All the birds of the air fell a-sighing and a-sobbing,/ When they heard the bell toll for poor Cock Robin. The avian flu that is devastating many Asian economies is an infectious disease caused by a type A strain of the influenza virus. The subtypes of influenza viruses 16 HA (haemagluttinin) and 9 NA (neuraminidase) are known to infect wild waterfowl, thus providing an extensive reservoir of influenza viruses perpetually circulating mostly in bird populations. While all birds are susceptible to infection with avian influenza viruses, the vast majority of these viruses cause no harm.
To date, all highly pathogenic outbreaks have been caused by viruses identified as of the H5 and H7 subtypes. Of these H5N1, still an uncommon form, can become highly pathogenic both for birds and for other species. This is not a recent discovery. First identified in Italy in 1878, H5N1 is characterized by sudden onset of severe disease, rapid contagion, and a mortality rate that can approach 100 per cent within 48 hours.
What is of great current concern is that some species of migratory waterfowl are now thought to be carrying this H5N1 virus and introducing it to new geographical areas located along their flight routes. During 2005, an additional and significant source of international spread of the virus in birds became apparent for the first time, but the phenomenon remains poorly understood. Scientists are increasingly convinced that at least some migratory waterfowl are now carrying the H5N1 virus in its highly pathogenic form, sometimes over long distances, and introducing the virus to poultry flocks in areas that lie along their migratory routes. Should this new predatory role of H5N1 be scientifically confirmed, it will mark a change in the longstanding stable relationship that more or less safely contained H5N1 within a natural wild-bird reservoir.
Evidence supporting an altered role emerged in mid-2005 and has since been strengthened. The death of more than 6,000 migratory birds infected with the H5N1 virus that began at the Qinghai Lake nature reserve in central China in late April 2005 was highly unusual and probably unprecedented. The outbreaks of the H5N1 avian influenza began in south-east Asia in mid-2003. In late July 2005, the virus spread geographically beyond its original focus in Asia to affect poultry and wild birds in the Russian Federation and Kazakhstan. In October 2005, the virus was reported in Turkey, Romania and Croatia. In early December 2005, Ukraine reported its first outbreak in domestic birds. Now it has travelled to India too. Elsewhere in Asia, the virus has become endemic in several of the initially affected countries.
Of the hundreds of strains of avian influenza A viruses, only four are known to have caused human infections: H5N1, H7N3, H7N7, and H9N2. The last three caused only mild infections. A notable exception is the dreaded H5N1 that has proved to be highly pathogenic for humans too, accounting for proportionately by far the greatest number of human cases of very severe disease ending in death. The 1918 influenza pandemic was never really understood, but H5N1 crossed the species barrier and infected humans on at least three occasions in recent years: in Hong Kong in 1997 (18 cases, six deaths), in Hong Kong in 2003 (two cases, one death) and in the current outbreaks that began in December 2003 and were first recognized in January 2004.
But of far greater concern is the risk that H5N1, given enough opportunities, will develop the characteristics it needs to start another influenza pandemic. The virus has met all prerequisites for the start of a pandemic save one: an ability to spread efficiently and sustainably among humans. However, what is feared by the scientists is that the virus can improve its transmissibility among humans either gradually or suddenly, during the co-infection of a human or pig, which could result in a fully transmissible pandemic virus, announced by a sudden surge of cases with explosive spread.
All evidence to date indicates that handling dead or sick birds is the principal source of human infection with the H5N1 virus. Especially risky behaviours are the slaughtering, defeathering, butchering and preparation for consumption of infected birds. In a few cases, exposure to chicken faeces when children played in an area frequented by free-ranging poultry is thought to have been the source of infection.
All avian influenza viruses are readily transmitted from farm to farm by the movement of live birds, people (especially with shoes and clothing contaminated) and vehicles, equipment, feed and cages. Highly pathogenic viruses can survive for long periods in the environment, especially when temperatures are low. For example, H5N1 virus can survive in bird faeces for at least 35 days at 4 degrees Centigrade. At the much higher 37 degrees Centigrade, H5N1 can survive, in faecal samples, for 6 days.
For this highly pathogenic disease, the primary control measures suggested are culling (mass killing) of all infected or exposed birds, proper disposal of carcasses, quarantining and rigorous disinfection of farms and implementation of strict sanitary measures. Obviously, control is far more difficult under systems in which most birds are raised in small backyard flocks scattered throughout rural and semi-urban areas.
While culling is seen by the World Health Organization as the first line of defence, vaccination of poultry in a high-risk area is considered a supplementary emergency measure, provided quality-assured vaccines are used and recommendations from the World Organization for Animal Health are strictly followed. Poor quality animal vaccines pose a risk for human health, as they may allow infected birds to shed the virus while still apparently disease-free.
Apart from being difficult to control, outbreaks in farmyard flocks are associated with a heightened risk of human exposure and infection. These birds usually roam freely as they scavenge for food, often mingling with wild birds and sharing water sources with them. Such situations create abundant opportunities for human exposure to the virus, especially when birds enter households or are brought into households during adverse weather, or when they share areas where children play or sleep.
Poverty exacerbates the problem: in situations where a prime source of food and income cannot be wasted, households frequently consume poultry when deaths or signs of illness appear in flocks. But the ghastly asymmetry, between the rich and the poor, in the use of culling is not the only human question that remains unanswered. Who gave Man the right to cull all farmyard animals' As I have mentioned, pigs too carry the virus and do so even more dangerously, for they can be ill in two ways with both the human flu and the avian one, making it dangerously easier for the dreaded species crossover. Are we thinking of killing them off too' One day not only farmyard fowls and animals but all birds may be suspect, then all other animals; finally perhaps some categories of the human species. Where will you draw the line' Even being a sinner myself, I dread to think: what more sins against Nature are in store for us' And then what would happen to our lifeline, the food chain'
For relief, let me end presenting two rather different responses from persons concerned with the avian flu: Jeffery Taubenberger and his colleagues have sequenced the genes of the influenza virus recovered from preserved lung tissues of a US soldier who died of influenza in 1918, and a flu victim whose body had remained frozen in the permafrost of Alaska since 1918. As reported in Nature (October 6, 2005), the 1918 genome turns out to resemble bird flu genomes more closely than those of human strains. The question Taubenberger raises is, if a bird flu virus actually created the pandemic of 1918, what clues does that give out for understanding its current reappearance' A concerned ministry babu in New Delhi reportedly told the national press, 'I am a vegetarian, but I am now eating chicken for the good of the country.'