Our best animal friends might not know it, but they could be putting us at risk from another influenza pandemic, researchers have warned.
Key to the worrying prediction is the increasing diversity of flu within dogs, and new evidence that the virus can jump from pigs into canines. These developments match the build-up to the H1N1 pandemic in 2009, which originated in birds.
Left unchecked, a similar scenario could evolve with dogs as the link between the animal kingdom and human beings, according to the team behind the new study – though as yet there's no evidence of transmission between us and our canine pals.
"The majority of pandemics have been associated with pigs as an intermediate host between avian viruses and human hosts," says one of the researchers, Adolfo García-Sastre, from the Icahn School of Medicine at Mt Sinai in New York.
"In this study, we identified influenza viruses jumping from pigs into dogs."
That follows the first identified case of a flu virus passing from a horse to a dog, fifteen years ago. Five years ago, scientists uncovered viruses jumping between birds and dogs. Now it seems we can add pigs to that list, and the timeline shows this is all happening relatively quickly.
The researchers sequenced complete genomes for 16 influenza viruses sampled from 800 dogs in Southern China between 2013 and 2015. They found matches with certain types of swine flu, and new variations on existing canine flu viruses.
Samples were taken from dogs visiting the vet with respiratory problems, and around 15 percent of the animals had influenza.
"What we have found is another set of viruses that come from swine that are originally avian in origin, and now they are jumping into dogs and have been reassorted with other viruses in dogs," says García-Sastre. "They are starting to interact with each other."
And so it was in 2009 – avian flu jumped to pigs, mixed with existing strains of the virus to create something different, then made the jump into unsuspecting humans. Because the flu strain was something new, we hadn't developed any immunity to it.
The next question is whether this mix of flu strains that develops in dogs is going to be harmful to the human population, if it ever makes the leap across – and we can't rule that out, considering how much time people and canines spend together.
As the viruses mix and become more diverse, the chances increase that eventually one will be able to infect a person. With that in mind, the research team is calling for steps to be taken to restrict the spread of flu viruses between dogs, with vaccination programmes one of the options to consider.
The dogs involved in the study were all from a single region in China, which is one reason not to panic yet – the picture may not be the same worldwide.
Another caveat is that for a pandemic to occur, the virus strain that jumps from dogs to humans would also have to be easily transmissible between people. Still, the researchers are advising caution.
"If there is a lot of immunity against these viruses, they will represent less of a risk, but we now have one more host in which influenza virus is starting to have a diverse genotypic and phenotypic characteristics, creating diversity in a host which is in very close contact to humans," says García-Sastre.
"The diversity in dogs has increased so much now that the type of combinations of viruses that can be created in dogs represent potential risk for a virus to jump to a dog into a human."
The research has been published in mBio.