Very new Vietnam possible case - Event Date: August 05 2007

Did wild birds bring avian flu to India?

N. Gopal Raj

THE OUTBREAKS of deadly bird flu that occurred among poultry in Maharashtra earlier this year could have been the result of two independent introductions of the virus, possibly by wild birds, according to research that has been published in the journal Current Science.

Tests carried out at the time by the government's High Security Animal Disease Laboratory in Bhopal had shown that the outbreaks in Maharashtra were caused by a strain of bird flu known as H5N1. This virus had ravaged poultry flocks in several East and South-East Asian countries since December 2003. To the world's alarm, the virus did not remain confined to birds but occasionally jumped to humans, often with deadly effect.

In the latter half of 2005, the virus spread to Europe and other parts of the world with frightening rapidity. Its journey out of eastern Asia is usually thought to have begun at Qinghai Lake in western China where large numbers of wild birds congregate before their autumn migration. Thousands of these wild birds suddenly died in mid-2005 and were later found to have been infected by the H5N1 strain.

Shortly afterwards, the virus appeared in Mongolia, Siberia, and Kazakhstan before moving on to countries in central Europe, western Europe, and West Asia.

By early February this year, chicken were dying of the virus in Nigeria. Shortly afterwards, India announced that the virus had been detected in samples taken from poultry in Nandurbar district in Maharashtra. Soon afterwards another outbreak was reported from nearby Jalgaon district, also in Maharashtra.

H.K. Pradhan and his colleagues at the Bhopal laboratory have been looking closely at the viruses isolated from the outbreaks at Nandurbar and Jalgaon. In particular, they studied the genetic sequence for a key viral protein known as haemagglutinin.

The haemagglutinin gene in the viruses from the Nandurbar and Jalgaon outbreaks were "not identical," suggesting that these outbreaks were possible due to "two independent populations of the virus introduced at two different times," noted the scientists in their Current Science paper. J.S.M. Peiris, a noted virologist at the University of Hong Kong, is a co-author of the paper.

The scientists also compared the haemagglutinin gene of the H5N1 viruses found in India to those of 30 H5N1 viruses found in east and south-east Asia, Europe, Eurasia, and Africa. The two Indian H5N1 viruses were genetically closest to the viruses isolated this year from swans in Italy and Iran. Such genetic similarity was "suggestive of spread of the virus to distant places through wild, aquatic bird migration," say the scientists.

These findings mirror those based on analysis of the H5N1 viruses responsible for outbreaks in Nigeria. The paper published recently in the scientific journal Nature suggested that there had been multiple introductions of the virus into the African country. Based on comparisons of the haemagglutinin gene, a team of scientists from Luxembourg, Nigeria, and the Netherlands said three lineages of the H5N1 virus had been "independently introduced through routes that coincide with the flight paths of migratory birds, although independent trade imports cannot be excluded."

The issue of how far wild birds are to be blame for the spread of the H5N1 virus, especially its movement out of eastern Asia, is a contentious one. Wild water birds are known to harbour a rich diversity of bird flu viruses. But these viruses are of low virulence that do not harm the wild birds. It was only when the flu viruses spread to other birds, such as chicken, that they mutate and became highly virulent.

FAO finding

The question has been whether migrating wild birds could also carry the dangerously virulent H5N1 strain over long distances when the virus appeared to be lethal to the birds themselves. A recent report prepared for the Food and Agriculture Organization (FAO) said there was evidence that several species of wild birds may be able to carry the virus without themselves becoming ill. "The search for the main healthy carrier species of H5N1 along these migratory routes is under progress," observed Marius Gilbert and fellow scientists in their report of February 2006.

Conservation groups question whether genetic similarities between the H5N1 viruses found in Nigeria and India with strains isolated in distant places can by themselves be taken as evidence implicating wild birds. "Genetic similarity says nothing whatsoever about the mode of transport between sites," points out Richard Thomas of BirdLife International.

A news feature published in a recent issue of Nature quoted Ward Hagemeijer, programme leader for avian influenza at Wetlands International, as saying that a Nigerian H5N1 strain matching one found only in wild European birds was not convincing evidence that wild birds were the cause of the outbreak. Many more H5N1 strains had been studied in wild birds and than in poultry. So a search for similarities between strains was far more likely to turn up a close relative in the wild bird database than in the database for poultry.

Dr. Richard Thomas makes a similar point about the finding that the Indian H5N1 strains closely resembled those found in dead swans in Iran and Italy. The most plausible source of the virus in the dead swans was contamination from poultry sources in the Black Sea and Caspian Sea regions, where the virus had been present for months prior to the wild bird outbreaks in Europe. "Yet no poultry samples are analysed from this region," he said in an email.

A "smoking gun" that could convincingly pin the blame on wild birds is lacking. In the early months of this year, nearly 5,000 samples were collected from wild birds in 12 African countries. The sample collection included countries that had just reported H5N1 infections. But the virulent H5N1 virus did not turn up in even a single sample.

Likewise, although some 1,100 samples taken from wild birds in India were reported to have been tested at the Bhopal laboratory, none was positive for the H5N1 virus, points out Taej Mundkur of Wetlands International. Moreover, before reaching Maharashtra, wild birds would have had to stop at various places further north in India as well as in neighbouring Pakistan, Bhutan, and Nepal. Yet no deaths of wild birds or poultry as a result of the virus were reported from places along the birds' migratory route prior to the outbreaks in Maharashtra, he pointed out.

The Government had checked and ruled out various routes for the H5N1 virus to have entered the country, such as through smuggled poultry, contaminated feed or poor quality vaccines, according to S.K. Bandyopadhyay, the Union Government's Animal Husbandry Commissioner. But it had not been possible to rule out wild birds and the findings of the Bhopal laboratory lent support to that view.

However, in order to establish that wild birds were indeed responsible, it was necessary that the same or very similar viral strains to those found in infected poultry also be isolated from wild bird samples in India, Dr. Bandyopadhyay told The Hindu . That had not been possible.

Even at the global level, unravelling the role of wild birds in the spread of H5N1 is not proving easy. Laboratory experiments show that certain wild ducks, if they became infected, would be capable of transmitting the virus. But healthy migratory birds that are actually carrying the virulent H5N1 virus have been difficult to find. More than 45,000 wild birds were tested between October 2005 and January 2006 in countries of the European Union, and not one had the virus. The virus has been detected only among dead and moribund wild birds in Europe.

Does that mean that migratory birds only act as carriers of the virus after they pick it up from infected poultry or other wild birds at an earlier stage in their journey? Or are some species of wild water fowl now able to act as `reservoirs' of the virulent H5N1, as they do with low-virulence forms of bird flu?

There was poor understanding of the H5N1 virus' presence in various wild bird populations, lamented Juan Lubroth, who heads FAO's infectious diseases group, when he addressed a scientific conference in May this year. The virus had been documented too frequently in dead wild birds — and too little in healthy wild bird populations, he pointed out. Limited financial resources, lack of local expertise to carry out such investigations, and the logistics of taking samples from wildlife had constrained the surveillance that was necessary of bird flu viruses in wild birds, he remarked.

Moreover, hitching a ride with wild birds is only one of the ways by which the virus is able to spread. "FAO considers that globalisation and international trade are definitely the main factors in the spread of the virus from one country or region to another, and that wild birds play only a partial role in this," said the organisation's deputy directory-general, David Harcharik, at the May conference.