Tuesday, April 9, 2013
Avian Influenza A (H7N9) - April 9, 2013
Well, I was not expecting to write to you so soon. As mentioned in my last post, I thought we were at the apex of the flu season and, as it turned out, that was the case as the incidence of influenza in the North American season began to diminish soon afterwards. I had hoped to send a note out now, not on the flu, but on Lyme disease, but some news has changed my plans.
As you may know, just as our season winds down, our next year’s flu is beginning to gather steam in the East and in the Southern Hemisphere. Specifically, China is starting to have flu issues. And as has occurred before, there is a new strain of avian flu that has infected humans there, named Avian Influenza A (H7N9). The local bad news is that, to date, 24 or so people have been laboratory confirmed infected with this flu, and 7 or so have died. This makes catching the flu a highly lethal proposition. (Of course it is very possible that others had this same flu and have not been laboratory confirmed and recovered, shifting these numbers, which is a conversation for another time.) More than likely you will be hearing a great deal about this if it grows or if the news media picks up the story. What they may not make clear is what this all means. Allow me to mention a few points I have made over the past years, so that you have a scaffolding on which to place that information; I apologize for repeating myself, but it may be useful.Yes, this does seem to be a novel flu strain to humans. And yes it does seem to pass from one species to another. However, the main threat to us, as a population, is not the bird to human spread, but the human-to-human spread. In other words, while you, a human, may catch it from a bird for example, and if you do catch it have a higher likelihood of mortality, that is not what drives a pandemic. For a pandemic to occur, human-to-human spread must occur, and must occur easily. The way I think about this, and the way I have written on this, is thinking of it as gates that are closed or open. The first gate relates to the ease of the flu virus transmission from bird to bird. (Here, you can think of bird as a proxy for another species, such as pig, etc., that can also pass a flu to humans.) The easier such transfer is, the more open that gate is and more likely for the virus to spread amongst birds, which is not so good for them, but does not yet greatly impact people.
The next gate is the bird to human transmission. How easy is that? If it is easy, then we have a local problem. In other words, the birds begin to pass the virus to people. And, as you recall our discussion, since the human’s immune system may not have a great deal of historical experience with that virus, then those local people may suffer greatly, and some may even die during that infection.
The next gate is the most important. How likely is it that a human infected with that virus can pass it to other humans. If that gate is open then the likelihood of epidemic is very high. The last gate is the natural history of that virus. It turns out that many times a highly lethal virus becomes less lethal as it progresses from individual to individual. As well, it may become less likely to pass from individual to individual. This has been noted before in other viruses, and I believe there are logical evolutionary reasons why this is beneficial to both virus and host species. (That is also something we can talk about another time.) However, if the lethality stays or increases, and it is easy to pass from human to human, then the chance of a pandemic becomes a reality.
Where are we now? We have found this novel virus, that came from one species to humans, and it turns out that many who catch it die. That is what we know. Other things we know: Once the virus has been identified, health authorities began surveillance on family members and others that were exposed to the individual people that caught/died from the virus. As of now, to my knowledge, no one has developed this flu. In other words, as of now, it does not seem as though the human-to-human transmission is easy or being seen at all, which is good news! That should be one of the most important variables to track. As well, we may find that over the next weeks the people who catch this flu have a less lethal version, which would be good news as well, which is what I expect to see.
That is where we find ourselves, as of now. If you are likely to hear anything about this virus, it is that some people died in China, from a flu that is new to humans. A sort of local story that is tragic to those involved but not to our species. If these variables change or gates open, the news will just stop there or gather a great deal of steam and hysteria. Until then, DON’T PANIC if you hear about this news.
Paul Herscu ND, MPH