April 25, 2013 4:00 pm

Why the WHO is calling H7N9 one of the ‘most lethal’ flu viruses so far

This picture taken on April 13, 2013 shows Beijing's first human case of H7N9 bird flu, a 7-year-old girl.

STR/AFP/Getty Images

TORONTO – In a mere few weeks, a strain of avian flu has infected 108 people and killed 23 in China. The birds aren’t showing any symptoms of illness making it difficult for scientists to track its origins. Meanwhile the virus seems to jump to humans easily.

It’s this combination of factors that has Canadian and global officials worried about the H7N9 bird flu that has emerged in China. The World Health Organization has even called the influenza “one of the most lethal” viruses it’s seen so far.

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Read more: WHO: New flu passes more easily from bird to human

Global News spoke with two Canadian doctors, who are leading infectious disease research in the country. Here’s what they had to say about the influenza the world is watching with bated breath.

Dr. Michael Gardam is director of infection prevention and control at University Health Network.

Dr. Joel Kettner is the medical director of the International Centre for Infectious Disease. He was Manitoba’s chief medical officer of health from 1999 to 2012.

Global News: Is it concerning to hear that the WHO is calling H7N9 one of the “most lethal” flu viruses so far? What makes it so dangerous?

Dr. Gardam: There is no doubt – this is by far the most worrisome strain. The reason is it’s come on the scene only in the last few weeks, and we’ve already got over 100 cases. We don’t know what the reservoir is very well, there’s a bunch of things we don’t know about it and it’s causing a lot of very, very severe illness. The H5N1 virus was more deadly, but it took us much, much longer to get to this stage. This is happening quite quickly so that’s why they have a lot of concern about it.

Basically what they’re saying is it’s more lethal because it seems to be infecting people much easier than other bird flu strains we’ve seen.

Dr. Kettner: Yes, it is appropriate to be concerned by such a statement, but this statement is based on preliminary information. It is probably too early to conclude how “lethal” this virus is until more is known about the spectrum of illness – exposure rates, infection rates, proportion with mild and severe illness.

The three factors that make an avian flu “dangerous” for humans are 1. Transmissibility from bird to human; 2. Contagiousness from human to human and; 3. Severity of disease in infected humans.

Read more: Taiwan watching travellers from China closely after first bird flu case surfaces

Global News: What are the chances of this bird flu making its way to Canada?

Dr. Kettner: It is too early to estimate this risk, but if this H7N9 behaves the way that the H5N1 has, the probability of a sustained presence of H7N9 in North American poultry and/or the sustained spread in humans seems very low at this time. The probability of isolated cases associated with travel from an affected location is higher, as has apparently already occurred in a traveller returning to Taiwan.

Dr. Gardam: Nobody knows. This is a new strain of H7N9, it’s never done this. I’m assuming this will not do it because the experience with H5N1 but it’s naïve to say that it can’t start spreading human to human.

Global News: What is the catalyst that could spark a global pandemic?

Dr. Gardam: The catalyst really is spreading from human to human. As long as it’s just in birds and hopping to humans on occasion, you’re going to get the same epidemiology that you have now. If it can efficiently transfer from human to human, you’ll have a pandemic. It’ll just take off because typically one person with influenza spreads flu to at least three to four people, depending on the virus and the setting where the exposure occurs. You can imagine how quickly you’d get into the millions and millions of people. That’s the piece that’s worrisome; the fundamental thing about this virus though is you don’t know when it’s going to happen until it happens.

(Dr. Gardam explains that influenza is “very sloppy” when it replicates itself – it’s always making mistakes. It could make a “good mistake” for the virus and alter itself to make it easier to spread from person to person. It could also recombine with other viruses, picking up components, of say H3N1 or H1N1, making it easier for humans to contract.)

Read more: Q & A: What you need to know about the bird flu in China

Dr. Kettner: The main factors that would determine whether H7N9 will evolve into a serious “global pandemic” are whether it can be spread sustainably from person to person and whether there is a significant degree of severity of illness. There has been no reported evidence of sustainable spread at this time.

(Kettner said that if migratory birds were carrying the virus that could also trigger an epidemic.)

Global News: In 2003, the H5N1 bird flu strain swept across Asia, killing 360 people worldwide, mostly after contact with infected fowl. What are the differences and similarities between this influenza and the H5N1 strain that ravaged poultry across Asia in 2003?

Dr. Gardam: H7N9 is not making the birds sick, so it’s making it very hard to figure out where it is. With H5N1, you just had to look and if all the birds were dead, you’d have a pretty idea that’s where H5N1 was. This time, it isn’t killing the birds so it makes it much harder to track. There’s a sizeable number of people who swear up and down that they weren’t in contact with birds. There’s still this missing element there we’re not figuring out.

(Gardam says that about 20 to 40 per cent of H7N1 patients deny any contact with birds.)

In the same breath, there’s no evidence of human to human contraction because we’d already have cases in Toronto. If it was efficient human to human spread, we’d know. So where did (patients) get it from? We’re not finding it in pigs, we’re not finding it in other species.

They both have a tendency to infect lung receptors rather than throat infectors which is probably why they cause much more severe disease. In a typical flu, the nose and throat areas are targeted; with avian strains they infect deeper down in the lungs, so they tend to cause much more serious disease. Humans don’t have antibodies against these – it’s not like we’ve been exposed to something similar in the past so if one of these viruses did get going, we’d all be susceptible to it.

Read more: How bad is the new bird flu strain that has killed people in China and is worrying scientists?

Dr. Kettner: It appears that severe illness from the H7N9 virus has been diagnosed relatively more frequently in older men compared to the pattern with H5N1. (IN H5N1, younger adults and children made up a greater proportion of the cases. This difference has led people to question why older adults are more susceptible to this virus and if there’s a biological reason.)

Global News: Since that encounter with H5N1, has there been safety mechanisms put in place to try to prevent bird flu from circulating amongst poultry and amongst humans? What do world officials need to do to protect and prevent?

Dr. Kettner: Appropriate responses include enhanced surveillance,  usual “common-sense” hygiene and other reasonable infection prevention and control precautions, avoidance of over-reaction and panic by keeping these events in perspective, and continued advocacy for improvements in nutrition and working and living conditions, reduction of poverty, and better capacity of and access to quality public health, primary and secondary care.

Dr. Gardam: If you under call it now and we have a pandemic, you dropped the ball and if you over call it now and you don’t have a pandemic, you’re fear mongering. You really can’t win in speaking about this or preparing for it. It’s really containment at this stage: the main control measure is to kill the birds. China’s also been vaccinating birds. If it’s able to transmit from person to person, there really isn’t anything realistically that you can do, it’s just going to spread. I’ve likened it to a tidal wave – it’s about surfing the tidal wave than trying to stop the tidal wave.

carmen.chai@globalnews.ca

© Shaw Media, 2013

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