Could Our Pets Cause the Next Pandemic?
Dogs are regularly infected with H3N8 and H3N2 canine influenzas, as well as a variety of other strains. Cats often catch dangerous respiratory infections from viruses like feline calicivirus and Felid alphaherpesvirus. Both also carry various bacteria and parasites that can pass to humans. Yet despite all the diseases affecting these animals and the countless, adorable interactions we share, there has not been a single instance in known memory where one of our beloved canine or feline companions has triggered a global pandemic in humans. Why not? And could it ever happen?
To explore the first question, it might help to compare our pets to the animal presently synonymous with emerging viruses: bats. Deadly infectious diseases like Ebola, Marburg, nipah, SARS, Lassa, and of course COVID-19 have all been linked to these furry, flying mammals. Unlike, dogs, cats, and most other mammals, bats have mutations that blunt their immune responses. Somewhat paradoxically, that seems to be a benefit – their immune systems keep viruses at bay while not overreacting, which can cause harmful collateral damage to bodily systems. As a side effect, this means that viruses and bats can co-exist – bats provide homes to various viruses while suffering few ill effects.
Unfortunately for other animals, that makes bats a breeding ground for dangerous diseases that can mutate and jump species. Many of the 1,200 bat species worldwide live in large colonies, which range in size from dozens of individuals to hundreds of thousands. These colonies are dense and intimate, making viral transmission a fact of life. Such a melting pot is a mixing pool for viruses where mutations abound. Bats can then come into contact with other animals or humans via predation, co-habitation, or hunting and share their viral interlopers.
Moving the discussion back to our prized pets, there are a few apparent factors that make it less likely that dogs or cats will spark a global infectious disease pandemic in humans, even though we regularly interact. For one, they are fairly separated from others of their species, making viral transmission more difficult and mixing less common. Nor do they interact very often with other animal species, provided they are kept on leash or indoors. Moreover, between grooming appointments, veterinary visits, and mandatory vaccinations, we regularly keep them clean and cared for.
All of this doesn't make our pets immune from triggering pandemics. It is possible. In research published back in 2018, scientists from Mount Sinai noted numerous instances of pig viruses recently jumping to dogs and creating new canine influenza viruses. If this could happen, perhaps other new viruses could emerge and jump from dogs to humans...
As for cats, numerous studies conducted over the last fifty years have found that they seem to be surprisingly at risk of being infected by us. Cats are sometimes susceptible to contracting seasonal influenza from their human owners, though they usually do not develop symptoms. In the wake of the H1N1 swine flu pandemic in 2009, one study found that a third of cats sampled had antibodies to the virus, indicating that they were infected at one point.
In return, cats share a curious parasitic infection with us. Toxoplasmosis infects at least half the world's cats and perhaps half of all humans, albeit with almost no ill effects. We can swap the parasite back and forth, but it can only reproduce in cats.
Cats are well known for hunting birds, killing as many as 3.7 billion annually. This proclivity makes them prone to contracting avian influenzas. H5N1 has been known to infect cats, causing fever, lethargy, and sometimes even death. In 2016, H7N2 avian flu jumped to cats in New York City and caused an outbreak among felines in the city's animal shelters. More concerningly, the virus jumped to humans. "One human infection was detected in a person who had close, prolonged unprotected exposure to the respiratory secretions of H7N2 infected, sick cats at an affected shelter," the CDC reported. Luckily, this H7N2 strain was mild and never gained the ability to spread between humans.
While it cannot be ruled out that our furry friends could give rise to a deadly pandemic in humans, the prospects seem unlikely at this point. So don't be afraid to cuddle your precious pets as you ride out the present SARS-CoV-2 pandemic at home.