Why are there so many COVID-19 outbreaks at pork plants?

As the coronavirus pandemic spreads, we've asked readers what they most want to know about its impact, prevention and treatment. This is an answer to one of those questions. You can find more answers here.

Why are there so many COVID-19 outbreaks at pork plants? Have there been outbreaks at other food plants?

Pork and other meat-processing plants have emerged as hotspots for the spread of the new coronavirus because employees work side by side in close quarters, making social distancing impossible.

The highly contagious virus is thought to be primarily transmitted by breathing in the respiratory droplets released by an infected individual, although it can also exist on surfaces, too.

Daily reports of plants closing because workers tested positive have called into question whether slaughterhouses can remain virus-free. According to experts, the answer may be no.

"It's not that people aren't trying. It's just that it is very difficult to control this illness," said Dennis Burson, an animal science professor at the University of Nebraska-Lincoln.

The list of companies dealing with infected workers has been growing every day at plants across the country:

- JBS USA announced April 26 that the JBS Packerland beef processing plant in Green Bay would be closed temporarily. The Green Bay Press Gazette reports that at least 189 COVID-19 infections had been linked to the plant.

- On April 24, Jennie-O closed two turkey plants in Willmar, Minnesota, after 14 workers tested positive for COVID-19.

- Tyson Foods' huge pork-processing plant in Waterloo, Iowa, was closed April 22 after numerous workers tested positive.

- Smithfield Foods announced April 12 that it was closing its pork processing plant in Sioux Falls, South Dakota, until further notice after hundreds of employees tested positive for the coronavirus.

- JBS USA's pork-processing plant in Worthington, Minnesota, which employs more than 2,000 people, was idled indefinitely April 20.

A week before the Worthington closure, officials with the United Food and Commercial Workers Local 663, the union that represents 1,850 workers at the JBS pork processing plant, had demanded the company scale back production to allow employees to work farther apart.

UFCW Local 663 President Matt Utecht has praised JBS' handling of the pandemic in Worthington, saying that "gloves, surgical masks, face shields, overcoats — these things came out quicker than in some other facilities."

But Utecht said in a statement that failure by the plant to give workers more space "will put our community and our nation's food supply at devastating risk."

In an attempt to protect workers, companies have started checking employee temperatures, staggering breaks and altering start times. Owners said they have also done more to clean plants, added more break space, slowed production lines so workers can spread out and added plastic shields between workstations.

Major companies, including Smithfield, Tyson, Cargill and JBS, said they have relaxed their attendance and sick leave policies to encourage sick workers to stay home and allow them to be paid.

Still, workplace safety expert Celeste Monforton said it's difficult to ensure people working so closely together won't spread the virus.

"The scary thing is you know if it could happen in one plant, the potential is there for a lot of other plants," said Monforton, a lecturer in public health at Texas State University. "Unless you're super vigilant, this is a recipe for disaster."

Star Tribune staff writers Adam Belz and Mike Hughlett, and the Associated Press contributed to this article.