Canine influenza virus (CIV) is a highly contagious viral infection that not only affects dogs, but cats as well. And new research says humans could one day be at risk, too.

According to a new study published in the journal mBio, scientists have discovered that domestic dogs are harboring flu viruses that have the potential to jump to humans. That’s a scenario previously thought highly improbable, if not impossible: no cases of a human catching canine influenza have ever been recorded.

New flu viruses are incubated in animals before they jump to humans.

The 1918 Spanish Flu pandemic that killed between 50 and 100 million people is believed to have started in birds. And the 2009 Swine Flu pandemic was transmitted to humans via pigs, killing an estimated 203,0000 people worldwide.

The two strains of CIV known to infect dogs in the US both jumped to canines from other species.

Canine H3N8 was first identified in racing greyhounds in Florida in 2004 and is thought to have mutated from an equine influenza strain that jumped from horses to dogs. Canine H3N8 has since been identified in dogs in most US states and the District of Columbia.

Canine H3N2 was first identified in the United Sates in 2015 after an outbreak of canine respiratory illness in the Chicago area. Previously, it had been isolated to South Korea, China, and Thailand, where it was believed to have originated as an avian influenza that jumped from birds to dogs.

Neither strain has been shown to infect humans.

But the newly discovered CIVs with the potential to make the jump to humans weren’t discovered in the US. They were found in China.

Researchers from the Icahn School of Medicine in New York City sequenced the genomes of 16 influenza viruses obtained from pet dogs in southern China between 2013 and 2015 and discovered they contained segments similar to the H1N1 strain responsible for the 2009 Swine Flu outbreak.

“The majority of pandemics have been associated with pigs as an intermediate host between avian viruses and human hosts,” said corresponding author Adolfo Garcia-Sastre, PhD, director of the Global Health and Emerging Pathogens Institute at the Icahn School of Medicine. “In this study, we identified influenza viruses jumping from pigs into dogs. We now have H1N1, H3N2, and H3N8 in dogs. They are starting to interact with each other.”

“This is very reminiscent of what happened in swine ten years before the [2009 Swine Flu] pandemic,” Garcia-Sastre noted. “The evolution of canine influenza viruses in Asian dogs is increasingly complex, presenting a potential threat to humans.”

The researchers say opportunities for interspecies transmission of influenza viruses abound in southern China, where diverse species are often raised in proximity and intermingle at live-animal markets. And changing cultural attitudes toward dogs in China mean more opportunities for contact between canines and humans—dogs are increasingly kept as pets, untold numbers roam the streets as strays, and many are still raised as meat for human consumption.

Most of the dogs sampled in the study were found in veterinary hospitals, where they had presented with respiratory symptoms. The 16 genomes sequenced from those dogs led to the identification of 5 new CIV genotypes previously unknown in canines.

But Garcia-Sastre told NEWStat that veterinarians and their clients can rest easy—at least for now: “At this moment, there is no reason to be alarmed, as the chances of dog-to-human transmission are very unlikely.”

However, he does note that, “The increase in influenza virus diversity in dogs is of concern, and we should increase our efforts in monitoring influenza in dogs to understand whether the viruses are changing or increasing in frequency.”

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