from a return of the virus, far below the 2013 levels. Such losses would occur if 10 percent of sow farms become infected, which Ackerman said was a "very real expectation" that would be devastating to producers.

From July 1 to Dec. 4, 2 percent of herds cumulatively reported new infections, according to an analysis from Bob Morrison, a professor at the University of Minnesota. That is down from 56 percent between July 2013 and June 2014 and 9 percent a year later.

For farmers seeking to deliberately expose their herds to the virus, one step is to identify infected hogs, so they can serve as "vaccine" donors. At Prestage Farms in Oklahoma, which sells hogs to Seaboard Corp, workers place pieces of rope into pig pens for hogs to bite. The rope is then tested for the virus, says Ron Prestage, who runs a division of the family-owned company.

If the disease is detected, workers scoop up manure from the pens to mix with feed for female breeding hogs, so they can pass on antibodies to piglets through their milk, said Prestage, who is also president of the National Pork Producers Council.

"They get a little bit of a belly ache and have diarrhea and then get over it," he said.