As cases of seasonal influenza surge, health officials are closely monitoring a growing threat — the potential fusion of human and bird flu strains.
This is because if the viruses mix, it could give rise to a highly-contagious and deadly new pathogen, sparking fears of a global pandemic.
Influenza cases have been on the rise in Canada in recent weeks, alongside reports of avian flu infecting birds such as Canadian geese. And as spring migration brings more birds into the country, they may potentially carry the avian flu virus with them.
The more human flu that is out there, the more probable it is that the virus could mix with the bird flu and create a new virus that has “pandemic potential,” explained Matthew Miller, director of DeGroote Institute for Infectious Disease Research at McMaster University.
He added that a bird flu pandemic is currently being prevented by its limited ability to spread from human to human.
“However, seasonal flu obviously spreads very well from human to human. The concern is that if an avian flu and a human flu were to mix their genetic material, the avian flu might maintain the severe disease that it causes but gain the ability to spread well between humans the way that seasonal flu can,” he said.
Currently, the human risk of bird flu remains low, but the Public Health Agency of Canada (PHAC) said every time the virus jumps to a new species, it raises the risk.
Human infection with avian influenza is rare and usually occurs after close contact with infected birds, other infected animals or highly contaminated environments.
In November, a 13-year-old girl from British Columbia was hospitalized with avian influenza and tested positive for H5N1 Clade B.2.3.4.4B, and Genotype D1.1, the same clade and genotype detected in wild birds in B.C., as well as in outbreaks at poultry farms in the province.
Get weekly health news
Kerry Robinson, the vice-president of the infectious diseases and vaccination programs branch of PHAC, said the agency continues to monitor evidence on avian influenza around the world.
“We do see that individuals that have exposure to animals, infected animals, do have a higher risk of avian influenza. But as a whole, the risk does remain low,” she said.
But she stressed that influenza viruses continue to evolve in animals and have the potential to adapt to humans as well.
How human flu could alter course of bird flu
The influenza virus has a genome made up of eight distinct RNA segments, much like individual cards in a deck, Miller explained. This unique structure allows influenza to rapidly exchange genetic material with other influenza viruses, enabling it to quickly acquire new traits and adapt.
Imagine a deck of cards, Miller said. A human flu virus has eight cards from the suit of clubs, while a bird flu virus has eight cards from the suit of hearts.
If both viruses infect the same cell — often in an animal like a pig, which acts as a mixing vessel — their genetic material can shuffle like a deck of cards. The new virus still needs eight cards, but now it might have a mix of clubs and hearts, meaning it carries genes from both the human and bird flu.
This is dangerous, he said, because the virus can quickly gain new traits, like the ability to spread more easily or become more deadly.
“This process is how prior flu pandemics in the past have started… two different influenza viruses infect one host,” Miller said.
“In the case of 2009, H1N1 swine flu. It was actually three viruses that mixed their gene segments or their deck of cards, an avian flu, a swine flu and a human flu. And that’s what led to the 2009 H1n1 pandemic.”
There’s no evidence that human and bird flu viruses have mixed. However, different strains of avian flu have already combined their genes.
For example, the current H5N1 virus causing outbreaks in poultry and dairy cattle in Canada and the U.S. originated from two different viruses: H5N6 and H5N8. Through genetic mixing between different strains of avian flu, this virus evolved into the H5N1 strain.
Symptoms may overlap
Although the likelihood of bird flu mixing with human flu decreases during North America’s summer months, when seasonal flu circulation is minimal, the risk does not disappear entirely, Miller said.
In the Southern Hemisphere, where the seasons are reversed, flu activity can still be prevalent, maintaining the potential for viral interactions.
“The other concern is detection and the ability to distinguish possible cases of avian influenza from seasonal human influenza,” Miller said.
“They typically cause respiratory infections and are characterized by common symptoms that we all think about, like coughing, sore throat, sometimes runny nose and sneezing. With the flu, we tend to see more systemic symptoms as well. So things like fever and fatigue and muscle ache.”
One notable difference in symptoms when someone is infected with bird flu is severe conjunctivitis or pink eye. This symptom is rare in seasonal influenza cases, making it a key warning sign for doctors that something unusual may be occurring, he noted.
And although the risk to the public remains low at the moment, Miller said, “The problem with risk for flu pandemics is that it’s not necessarily a slow gradient of increased risk over time. We could go from sort of like very, very low-risk today to immense risk a week from now.”
— With files from Global News’ Katherine Ward
