As more packing plants and large farms begin “whole herd testing” of their employees, initial PCR test results are showing far more positive COVID-19 test results than expected with upward of 90% being asymptomatic.
“It's a huge relief for people to know,” says Tim Loula, DVM of Swine Vet Center in St. Peter, Minn. “Now that they are testing entire populations, they're finding all these people who are positive, and they're going wow, that's a big relief. I was worried every day going to work and now I'm not because I was positive already and I feel great.”
Loula says the antibody tests are important to further understand the negative PCR tests. Have these people never been exposed, or were they exposed and now have protective antibodies that have eliminated the virus? This is the next piece in getting the U.S. back to work.
Understanding how coronavirus antibodies work in swine could offer human doctors insight as they battle COVID-19.
Although there are varying opinions on antibodies at this stage, Loula says you can’t deny antibodies have been key to eradicating other coronaviruses in swine. He points to the example of porcine epidemic diarrhea virus (PEDV), a novel coronavirus that upset the swine industry in 2013 and 2014.
When PEDV reared its ugly head in the U.S. for the first time, the swine herd had no immunity to this virus strain that was more than 99.5% similar to a Chinese strain, explains Cassandra Jones, associate professor in the Department of Animal Sciences and Industry at Kansas State University.
“It spread quickly and was incredibly detrimental. There was 80% to 100% mortality in suckling pigs until immunity was established or the disease was walked out of different herds, resulting in 3.2% loss to the U.S. pig herd – a $900-million to $1.8-billion economic impact,” Jones says.
Although the U.S. has recognized other coronaviruses in the swine population, such as transmissible gastroenteritis (TGE), respiratory coronavirus and delta coronavirus, Loula says the U.S. herd was completely naïve to PEDV, making it especially vulnerable just like the human population is to COVID-19.
“Because of the severity of PEDv, most producers in the upper Midwest and Canada set out with a goal to rid their herds of the disease (eradication). There were almost no examples of PEDv eradication internationally, although TGE had been successfully eradicated from most U.S. swine herds,” Loula says.
Coronavirus antibodies played a key role in the elimination of the virus, he explains. Veterinarians used a documented herd closure plan of “Load – Close – Homogenize.”
Loula explains: Isowean sow farms filled up sow farms with four to six months’ worth of replacement gilts. The farm then closed, meaning no more naïve animals (gilts or boars) could be added to the sow farm until the farm was deemed negative. Finally, the farm homongenized by naturally exposing every animal on the farm to the virus, with a variety of methods commonly used in the swine industry.
The result? Within 12 to 36 hours after exposure, 60% to 70% of the animals began to exhibit clinical signs of PEDv.
“The other animals remained asymptomatic despite multiple exposures to the virus (similar to what we’re beginning to see in the human population with COVID-19), but when tested weeks later with an antibody test, all became antibody-positive and were immune,” Loula explains. “The sow herd develops protective antibodies. When sows farrow, the newborn piglets ingest colostrum that contains protective immunity against PEDv.”
Pigs were then weaned at approximately 21 days of age and moved to another site (nursery or wean-to-finish facility). The sow unit was thoroughly cleaned and sanitized during this closure, including all equipment and divider boards. Loula says many disinfectants have been shown to be effective against PEDv. Some producers even fumigate using a long-lasting product with residual activity.
Eradication of PEDv was confirmed by introducing naïve sentinel animals and observing for 30 days, then doing serology (ELISA test) to confirm there has been no exposure. Farms also took rectal swab samples and tested weekly until there were four to six consecutive negative results, he says.
“After confirmation of negative status, naïve gilts can again be brought into the herd demonstrating the disease has been eradicated using hygiene and herd immunity (antibodies),” Loula says.
Since 2013, PEDv elimination has been accomplished on hundreds of thousands of sows with most farms being PEDv-free since 2015.
“PEDV coronavirus antibodies are protective enough to protect individual sows as well as their piglets through sow colostrum, allowing for elimination of the virus from sow herds. Asymptomatic animals in the herd still developed sufficient antibodies to rid themselves of the virus, similar to sows that exhibited clinical signs,” Loula says.
More Tests Needed
More human antibody testing is needed on entire populations (farms, businesses, packing plants) to further understand the amount of exposure to COVID-19 that has occurred asymptomatically. Loula believes it’s a critical piece in the puzzle to alleviate fear and apprehension in people so they can get back to work.
In Wuhan, China, where COVID-19 originated, he was surprised to learn there was a day last week where zero people were in Wuhan hospitals for COVID-19.
“I would not have predicted that it would get that much better that soon, unless there is an incredible amount of immunity that's going on,” Loula says.
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