Influenza, a respiratory illness caused by viruses that infect the nose, throat and sometimes the lungs, can cause mild to severe illness, and even prove fatal, which is a good reason to get a shot of the latest flu vaccine each year.
But one of the bewildering — and scary — things about the flu is that it's a protean enemy, one that can assume different forms. A flu virus is capable of reassortment, in which it picks up pieces of other flu strains that have infected the same cell, and of evolving by mutations. (Here's an article from Journal of Virology that explains how those processes work.) As it does this, it's also capable of changing enough that it can jump from "reservoirs" of host animals, such as wild birds, poultry and pigs, to humans. The H1N1 influenza pandemic in 2009-2010, for example, was caused by a virus that turned out to be a genetic hodgepodge, fashioned from bits and pieces of North American, European, and Asian swine viruses, with some North American avian influenza and a segment of human influenza mixed in for good measure.
And now, a study published June 5 in the journal mBio contains the worrisome news that in the future, we may have more to worry about than just swine, poultry or avian flu. Researchers have found a set of flu strains, avian in origin, that have jumped from swine to dogs in China, and then have reassorted with dog viruses, to produce several new canine flu strains in that country.
"I think the most important concept is that a few years ago, dogs were not considered a major host of influenza virus infections," explains Adolfo Garcia-Sastre, the study's corresponding author. He's director of the Global Health and Emerging Pathogens Institute and principal investigator at the Center for Research on Influenza Pathogenesis, Icahn School of Medicine at Mount Sinai in New York City.
"Most of the hosts we were worried about for generations of potential viruses that could infect humans were poultry viruses, bird viruses and swine viruses," Garcia-Sastre says. "But the situation seems to be changing."
No Documented Cases So Far
That doesn't mean it's time to panic if you just let Fluffy or Champ lick your face. Garcia-Sastre cautions that the findings don't imply any imminent danger that a canine flu strain could jump from dogs to humans. According to the Centers for Disease Control and Prevention, there are no documented cases of such a transmission, and canine flu would have to undergo more evolutionary changes for that to be possible. However, "there are now more and more developing varieties of the virus in dogs," Garcia-Sastre says. And the more viral diversity, the greater the possibility that something that's seemingly unlikely could happen.
Veterinarian Jonathan Runstadler, a professor in the Department of Infectious Disease and Global Health at the Cummings School of Veterinary Medicine at Tufts University, says in an email that the new study's findings are significant.
"The authors describe a suite of viruses they have characterized in dogs which originate from multiple sources at multiple time points, creating a more complex ecology than has previously been documented in dogs," Runstadler writes. "In addition, several of these viruses are close to strains that are already circulating in humans and which therefore may present a higher zoonotic risk [or chance of making the jump from animals to people]."
Why canine flu has never evolved into a form that could jump to humans is unclear. "The influenza viruses that have been documented previously in dogs have been a limited circulation of individual strains which did not show strong zoonotic risk for human infection," Runstadler writes.
But in China, there's a complex ecosystem that includes not just pet dogs and strays, but canines raised for their meat. "All this suggests potentially more opportunities for several viral strains to mix, circulate and undergo reassortment which we believe increase the chance of spillover to human hosts, given enough contact and opportunity," according to Runstadler.
Species-to-species Jumps Rare
Edward Dubovi, a microbiologist and professor in the Department of Population Medicine and Diagnostic Sciences at Cornell University's College of Veterinary Medicine, says in an email that species-to-species jumps are rare, because it requires multiple changes in the virus. "As an example, equine flu H3N8 can infect dogs, but the infection never spreads to another dog with one exception," Dubovi writes. "In 2004 an equine origin H3N8 was found in dogs. A number of genetic changes were identified that distinguished this from normal EIV [equine influenza virus]. In the 14 years since, every H3N8 found in sick dogs has a genetic profile linked to a single jump prior to 2004."
"The opportunity for other jumps was there, but the unique changes needed to do it successfully are exceedingly rare by way of mutations. Jumps due to reassortants are a different story as to frequency."
According to Dubovi, it's unlikely that canine flu would jump from dogs to humans in the U.S., where it would take multiple mutations. But in China, he says, "a reassortant jump could occur as the opportunity is there on a daily basis with flu circulating in the mixed animal markets. We don't have that high risk activity in the U.S."
One way to possibly nip the problem in the bud would be to vaccinate dogs against the flu. "Vaccination reduces the viral load in the environment thereby reducing chances of co-infection," Dubovi explains. "There is currently no downside risk to vaccination as these are killed products." Unfortunately, though, "cost is a factor, which would likely hamper use in Asia where it would do the most good."