January 28, 2004

Sneezing fowl

Bird flu: it's this year's Asia disease.
Last year it was SARS, before that dengue fever. There's always something. So let's look at the WHO and what they say about it.

Avian influenza is an infectious disease of birds caused by type A strains of the influenza virus. The disease, which was first identified in Italy more than 100 years ago, occurs worldwide. All birds are thought to be susceptible to infection with avian influenza, though some species are more resistant to infection than others.

...

Direct or indirect contact of domestic flocks with wild migratory waterfowl has been implicated as a frequent cause of epidemics. Live bird markets have also played an important role in the spread of epidemics.

...

In the absence of prompt control measures backed by good surveillance, epidemics can last for years. For example, an epidemic of H5N2 avian influenza, which began in Mexico in 1992, started with low pathogenicity, evolved to the highly fatal form, and was not controlled until 1995.

All type A influenza viruses, including those that regularly cause seasonal epidemics of influenza in humans, are genetically labile and well adapted to elude host defenses.

...influenza A viruses, including subtypes from different species, can swap or “reassort” genetic materials and merge. This reassortment process, known as antigenic “shift”, results in a novel subtype different from both parent viruses. As populations will have no immunity to the new subtype, and as no existing vaccines can confer protection, antigenic shift has historically resulted in highly lethal pandemics. For this to happen, the novel subtype needs to have genes from human influenza viruses that make it readily transmissible from person to person for a sustainable period.

Conditions favourable for the emergence of antigenic shift have long been thought to involve humans living in close proximity to domestic poultry and pigs. Because pigs are susceptible to infection with both avian and mammalian viruses, including human strains, they can serve as a “mixing vessel” for the scrambling of genetic material from human and avian viruses, resulting in the emergence of a novel subtype. Recent events, however, have identified a second possible mechanism. Evidence is mounting that, for at least some of the 15 avian influenza virus subtypes circulating in bird populations, humans themselves can serve as the “mixing vessel”.

There's plenty more. Hong Kong had a bird flu scare i 1997, where 18 people were infected and 6 died. They culled 1.5 million chickens in 3 days, probably averting a wider pandemic. The bad news is another pandemic is expected, following the big one in 1918-19 and others in 1957 and 1968. And there's this:
at least four months would be needed to produce a new vaccine, in significant quantities, capable of conferring protection against a new virus subtype.
End result: it's potentially scary but so long as the various countries infected slaughter their poultry populations quickly it can be contained. Of course it doesn't bode well that Asian nations have a bad track record when it comes to infectious diseases. In this case Thailand, where it seems to have started, deliberately kept quiet about it for a few weeks. Hopefully Governments are all aware of the need to deal with things quickly. But as I said, they don't have a good track record.

The lessons of SARS weren't so obvious, apparently. History repeating itself.

UPDATE: Adam has more.

Posted by Simon at January 28, 2004 12:20 PM | TrackBack
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