Why do pigs exposed to the coronavirus not get sick?

TORONTO -- Pig cells respond differently than human cells when exposed to SARS-CoV-2, the virus that causes COVID-19, a study out of Iowa State University suggests, finding that could help with the eventual development of new methods of treatment for the disease.

The ability and potential consequences of SARS-CoV-2 to infect other species and mutate into new strains have been an ongoing concern during the pandemic.

Earlier this week, samples taken from white-tailed deer in southwestern Ontario were found positive for COVID-19. And in Hong Kong, the government announced a massive cull of thousands of pet hamsters and other small animals due to worries that they could carry and transmit the virus to humans. Cats and mink are also known to carry the virus.

Unlike other animal species, however, pigs appear resistant to infection and do not transmit the virus to other animals, previous research has found.

Pigs live in close proximity to humans and are known to host coronaviruses and incubate other types of viruses like influenza, so in the early days of the pandemic they were an obvious area of focus and study, according to the journal Nature. But research found that pigs appear largely resistant to infection.

Scientists at Iowa State wanted to know why, so they introduced the virus to cultured pig and human respiratory epithelial cells to study how cells responded to infection. The results were published in the peer-reviewed online journal, Cell Death Discovery.

“When we looked under the microscope there was an interesting phenomenon going on inside the cells,” Rahul Nelli, a research assistant professor of veterinary diagnostic and production animal medicine with Iowa State’s College of Veterinary Medicine, said in a press release statement.

“The nuclei of the infected pig cells were starting to shred into fragments but not uninfected pig cells.”

This process is a key sign of apoptosis, a controlled cell death that results in minimal tissue damage and limits viral replication, all of which also helps protect against severe illness. Infected cells are quickly killed off without prompting an over-reaction from the immune system.

Pig cells were approximately 100 times more likely to go through apoptosis than human cells, which tend to undergo necrosis instead, researchers found. Necrosis is a less controlled type of cell death that can provoke a strong hyperimmune response.

“We don’t want to over-conclude, but this response is probably something intrinsic to the pig immune system that is innate and not acquired,” said co-author Luis Giminez-Lirola, an associate professor with Iowa State’s College of Veterinary Medicine.

“The idea is to kill the virus subtly but fast enough so there’s not an excessive immune response triggered.”

Nelli and Gimenez-Lirola, who have both studied coronaviruses in pigs for a number of years, say further study could one day result in treatments that would trigger apoptosis in human cells.