The Hindu : A fearful flu

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Saturday, Jan 24, 2004

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A fearful flu

By N. Gopal Raj

FOR A time it seemed that the Severe Acute Respiratory Syndrome (SARS) had reared its ugly head once again. But this SARS strain appears to be milder than the one that spread global panic last year. Instead, a more potent threat has health officials worried — influenza. A `bird flu' has successfully infected humans, killing all five confirmed cases in Vietnam. Now, Thailand says two of its citizens have been infected. So far, people have picked up the infection only through close contact with poultry. But if this virus develops the ability to pass easily between people, it could start another influenza pandemic.

There were four influenza pandemics in the 20th century. The great pandemic of 1918-1919 claimed 40 million to 50 million lives worldwide, more than four times the number killed in the First World War. Experts say that another pandemic is inevitable and perhaps even imminent. The `bird flu' virus presents a real threat because people would have no immunity against this new strain.

Influenza strains from birds were involved in setting off pandemics during the past century. But hitherto they never directly infected humans. Instead, pigs were the medium for bird and human influenza strains to exchange genes. This process of reassortment created new strains that could not be recognised by human immune systems.

It was in 1997 that a strain of bird flu called H5N1 was first found to have infected humans directly. Six of the 18 persons infected in Hong Kong died. Within three days, Hong Kong's officials destroyed all the poultry in the city, numbering one and a half million birds, probably preventing a pandemic from breaking out.

But H5N1 has reappeared. Outbreaks of the virus infection among poultry occurred in South Korea in mid-December 2003, subsequently in Japan and Vietnam, and most recently in Thailand. Poultry in these countries have been slaughtered in their millions in an effort to stop the epidemic. "Of all the avian influenza viruses, which normally cause infection in birds and pigs only, the H5N1 strain may have a unique capacity to cause severe disease, with high mortality, in humans," says the World Health Organisation.

The WHO has sounded a grim note of warning, saying the situation could worsen. The simultaneous occurrence in several countries of large epidemics of H5N1 influenza in domestic poultry was historically unprecedented. The H5N1 strain may be more widely established in bird populations and in the environment in this part of the world than presently appreciated, says the WHO. Wild migratory water birds, most notably wild ducks, are the natural reservoir of bird influenza strains and can pass on the disease to more poultry flocks. "The potential for further spread of ongoing poultry epidemics, both within affected countries and to other countries, is therefore great," observes the WHO. This in turn increases the risk of a highly contagious human influenza strain evolving, which could set off a global pandemic.

The 1997 H5N1 event showed that poultry, and not just pigs, could act as intermediate hosts, say influenza experts Richard Webby and Robert Webster in a review published recently in Science. Sequencing of the H5N1 strain implicated in the current outbreak has shown that all its genes are currently of avian origin, which probably accounts for its inability to be transmitted from one human to another. "The only thing that has saved us so far this century is that these viruses haven't acquired the ability to transmit human to human," Dr. Webster has been quoted as saying in one press report.

But this could change if H5N1 strains become established in poultry and humans are also infected, warns the WHO. H5N1 mutates rapidly and also has a propensity to acquire genes from other viral strains. The spread of infection in birds increases the opportunities for direct infection of humans. If more humans are infected, the greater the chances of their serving as `mixing vessels' where human and bird strains exchange genes. Such reassortment could lead to the emergence of a novel influenza strain with sufficient human genes to be easily transmitted from person to person. "Such an event would mark the start of an influenza pandemic," points out the WHO. The hope is that culling farm birds and animals quickly will stop such a pandemic strain arising.

"If H5N1 influenza became pandemic in human beings, vaccination is not an option," according to an editorial in The Lancet. The WHO has announced that it is speeding up development of a new type of vaccine using genetic engineering, as existing vaccines would be ineffective against H5N1. But as this vaccine was untested, time-consuming clinical trials were unavoidable. Antiviral drugs were expensive and not effective enough. As influenza was far more contagious, quarantine measures which controlled SARS last year would not work. "In view of the high mortality of human influenza associated with this strain, the prospect of a worldwide pandemic is massively frightening," comments The Lancet editorial.

B. Lalitha Rao, Deputy Director at the National Institute of Virology in Pune and an expert on influenza, points out in a recent journal paper, "influenza surveillance activity in India is extremely limited." Such surveillance and detection would allow health authorities to implement preventive measures to control the spread of the disease.

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