How worried should we be about bird flu? What to know about H5N1

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When faced with the potential outbreak of a serious contagious disease, communication expert Peter Sandman once wrote, “the trick is to get people ready for something that might be really bad, while simultaneously reminding them that it might also be no big deal.”

Which is it for bird flu? It’s too soon to know. But H5N1’s spread among U.S. dairy cattle has infectious disease specialists rattled.

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“I’m extraordinarily sympathetic to the general public sentiment that, like, ‘Oh my God, we’re so sick of infectious diseases.’ We don’t want alarmism,” said Matthew Miller, Canada Research Chair in viral pandemics.

“The best way to avoid alarmism is to avoid human infections, and do the low-hanging fruit stuff really well,” he said, including more robust biosecurity measures in settings like farms and educating people who hunt, “because we know a lot of animals that are harvested are at high risk (of infection with H5N1) as well,” like foxes and ducks.

The highly pathogenic avian influenza H5N1 virus hasn’t been detected in dairy cattle in Canada, or in 600 retail milk samples tested as of June 18. The risk of transmission to humans remains low and commercially sold milk and milk products remain safe, according to the Canadian Food Inspection Agency.

But an outbreak that began with reports of sick cows — and dead wild birds — on dairy farms in Kansas and Texas in March has now spread to 131 herds across a dozen American states. Three human cases (one in Texas, two in Michigan, all with mild symptoms) have been linked with the ongoing outbreak in U.S. dairy cows.

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Influenza viruses evolve and shift all the time. What has scientists unnerved is that H5N1 is mutating to more easily infect mammals. “I’m very worried about pigs,” Miller said. Pigs can act as mixing vessels, allowing bird flu to swap genetic material with human flu viruses, making it much better adapted to transmit in humans.

Finland is set next week to become the globe’s first nation to start administering bird flu vaccines, beginning with farm workers.

How worried should people be? Here’s what we know about the H5N1 virus and its potential threat to humans.

H5N1 isn’t new

H5N1 was first detected in birds in 1996, in southern China and Hong Kong. Since then, it has become endemic in bird populations, wiping out millions of wild and farmed birds. H5N1 has infected at least 889 people since 2003, killing a high proportion (52 per cent) of those it infects.

People mostly get infected after contact with a live or dead infected animal’s bodily fluids, like saliva, blood or feces.

Human-to-human transmission of H5N1 is rare. But the high ratio of deaths to cases suggests bird flu “could cause a major public health emergency if human exposure and viral evolution lead to sustained person-to-person transmission,” according to an editorial published in the BMJ.

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What can H5N1 do to humans?

Seasonal flu is generally restricted to our lungs. It particularly likes our upper airways, Miller said, which is good, because lower lung infections tend to lead to more severe infections, like pneumonia.

But H5N1 doesn’t linger in the lungs. The host factors it relies on to replicate in cells are found in almost all human tissues, Miller said. “It can infect all kinds of other organs, including our brain.” Humans infected by H5N1 have developed complications ranging from encephalitis (inflammation of the brain) to coma.

The case fatality rate (the proportion of people with confirmed infections who die) has exceeded 50 per cent, but infections are likely being missed, if people had no, or unremarkable symptoms. The true death rate is likely lower. One estimate by scientists at the Public Health Agency of Canada put the infection fatality rate closer to 14 to 33 per cent, which would still present “a truly dreadful scenario” in the event of a pandemic with sustained person-to-person transmission, the researchers said.

Critical illness is higher in children and young adults.

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“A concerted and dedicated effort by the international community to avert a pandemic through combating avian influenza in animals and humans in affected countries needs to be a global priority,” the authors wrote in 2008.

How much would H5N1 have to change to trigger a human pandemic?

It’s still unclear how the first cows in Texas got sick. But cows are likely being infected via contaminated milking machinery, not passing the virus through airborne droplets from cow to cow, virologist Dr. Thomas Peacock told the BBC.

“A full set of adaptions” would be needed for human-to-human, airborne transmission, the British researchers wrote in the British Medical Journal. But as the virus spills from birds to more and more mammal species, including, sporadically in Canada, racoons, striped skunks, red foxes, cats and dogs, “people are exposed more than ever, providing a human land of opportunity for viral mutation and recombination.”

“There are two ways the virus can learn to transmit in humans,” said Miller, scientific director of the Michael G. DeGroote Institute for Infectious Disease Research at McMaster University in Hamilton.

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“It adapts by itself, which I would call the slow, and probably less risky road.” The “high-risk super highway” would be for the virus to recombine with a human virus, “or some other mammalian virus that’s better suited to human transmission.”

What happens if an H5N1 infected milk sample is found in Canada?

“Then we’re in trouble,” Miller said. “Because if we find an infected milk sample, then the virus is almost certainly pretty widespread, because we know how rapidly the virus can spread via contaminated milking equipment.”

“The CFIA (Canadian Food Inspection Agency) is taking an approach to testing commercial milk that gives them some insight into where the milk comes from,” Miller said. “Intuitively what they would do is start looking back at the farms that were the supplier of the milk that tested positive in order to identify infected animals.”

CFIA has also put in place mandatory testing measures for cows coming in from the U.S.

What about H5N1 vaccines?

Commercial H5N1 vaccines are available, though there is no avian influenza vaccine currently available in Canada for public use, according to Health Canada’s website. The U.S. has stockpiled H5N1 vaccines, and other countries are soliciting tenders for emergency doses. “I’m not aware that Canada has done that yet but quite certain that it is being considered at this point,” Miller said. Canada has procurement contracts in place with manufacturers that make H5N1 vaccines.

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“I’m acutely aware of public sentiment around vaccines right now, and resistance to vaccination,” Miller said. But the H5N1 vaccines use the same technologies used to make seasonal flu shots, which are updated every year to match the circulating viruses.

The risk to humans isn’t high enough to justify a mass H5N1 vaccination campaign. “My preference is that we don’t find ourselves in that scenario,” Miller said.

“The best thing we could do is adopt a strategy like Finland is using and offer vaccine to those at high occupational exposure risk to prevent a widespread human outbreak.”

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