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Chicken Flu: How Much Of A Threat? By Amit Sen Gupta

THE recent epidemic termed “chicken flu” has once again underlined how fragile humankind’s mastery over disease really is. The chicken flu epidemic, affecting poultry stocks, has now spread across most of East Asia and spread to other countries too is a distinct possibility. Its incidence has been officially reported in Korea, Vietnam, Japan, Thailand, Cambodia, Hong Kong, Laos, China, Indonesia and Pakistan. The disease is caused by a virus that belongs to the influenza family, and primarily affects birds. While wild birds are also affected, domesticated birds like chicken are affected more seriously.

The disease is not new, in the sense that variants of the same virus are known to have devastated poultry farms in different parts of the world in the past. What is, however, particularly worrying as regards this epidemic is the ability of the virus to attack human beings to- i.e. it seems to be able to “jump species” and cause disease in human beings. This coupled with the fact that influenza epidemics among humans caused by new viruses have been known in the past to kill thousands, even millions, has set alarm bells ringing across the world.

WHAT IS CHICKEN FLU?

While the virus responsible for chicken flu is part of the family of viruses that cause influenza, the recent epidemic has been caused by a new variant. In fact, the periodic incidence of epidemics caused by the influenza virus, is explained by the ability of the influenza virus to mutate (i.e. change its characteristics), thereby evading the immune systems of human beings. When a disease is around for a long time, most people develop immunity to the virus (or other disease causing organism). Viruses try to outwit us by constantly changing themselves, and “fooling” our immune systems.

The scientific term for Bird Flu is “Avian Influenza”. It normally infects only birds and, less commonly, pigs. While all bird species are thought to be susceptible to the infection, domestic poultry flocks are especially vulnerable. The disease in birds has two forms: the first causes mild
illness, sometimes expressed only as ruffled feathers or reduced egg production. The second form, known as “highly pathogenic avian influenza” was first recognized in Italy in 1878. It is extremely contagious in birds and rapidly fatal, with a mortality approaching 100 per cent. Outbreaks of avian influenza, especially the highly pathogenic form, can be devastating for the poultry industry and for farmers. Economic consequences can be especially devastating in developing countries where poultry raising is an important source of income - and of food - for impoverished rural farmers and their families.

HOW THE DISEASE SPREADS

Since mid-December 2003, a growing number of Asian countries have reported outbreaks of highly pathogenic avian influenza in chickens and ducks. Infections in several species of wild birds and in pigs have also been reported. The rapid spread of highly pathogenic avian influenza, with outbreaks occurring at the same time in several countries, is historically unprecedented and of great concern for human health as well as for agriculture. Particularly alarming, in terms of risks for human health, is the detection of a highly pathogenic strain, known as “H5N1”, as the cause
of most of these outbreaks. H5N1 has jumped the species barrier, causing severe disease in humans, on two occasions in the recent past and is now doing so again, in gradually growing numbers, in Vietnam and Thailand.

Within a country, the disease in birds spreads easily from farm to farm. Large amounts of the virus are secreted in bird droppings, contaminating dust and soil. Airborne virus can spread the disease from bird to bird, causing infection when the virus is inhaled. Contaminated equipment, vehicles, feed, cages or clothing - especially shoes - can carry the virus from farm to farm. Droppings from infected wild birds can introduce the virus into both commercial and backyard poultry flocks. So called “wet” markets, where live birds are sold under crowded and sometimes-unsanitary conditions can be another source of spread.

The disease can spread from country to country through international trade in live poultry. Migratory birds, including wild waterfowl, sea birds, and shore birds, can carry the virus for long distances and have, in the past, been implicated in the international spread of highly pathogenic avian influenza. Migratory waterfowl - most notably wild ducks - are the natural reservoir of bird flu viruses, and these birds are also the most resistant to infection. They can carry the virus over great distances, and excrete it in their droppings, yet develop only mild and short-lived illness.

MAJOR CONCERNS

As discussed earlier, the major concern about the epidemic is that most of the recent outbreaks have been caused by the highly virulent H5N1 strain of the virus. There is mounting evidence that this strain has a unique capacity to jump the species barrier and cause severe disease, with high mortality, in humans. To date, Vietnam has reported 23 confirmed cases of human infection, of which 15 have been fatal.

A second and even greater concern is the possibility that the present situation could give rise to another influenza pandemic in humans. Scientists know that avian and human influenza viruses can exchange genes when a person is simultaneously infected with viruses from both species.
This process of gene swapping inside the human body can give rise to a completely new subtype of the influenza virus to which few, if any, humans would have natural immunity. Moreover, existing vaccines, which are developed each year to match presently circulating strains and protect
humans during seasonal epidemics, would not be effective against a completely new influenza virus. If the new virus contains sufficient human genes, transmission directly from one person to another (instead of from birds to humans only) can occur. This could give rise to the next great
influenza pandemic. An influenza pandemic is a global outbreak of influenza and occurs when a new influenza virus emerges, spreads, and causes disease worldwide.

The last century has seen three such pandemics caused by the influenza virus. All of them spread worldwide within 1 year of being detected. The most devastating was termed the “Spanish Flu” and in 1918-19 killed an estimated 20 to 50 million people worldwide (i.e. possibly more than the
number who died in the First and Second World Wars combined!). This was followed by the “Asian Flu” in 1957-58 and the “Hong Kong Flu” in 1968-69 – each killed more than a million people globally.

NO ROOM FOR COMPLACENCY

Fortunately till date human-to-human transmission of the virus has not been reported. All the infected humans got the infection through the close handling of infected birds. A new virus adapted for efficient human-to-human transmission would spread very rapidly, and health authorities would know very quickly that a completely new virus had emerged.

There is no evidence, to date, that this has occurred. But it is virtually impossible to predict when and if this will happen. We do not really know if we stand at the doorstep of a new influenza pandemic, or if we can control such a pandemic if there is an outbreak.

Influenza viruses are highly unstable and their behaviour defies prediction. The first priority, and the major line of defence, is to reduce opportunities for human exposure to the largest reservoir of the virus: infected poultry. This can be achieved through the rapid detection of poultry outbreaks and the emergency introduction of control measures, including the destruction of all infected or exposed poultry stock, and the proper disposal of carcasses.

The most important control measures are rapid destruction (“culling”) of all infected or exposed birds, proper disposal of carcasses, and the quarantining and rigorous disinfection of farms. The virus is killed by heat (60 degrees C for 30 minutes) and common disinfectants, such as fomalin and iodine compounds. However, the virus can survive, at cool temperatures, in contaminated manure for at least three months. In water, the virus can survive for up to four days at 22 degrees C and more than 30 days at 0 degrees C. For the highly pathogenic form, studies have shown that a single gram of contaminated manure can contain enough virus to infect 1 million birds. Restrictions on the movement of live poultry, both within and between countries, are another important control measure.

Fortunately, till date the epidemic has not affected poultry stocks in India - though the outbreak in Sindh, in Pakistan, means that it is practically at our doorstep. There is really no room for complacency. One can only hope that the “bird flu” epidemic will be contained globally and that it will not trigger the next influenza pandemic among humans.

29th February 2004