SARS-CoV-2 originally jumped from animals to humans. So too did the virus responsible for SARS and MERS. There is now abundant evidence that humans can transmit SARS-Cov-2 to domestic pets and other animals. The more people infected the greater the risk of creating a permanent animal reservoir of infection; one that may pose a threat for generations.

Humans can infect other animals with the Covid-19 virus. Human can infect cats and dogs, lions and tigers, and now mink. Infected tigers can transmit the virus to lions. Min can transmit SARS-Cov-2 for one to another with devastating effects on entire colonies. Now it appears at least some animals can give the virus right back to us in what I call viral ping-pong.

Beginning early April 2020, large scale infections occurred in at least two mink farms in the Netherlands. Sequence analysis of the viral genes showed that while the virus spread rapidly within farms, even among animals housed in separate cages, it did not spread between farms. Subsequent studies showed that cats in and around the mink farms were also infected. Whether all cats were infected by contact with mink feces or respiratory droplets or infected by contact with one another is not known.

Then came the discovery that two mink farmworkers were infected. Genome sequencing established that the virus jumped back into humans from the infected mink

What are the implications of intraspecies jumps? SARS-CoV-2 can infect mink, domestic cats and dogs, ferrets hamsters, tigers, lions, and bats and macaque monkeys. There is some evidence that pigs, mice, and rats may also be able to be infected. Likely, wild species closely related to these animals can also be infected including coyotes, wild dogs, feral feline species, wild boar, pangolins, and civets. With time I am certain that the list will grow.

Should we worry about being infected by our pets and livestock? Perhaps. Soon more than ten million people worldwide will have been infected, each a possible source of animal infection. We know that the virus can jump from animals to humans, that is presumably how both SARS and Covid-19 infections began. Camels continue to infect humans with MERS eight years after the first cases were reported. We are fortunate that the MERS virus spreads slowly from person to person.

There are too many unknowns to estimate the degree of risk of human infection from animal reservoirs of Covid-19. We do not know the efficiency of transmission for humans to animals although it appears to be low for dogs and cats. We do not know the efficiency of transmission between animals, although we have seen that for mink animal to animal transmission can be very efficient. We are uncertain about the routes of infection, whether it is airborne, oral/fecal, contaminated water, or all three. We are still learning about all the different types of animals that can be infected and transmit the virus back to us.

What is clear is that there is a very real possibility that SARS-CoV-2 may establish deep reservoirs of infection in our pets, livestock, and feral animals that live around us. It is time to begin detailed surveillance of animal populations for evidence of infection. It may be time to be wary not only of our fellow humans but our pets and livestock as potential dangers.

This article originally appeared on Forbes.