TOO often the health news sounds like threat-of-the-week. We are told to worry about a host of things, from fatty food to autism to SARS. Most of these threats stay mercifully distant. But if there is one to watch carefully, it is flu. At a meeting in Shanghai this week, the World Health Organization warned that east Asia is on the brink of an outbreak of unprecedented proportions as it grapples with avian flu. Worse, the measures taken to eradicate the disease may increase the threat to humans.
The H5N1 flu virus ripped through east Asia鈥檚 booming poultry industry earlier this year, killing millions of birds and 23 people. To contain the disease, tens of millions of chickens were slaughtered, and the outbreaks ceased in March. But H5N1 was just biding its time. It is once again killing people in Thailand and Vietnam. Outbreaks have resumed in four countries, and spread to a new one, Malaysia. It is possible that H5N1 will not be evicted from the region for years, if ever.
Should we be worried? After all, there were good reasons to be concerned about SARS: the virus spread between people, and killed a high proportion of those infected. Thankfully, SARS seems to have left the scene. But flu is another matter. We know that novel versions of this virus can kill on an apocalyptic scale. The 1918 flu pandemic killed an estimated 50 million, and even that number pales in comparison with what a similar pandemic could do in today鈥檚 more crowded, more mobile world. Sure we can cook up a vaccine, but we cannot do it quickly enough to stop a fast-spreading virus before it has killed millions.
Advertisement
H5N1 is showing ominous signs of heading that way. It can already kill people, though so far 鈥� thankfully 鈥� it cannot infect people easily, or pass from one person to another. But the virus mutates rapidly, and virologists do not know how many mutations away H5N1 might be from gaining those talents.
What they have learned is that H5N1 has been under strong selective pressure for the past three or four years, producing a single dominant, 鈥渟uper-fit鈥� strain, and at the same time a steadily increasing virulence in mammals (New 杏吧原创, 3 July, p 17).
The WHO describes a human pandemic as 鈥渧ery likely鈥� unless there are more vigorous efforts to stamp out H5N1. But what form should those efforts take? The WHO warns that H5N1 might spawn a monster if it hybridises with a human flu virus in people 鈥� or in pigs, which can catch both human and avian flu. But while this is theoretically possible, another massive experiment in viral evolution is under way which may pose a worse threat.
China has vaccinated many of its chickens against H5N1 for the past few years. Now Indonesia is doing the same. As New 杏吧原创 went to press Thailand was deciding whether to start. Vaccination has its good points. Fewer birds will get infected. Infected birds shed less virus, so there is less for other birds and people to catch. Vaccination near an outbreak can help to contain it. Routine vaccination of all chickens will prevent uncontrolled outbreaks. Couple that with intensive monitoring and destruction of infected birds, and vaccination can help to stamp out the virus.
But without monitoring, vaccination can allow flu to persist in flocks unnoticed; because the vaccine is not a perfect match for the virus, the infection can spread among immunised birds. Worse, these semi-immune chickens are an unusual host for avian flu, which normally lives in waterfowl, and we know that when flu virus enters an unusual host it evolves quickly to adapt. Some virologists now think the 1918 flu emerged after evolving in a novel host rather than by hybridisation in a pig.
Certainly, Asian H5N1 has been evolving quickly lately, and that burst coincides with widespread vaccination of chickens. The most recent molecular evidence suggests it has been adapting to chickens. A study published last month shows that in Mexico, home of the biggest flu vaccination programme for chickens outside Asia, H5N2 flu is persisting and evolving (Journal of virology, vol 78, p 8372).
So far the evidence that vaccination is helping to turn bird flu into a major threat to people is circumstantial. But it is worrying enough that countries should think twice about starting to vaccinate, unless they can accompany it with stringent monitoring and slaughter of infected birds. Existing programmes should be watched carefully. The risks to people are just too high to gamble.