NC State College of Veterinary Medicine Epidemiologist on H7N9 Outbreak in China
Dr. Barret Slenning, an associate professor in the NC State University College of Veterinary Medicine, is an epidemiology expert and leader of the CVM Biosecurity Risk Management Group who has been following the emerging H7N9 outbreak in China. He answered a few questions related to the avian influenza strain, noting that new information is being learned on a daily basis about H7N9.
A version of this interview was prepared for the NC State Abstract by Tracey Peak.
[section_subtitle] Is the new H7N9 Avian Influenza strain related to H5N1 that was a concern for a few years ago?[/section_subtitle]
As the agent is researched more, this conclusion may change, but to my knowledge, none of the genetic components of this Type A H7N9 have been directly tied to H5N1. Of course, multiple generations ago, there could be some common ancestors – go back far enough in time and you are just about guaranteed to find them – but as far as I know there are no recent connections.
Remember that Type A influenzae have eight segments to their genome. If you are unlucky enough to be infected with two strains of Type A flu, the viruses can swap those genetic segments and create a brand new virus. That’s what we call reassortment. So this H7N9 could well have had its origin in a bird somewhere that was coinfected with an H7n? virus and an H?N9 virus. Talk about a sucky day.
[section_subtitle] So what do we know about H7N9? [/section_subtitle]
It appears that this virus has only avian-sourced genetic components. That makes it different from the others with which we’ve been concerned the last decade or so (H5N1, H1N1, H3N2), which were mixes of avian, swine, and human components. Understand, however, that the general scientific consensus is that the 1918 so-called ‘Spanish flu’ was an avian-only virus that jumped to people.
This is a good news / bad news story. The good news is that avian adapted influenzae tend to have more difficulty directly infecting humans than do swine adapted pathogens. Humans have a different mix of potential target cells in our respiratory tracts than do birds, so the avian adapted strains have difficulty infecting us. This is why the CDC and WHO are ‘concerned’ but are not waving their arms about in panic. Additionally, the Chinese CDC (and soon our own CDC) is making the basic components for a vaccine against this specific strain, but they are unlikely to manufacture it in quantities unless it shows signs of making more common jumps to people.
The first bad news part, however, is that for avian-only sourced influenzae it is much more likely that humans have not been previously exposed, and therefore, we are unlikely to have much immunity to them. Therefore, these viruses – if they get into people – can be tougher on us.
The second bad news part of this story is that this specific strain of H7N9 appears to be not so pathogenic in birds. That is, it does not cause severe disease in birds. This means that birds don’t show many clinical signs, making it harder to detect, and that infected birds will likely be fairly capable of going about their regular business – spreading the agent amongst themselves. H5N1 kills domestic birds quickly, so we are pretty quick at finding it. This one has a much better chance of infiltrating into multiple types of birds without giving us many clinical signs as warning.
[section_subtitle] Is there any relationship between the discovery of more than 16,000 dead pigs and more than 1,000 dead ducks found in several of China’s rivers and the H7N9 outbreak? [/section_subtitle]
Authorities have identified a few diseases in these pigs but I have heard nothing about them being infected with this H7N9 strain. That does not mean that it cannot happen. Pigs may be more susceptible than humans to H7N9 as they have more transitional cells lining their respiratory tract that avian-sourced influenza can invade. [Chinese authorities report tracking new cases of H7N9 to five live bird markets in the eastern part of the country].
[section_subtitle] H7N9 is not expected to be effective in being transmitted from person to person but people are getting sick? [/section_subtitle]
We have pretty good evidence, at the moment, that this H7N9 agent is poorly transmissable from person to person. You will see the abbreviation ‘H2H’ used: “human to human”. If it is going pandemic you may see experts talk about ‘H2H2H’, suggesting that it passed from one person to another, and then that person transmitted to a third person. That’s when you can assume it’s going big.
The first bit of evidence that this H7N9 does not transmit well from person to person: Of the 530 close contacts between confirmed H7N9 patients and others as of April 9, none have resulted in transmission and creation of new cases. In regular every-day seasonal flu you’d expect 20-30% transmission rates to close contacts. So, in normal flu we’d expect 530 cases to result in more than 100 new cases, and they’ve identified none so far with this H7N9. No doubt we will eventually find some, but it argues that this thing has a really tough time going from person to person.
The other bit of evidence relates to the demographic profile of who is currently getting sick in China. Of the first 14 cases, 13 were adults; only 1 was a child. Kids are near ideal flu transmitting vessels – they congregate together, they have little immunity, they touch themselves and each other constantly – and in most outbreaks they are a big part of the case numbers. Here, it does not appear to be so, which suggests that the virus is coming from an exposure other than human to human.
[section_subtitle]If the new strain is caused by a recombination of different avian viruses, what’s the likelihood that the same recombination could happen in more than one geographic location? [/section_subtitle]
We think we understand some of the basic requirements for these things to reassort in birds: You need a compatible environment (humid, not too cold, not too hot), you need stressed populations (crowded, food distressed, other diseases), and you need those populations to get into very close contact (crowded by humans or by occupying small ecological niches).
In Southeast Asia, a place that stays fairly warm and humid throughout the year, a common marketing system is called the ‘wet bird’ or ‘live bird’ market.In this system the customer goes to an open air market filled with farmers who have domestic birds (chickens and ducks) and wild caught birds (gulls, geese, ducks, etc.) alive in cages stacked on each other. The customer selects a bird, and either the selling farmer or a central part of the market kills the bird, defeathers it, guts it, and gives it to the customer as a ‘warm’ bird. The customer then goes home and prepares it for a family meal.
Such systems are about perfect for allowing different viruses to mix and reassort. If you remember the last few minutes of the movie “Contagion”, that is pretty much the system they showed as being the source of the global killer virus. It is how SARS got its start. It is how several of the influenzae have jumped species.
We have live bird markets in the U.S., most commonly along the East Coast, but they exist elsewhere. They tend to be small, however, and health regulations keep them from being quite as ideal for viral transmission as we see in Southeast Asia. Furthermore, for much of the year the weather is not compatible for high levels of transmission.They are a concern, but because they are economic units and are tied with cultural norms within our diverse population they continue to operate.
Other areas/situations where you are prone to seeing birds congregating in numbers sufficient to promote cross-infections and reassortment occur during migrations. Birds of several species will often stop in transit on the same lake or the same island beaches. This kind of close contact allows a flu virus numerous opportunities to jump ship. This is what is thought to have allowed H5N1, after having spent about eight years running up and down the eastern part of Southeast Asia, to jump across to the mid-East and Europe in Jan-Mar of 2005.
[section_subtitle] For more information: [/section_subtitle]
World Health Organization Disease Outbreak News
New England Journal of Medicine First Report of H7N9 Human Infection
