Bird flu: shorter interval between outbreaks

Published in The Hindu on February 8, 2007

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It was found that the strain that affected but did not kill the ducks were mutants. — Photo: Wikipedia

A second instance of avian influenza spreading across the globe and to countries far away from Southeast Asia, the `epicentre’ for the virus, seems real.

The lethal H5N1 virus has been recently found in a farm in Suffolk in Britain. Many countries where bird flu outbreaks were reported last time have been affected this time as well.

Starting in Southeast Asia, the H5N1 virus found in birds in Britain, Hungary and Russia, Egypt and Nigeria, shows some kind of a similar spread across countries. No outbreak of the virus has been reported in India till date. Indonesia appears to be the worst hit; Five of the six humans infected with the virus have died. In Vietnam, three outbreaks occurred in December in Ca Mau province and one in Bac Lieu province. China has not reported any outbreak so far.

While the flu has been marching from one country to another infecting birds and in some cases, humans, one fact is becoming apparent as long as the virus persists in birds, be it poultry or wild birds, avian flu outbreaks are bound to recur. Only the time interval between outbreaks cannot be determined for sure.

Short interval

The short interval between the end of the last outbreak in 2006 and the start of the latest outbreak a few months later indicates that the virus is not only prevalent in birds, preventive measures may have actually helped the spread of the virus.

A report in Nature (July 2006) indicates the presence of resistance of one of the mutations to the antiviral drug amantadine. The Centers for Disease Control and Prevention (CDC) Atlanta, in its report points out that the virus has developed resistance to not only amantadine but also to rimantadine. CDC cites another study that highlights the ability of ducks to spread more of the virus and for a longer duration without actually falling sick.

It may be remembered that when H5N1 first adapted to chicken, they proved lethal to ducks. But sometime in 2003, ducks showed signs of harbouring the virus without actually dying. According to New Scientist (May 2006), this phenomenon was actually seen in the field in October 2004. Scientists found one in five ducks in Vietnam that were infected by the virus were shedding them in their faeces. It was found that the strain that affected but did not kill the ducks were mutants. Such mutants must have then infected migratory birds that acted as carriers.

Routes of transmission

So were the migratory birds responsible for all the bird flu outbreaks across the globe? Not really. In most countries in East Asia (except Japan), the poultry got the virus directly from infected chicken and not from migratory birds; most European and African countries got it from migratory birds (Qinghai strain). The strains found in birds in Asia and Europe were different, making the route of infection quite clear.

Though quite early to say with surety, officials in Britain have claimed that the infection found in farm birds did not come from wild birds. But as the article in New Scientist (May 2006) points out, there is a possibility that the virus originally carried by wild birds has been moving back and forth from poultry to wild birds.

That brings out the importance of controlling the virus in poultry as it is here that most human cases keep coming up. Exceptions exist, though. The Anhui province in China where a 37-year-old man who got infected but recovered in December last year did not apparently have any outbreak in poultry.

Mass vaccination without other stringent control measures may not be the best strategy. Mass vaccination could have only helped the birds to spread the virus without the symptoms showing up in the vaccinated birds, reports New Scientist (January 2007).