Could Dogs Become the Source of the Next Flu Pandemic?

As of May 2018, there have been 61 positive cases of dog flu in New York City. And with the exceptions of Hawaii, Alaska, and North Dakota, at least 1 strain of canine flu (H3N2 or H3N8) has been documented in every state—with the majority of states reporting cases of both strains. Now, researchers are speculating whether dogs could be a potential reservoir for a future influenza pandemic. This is according to a new study just published in the journal mBio that demonstrated that the influenza virus can jump from pigs into dogs and that influenza in dogs is becoming increasingly diverse.

"The majority of pandemics have been associated with pigs as an intermediate host between avian viruses and human hosts. In this study, we identified influenza viruses jumping from pigs into dogs," said one of the study’s authors, Adolfo García-Sastre, PhD, 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 Mt. Sinai, New York City.

Putting Mammals Under the Microscope

The investigators explained that although wild aquatic birds are currently considered the primary natural reservoir host for influenza A viruses, mammals have emerged as critically underrecognized sources of influenza virus diversity. They cited that pigs, for example, were the source of the 2009 H1N1 pandemic. According to the Centers for Disease Control and Prevention, there were approximately 60.8 million cases, 274,304 hospitalizations, and 12,469 deaths related to H1N1 flu in the United States during the first year the virus circulated.

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The investigators also pointed to 2 instances within the past 2 decades where dogs emerged as important influenza A virus hosts: H3N8 viruses were introduced from horses into dogs in the United States in 2004, and H3N2 was introduced from birds into dogs in southern China from May 2006 to October 2007.

Detecting a New Strain

Taking these recent outbreaks into account, the investigators analyzed the genomes of 16 influenza viruses from dogs in Southern China from 2013 through 2015 that presented at veterinary clinics for respiratory problems. They found that the genomes contained segments from 3 lineages that circulate in pigs: North American triple reassortant H3N2, Eurasian avian-like H1N1, and pandemic H1N1.

"In our study, 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,” the authors summarized. “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 10 years before the H1N1 pandemic."

The new virus identified in the study is H1N1, but because it comes from pigs and is of avian origin, it is different antigenically from the H1N1 strains that were seen in the pandemic and has a different origin than that of the previous H1N1 found in humans. As such, the authors recommend that future studies focus on further characterizing the virus and assessing whether humans have existing immunity. Even with the possibility that humans might be immune, the investigators concluded that the continued expansion of influenza A virus diversity in dogs combined with high human contact rates requires enhanced surveillance and ongoing evaluation of emerging pandemic threats.

"There are attempts to restrict influenza virus in pigs through vaccination and one could consider vaccination for dogs," Dr. García-Sastre said.