When scientists search for the origins of a novel influenza A outbreak, they often trace the virus back to birds or pigs. These animals act as reservoirs, hosts that allow diverse flu viruses to swap genome segments, evolving into new flu viruses that could potentially jump to humans.
Now, an international team of researchers has identified another animal that could harbor the makings of a human influenza pandemic: dogs. Their recent findings, published in mBio, suggest that flu viruses can evolve in canines to become increasingly complex. This represents a possible future threat, though experts don’t see imminent danger.
In the 2000s, dogs began catching the flu. As documented in published studies, influenza has jumped from horses to dogs in the United States and from birds to dogs in Asia. But the genetic diversity of canine flu viruses appeared relatively limited, with little potential to spread to humans.
A team of researchers from the Icahn School of Medicine at Mount Sinai in New York, Guangxi University in China, and the Bethesda, MD-based National Institutes of Health thought that dogs in southern China deserved a closer look because they are routinely exposed to other host species, including humans. “There is a large density of dogs, a large density of pigs, and a large density of chickens and ducks in China, in many cases in close contact,” says microbiologist Adolfo García-Sastre, study coauthor and director of the Global Health and Emerging Pathogens Institute at the Icahn School of Medicine.
The study took place in the Guangxi Zhuang Autonomous Region where dogs often come into contact with birds and pigs at live-animal markets. And people encounter street dogs, keep dogs as pets, and raise them for food. From 2013 through 2015, the team collected nasal swabs from 800 dogs, primarily pets suffering from respiratory conditions in veterinary clinics. Over 100 dogs tested positive for the flu. The team then isolated and sequenced the complete genomes of 16 of the influenza A viruses that they detected.
The researchers pieced together the evolutionary history of these viruses by creating a phylogenetic tree. Viruses with more similar gene segments clustered on branches closer together. The tree also included influenza virus genomes that other scientists previously isolated from humans and animals in Asia. This analysis allowed the team to trace the history of each virus as it moved through different host species and evolved over time.
They discovered that at least two swine H1N1 influenza viruses had jumped from pigs to dogs in Guangxi. But these viruses were genetically distinct from the H1N1 virus that caused the swine flu pandemic in 2009, as well as the one that caused the Spanish flu pandemic in 1918.
They also found that three novel flu viruses had formed through the mixing of these pig-origin viruses with previously circulating canine viruses that had originated in birds. “An increase in genetic diversity of viruses is always a concern,” says García-Sastre. “It increases the potential to acquire new properties like being able to infect a new host.”
This level of mixing and matching of viral genome segments within canines is “interesting and unusual,” says veterinary virologist Colin Parrish of Cornell University’s College of Veterinary Medicine, who was not involved with the study. “We’ve sequenced and looked for influenza in dogs in the United States and we’ve never seen anything other than classic canine influenza.”
The authors note that many questions remain, including whether these new viruses will continue to pass from dog to dog, how far beyond Guangxi they’ve reached, and whether canines are also carrying other novel flu viruses. Parrish and the authors all say that more surveillance of canine flu viruses is needed to better assess the potential for future spread to humans, including in dogs raised for food and street dogs. These dog populations could potentially function as viral reservoirs that spillover into pet dogs.
So should pet owners be concerned about catching flu viruses from dogs? “I don’t think there is a major reason to fear,” says García-Sastre. “Up to now, we don’t think that they are able to jump easily to humans.” And there has not been a documented case of this jump occurring in China or elsewhere, as noted by the US Centers for Disease Control and Prevention.
Dog owners shouldn’t be alarmed, agrees ecologist and evolutionary biologist Benjamin Dalziel of Oregon State University, who specializes in infectious disease, and was not involved in the study. “However, I think the study’s findings are an alarm bell for scientists,” he says. “Clearly more surveillance and analysis of influenza in dogs is needed because dogs are functioning more like mixing vessels for influenza evolution than we thought.”