As the deadly H5N1 bird flu continues its global spread, wiping out colonies of sea lions, decimating bird populations by the millions and even reaching Antarctica for the first time, concerns persist regarding its potential risks to human health.
The latest development emerged after the U.S. Centers for Disease Control and Prevention (CDC) on Monday reported a human case of avian influenza in a person who had contact with dairy cows in Texas presumed to be infected with the virus.
“So this highly pathogenic avian influenza has been circulating around the world at a very high rate for a number of years now,” explained Matthew Miller, the director of the Degroote Institute for Infectious Disease Research at McMaster University.
However, the fact that it has never before spread to dairy cattle raises significant concerns, he said.
“The thing that makes this concerning as it pertains to risk to humans is that we know humans are at most risk of viruses jumping into them when they’re in close proximity to animals that are infected. And indeed, this case in Texas appears to have derived from a worker on a cattle ranch who is in proximity to an infected cow,” he told Global News.
“It is a relatively new development that changes the risk evaluation.”
Although human cases remain rare, health experts caution there is a heightened risk of the bird flu evolving to infect humans more readily.
Avian flu, also known as the bird flu, is a disease caused by influenza viruses that spread among wild aquatic birds and can infect domestic poultry and other animal species. The viruses are distinct from the ones that cause the flu in humans, but they are related.
The H5N1 avian influenza virus first emerged in 1996 in southern China and has been causing bird outbreaks around the world since then.
Since 2020, a variant of these viruses belonging to the H5 clade 2.3.4.4b has led to an unprecedented number of deaths in wild birds and poultry in many countries in Africa, Asia and Europe. In 2021, the virus spread to the U.S. and Canada, and in 2022, to Central and South America, according to the World Health Organization (WHO).
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Globally, H5N1 has infected many mammals, including foxes, pumas, skunks, and both black and brown bears in North America. The avian flu has already reached new corners of the world in recent years and is now present in penguins living in Antarctica and polar bears in Alaska.
The virus usually infects the gastrointestinal tract of birds, and it is shed when the bird defecates, Miller said.
“When there are migrating ducks and geese over grazing pastures, for example, if those birds defecate, it can contaminate soil, it can contaminate drinking water, and that can lead to other animals becoming infected. Of course, in many cases, those infected birds may also die. And if scavenger animals, eat those birds, that’s another way that they can become infected,” he explained.
What is the risk to humans?
Human cases of H5N1 have primarily occurred due to zoonotic transmission through direct contact with infected birds (dead or alive) or contaminated environments. Human-to-human transmission is extremely rare, Health Canada stated on its website.
The most recent death happened last month in Vietnam, the WHO reported. A 21-year-old died on March 23 after he tested positive for H5N1. The man had no history of contact with dead or sick poultry or anyone with similar symptoms. However, health officials said he was reportedly trapping wild birds in his hometown.
“In Canada, the risk to the general public is still very low,” Miller said.
However, he believes the cases involving infected dairy milk warrant “increased attention,” especially from people whose occupations put them in close exposure to wild animals and farm animals.
“The real problem historically, with highly pathogenic avian flu infections in humans is that despite relatively low transmission rates, they can cause much more severe illness than a typical influenza infection,” he cautioned.
“Previous outbreaks of this avian flu have had death rates that are in some cases higher than 30 per cent, which is extraordinarily high by any virus standards.”
Another distinctive feature of this virus, he noted, is that unlike most seasonal influenza strains that primarily affect human lungs, highly pathogenic avian influenza can infect various organ systems in our body. In some instances, it may even reach the brain, resulting in severe complications such as encephalitis, which poses significant challenges for treatment.
Should we be concerned about milk?
On March 25, U.S. federal officials announced the detection of avian influenza in laboratory samples obtained from some affected cows in Texas and Kansas. Days later, federal officials said they had confirmed the presence of the virus in a herd in Michigan, and suspected more cases in cows in New Mexico and Idaho.
Officials said they believed the cows had contracted the virus from wild birds, but that transmission among the cattle “cannot be ruled out.”
It is the first time the disease has been found in dairy cattle, according to the American Veterinary Medical Association (AVMA).
Shayan Sharif, a professor and associate dean with the Ontario Veterinary College at the University of Guelph, called this transmission “quite surprising” and “rather unexpected.”
“But this virus has shown to be quite versatile. It can do a lot of different things. And maybe we should not have been surprised to see this virus jumping from birds to cattle,” he said.
Previously, Sharif said many infectious disease specialists had the impression that the H5N1 virus couldn’t transmit to cattle, but “many of our theories and hypotheses have been proven wrong. So I am quite concerned about what this virus is capable of doing in the future.”
Miller also believes that Canadian farmers are exercising increased vigilance over their cattle in light of these cases. With the onset of spring, migration will bring waterfowl back to these areas, heightening the risk.
However, Miller reassured there is no risk of contracting the virus from milk if it’s pasteurized.
“It’s the reason we pasteurize, to kill any sort of germs that might be present in the milk. And that’s why pasteurization is so widespread and it’s been so important in preventing infections,” he said. “And, in general, there’s not a risk of contamination of meat products either. So that’s not something that I think people need to be concerned about.”
How to prevent future outbreaks
Miller and Sharif stress that the most effective strategy for preventing widespread avian flu outbreaks is to minimize its chances of crossing from animals to humans.
As Miller explained, the longer the virus has opportunities to jump from animals to humans, the more likely it is to develop the ability to infect people.
“Then it has an opportunity to transmit between people. That creates a risk of widespread outbreaks or pandemics. And thankfully, as far as we know, this virus does not do that efficiently, and we want to keep it that way.”
Sharif noted that because avian flu operates similarly to COVID-19, many of the practices implemented during the pandemic can be applied here. This includes minimizing contact with infected animals, ensuring thorough handwashing and practising disinfection whenever possible.
Another strategy to help prevent an outbreak involved vaccination.
“We currently don’t have any vaccination plan for poultry or for cattle in Canada,” Sharif said. “But time will tell whether or not there is going to be a safe and efficacious vaccine available for poultry and cattle.”
— with files from Global News’ Katherine Ward and Reuters
