The process of ‘spillover’ — the passing of a virus from animals to humans — has been central to the COVID-19 story since the beginning, as scientists and politicians have argued a long time.

Now, scientists worry that as more humans infect their pets and other animals with COVID — a process known as ‘spillback’ — these animals could function as bioreactors, creating new variants that they will pass back to humans. Some even suggest that the highly transmissible Omicron variant has its originals in a spillback and spillover handoff.

"Cross-species transmission generally results in the rapid adaptation of the virus to the new host, and repeated transmissions may hasten viral evolution and novel strain emergence,” an interdisciplinary team of scientists at Colorado State University and the University of Pennsylvania wrote in a study recently published in 'PNAS'.

Others go further, with a new study suggesting that the highly transmissible Omicron variant had its origins in a spillback and spillover event.

Spillback is common

Spillback of COVID-19 has returned to the news this week as Hong Kong authorities struggled to contain an outbreak of COVID-19 in hamsters—which was traced back to a local pet shop—and ordered new owners of hamsters to return their pets in order for them to be killed.

Spillback from human to animal has been present throughout the pandemic. A new study published this week by the University of Pretoria, South Africa, found that lions and pumas at a private zoo in South Africa got severe COVID-19 from asymptomatic zoo handlers. In November, Lincoln Children’s Zoo in Nebraska lost three snow leopards to the disease.

In total, the World Organization for Animal Health recorded 625 COVID-19 outbreaks in animals, infecting 17 species in 37 countries up to the end of December 2021.

Spillover is rare—but can be dangerous

Viral spillover from animals to humans is normally a rare occurrence, however. The first and only recorded spillover of COVID-19 occurred after the huge November 2020 outbreak of the virus in mink farms in Denmark, which prompted the government to order the cull of 17 million minks.

On Nov. 5, Danish public health authorities reported the detection of a mink-associated SARS-CoV-2 variant with a combination of mutations not previously observed. Danish authorities have since said the new strain of COVID-19 has likely been eradicated.

"[I]n most cases, the virus has passed from an infected human to the animal through close contact," said British Veterinary Association (BVA) president Justine Shotton. "The main route of transmission for COVID-19 remains from human-to-human, and it is extremely rare for animals to cause transmission to humans."

When spillover from animal to human does occur, it can be dangerous. Studies have found that new variants could emerge from groups of animals with the COVID-19 disease, reigniting concerns that our furry friends might be incubating mutations for humans to catch.

The team of scientists at Colorado State University and the University of Pennsylvania noted their results “highlight the potential for human reinfection with new viral variants arising in species in close and frequent contact with humans.”

Omicron came from…mice?

Researchers at the Chinese Academy of Sciences in Beijing went a step further, using genetic analysis to suggest the Omicron variant of COVID-19, spreading rapidly across the world, acquired its array of mutations while spreading within a mouse population for more than a year.

“Collectively, our results suggest that the progenitor of Omicron jumped from humans to mice, rapidly accumulated mutations conducive to infecting that host, then jumped back into humans,” their paper, published in the Journal of Genetics and Genomics, found.

The good news however, is that there are no documented cases of cats or dogs transmitting the virus to humans. While one study of 76 pets found that 17.6% of cats and 1.7% of dogs tested positive for COVID-19 after living with a person who had the virus, there are no signs indicating that cats or dogs can give it to humans.

The best protection against spillover and spillback may be the same ones currently in use. Marietjie Venter and Katja Koeppel, two professors at the University of Pretoria who authored the zoo study, noted mask wearing and infection control were important to containing the spread of the virus at zoos: “These measures are also important because of the risk of new variants emerging if the virus establishes itself in other animal reservoirs; these variants could be transmitted back to humans.”

BVA president Justine Shotton said, “We would like to reassure pet owners that there is no need to panic. If you have COVID, you should limit contact with your pets as much as possible and if you do need to handle your pets, make sure you wear a face covering and wash your hands before and after. If you are worried about any unusual symptoms in your pet, call your vet for advice.”