What is bird flu?
Bird Flu or Avian influenza is a very contagious disease caused by viruses that normally only infect birds and poultry and also less commonly pigs. All bird species are susceptible, especially domestic poultry, which are very vulnerable to Bird Flu infections that can rapidly reach epidemic proportions. The disease in birds has two forms: The first causes mild illness with the symptoms being ruffled feathers or reduced egg production. The second symptoms are more serious and known as "Highly Pathogenic Avian Influenza". This is a extremely contagious disease for birds and rapidly fatal with a mortality of 100%. Birds and poultry can die on the same day that symptoms first appear.
What is the current situation?
Since mid-December 2003, a growing number of Asian countries, including Thailand, have reported outbreaks of the highly pathogenic avian influenza in chickens, ducks and other poultry. Infections of Bird Flu in several species of wild birds and in pigs have also been reported. The rapid spread of the Avian Influenza, with outbreaks occurring at the same time in several countries in Asia is historically unprecedented and of great concern for human health and agriculture industry.
Are humans at risk?
The detection of the highly pathogenic Avian Influenza strain, known as "H5N1" is the cause of most of the outbreaks and is very alarming for human health. The "H5N1" virus has jumped the species barrier and caused severe disease in humans on two occasions recently and is now doing so again, in gradually growing numbers of humans in Vietnam and Thailand.
Can the current outbreaks affect humans in large numbers?
Most, though not all of the major outbreaks of Bird Flu recently reported in Asia were caused by the "H5N1" virus strain, which seems to have a unique capacity to mutate and jump the species barrier, causing severe disease, with high mortality for humans. There is also the possibility that the present situation could give rise to another influenza pandemic in humans. Scientists say 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 influenza virus to which most humans do not have natural immunity.
Would existing vaccines be effective?
In the case of a new subtype of influenza virus, existing vaccines which are developed each year to match the circulating virus strains and protect humans, would not be effective. If the new influenza virus contains sufficient human genes, transmission from person to person, instead of from birds to humans only, can start to occur. When this happens, a new influenza pandemic or epidemic can begin. Most alarming would be the situation in which human-to-human transmission resulted in successive generations of severe disease with high mortality. In the years 1918-1919 a completely new influenza virus subtype emerged and spread around the world in 4 to 6 months. Several waves of infection occurred over 2 years, killing an estimated 40-50 million persons in the world.
Has there been human-to-human transmission now?
No. World Health Organization teams in Vietnam and Thailand are conducting studies to detect the earliest stage of human-to-human transmission of the disease. These studies are also expected to shed some light on the origins and characteristics of the currently circulating "H5N1" virus strain. A new virus for human-to-human transmission would spread very rapidly and health authorities would know very quickly that a completely new virus had emerged. Till now there is no evidence of that this has occurred yet.
Does human infection with H5N1 happen often?
No, very rarely. The first documented human infections with the "H5N1" avian strain occurred in Hong Kong in 1997. In that outbreak, 18 persons were hospitalized and 6 of them died. The source of infection in all cases was traced to close contact with diseased birds on local farms (1 case) and contact with chickens in live poultry markets (17 cases).
Is there a vaccine effective against H5N1 in humans?
No. Currently available influenza vaccines will not protect against the disease caused by the "H5N1" strain in humans. The WHO (World Health Organization) is working to develop a prototype "H5N1" virus for use by leading vaccine manufacturers. An available vaccine prototype virus, developed using the 2003 strain of "H5N1" (which caused the 2 human cases in Hong Kong), cannot be used to expedite vaccine development. Initial analysis of the 2004 virus, conducted by laboratories in the WHO network, indicates that the virus has mutated significantly since the year 2003.
Are there drugs available for prevention and treatment?
Yes. Two classes of drugs are available. These are the M2 inhibitors (Amantadine and Rimantadine) and the neuraminidase inhibitors (Oseltamivir and Zanimivir). These drugs have been licensed for the prevention and treatment of human influenza in some countries, and are thought to be effective regardless of the causative virus strain. However, initial analysis of viruses isolated from the recently fatal cases in Vietnam and Thailand indicates that the viruses are invariably resistant to the M2 inhibitors. Further testing is under way to confirm the resistance of Amantadine. Network Laboratories are also conducting studies to confirm the effectiveness of neuraminidase inhibitors against the current "H5N1" virus strains.
What are the control measures in birds?
The most important control measures are rapid complete destruction, called "culling" or "stamping out", of all infected or exposed birds plus proper disposal of carcasses, and the quarantining and rigorous disinfection of affected poultry farms. The virus can be killed by heat (56 degrees Celsius for 3 hours or 60 degrees Celsius for 30 minutes) and common disinfectants, such as formalin and iodine compounds. The virus can easily survive in contaminated bird-manure for at least 3 months. In water, the virus can survive for up to 4 days at 22 degrees Celsius and more than 30 days at 0 degrees Celsius. 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 and manure, both within and between Asian countries is another important control measure.
What are the consequences of outbreaks in poultry?
Outbreaks of Avian Influenza or Bird Flu can be devastating for the poultry industry and for farmers. An outbreak of highly pathogenic avian influenza in the United States in 1983-1984, largely confined to the state of Pennsylvania, resulted in the destruction of more than 17 million birds at a cost of nearly 65 million US-dollars. When outbreaks become widespread within a country, control can be extremely difficult. For example, an outbreak that began in Mexico in 1992 was not completely controlled until the year 1995.
How do outbreaks of avian influenza spread within a country?
Within a country, the disease 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 farm equipment, vehicles, poultry-feed, cages or clothing and especially shoes can carry the virus from farm to farm. The deadly virus can also be carried on the feet and bodies of animals, such as rodents, which act as "Mechanical Vectors" for spreading the disease. Flies may also act as mechanical vectors. Droppings from infected wild birds can introduce the virus into both commercial and backyard poultry flocks. The risk that infection will be transmitted from wild birds to domestic poultry is greatest where domestic birds roam freely and share the same water supply with wild birds, or use a water supply that might become contaminated by droppings from infected wild bird carriers. So called "Wet" or "Live" markets, where live birds are sold under crowded and unsanitary conditions can be another important source of spread.
How does the disease spread from one country to another?
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 and most notably wild ducks, are the natural reservoir of Bird Flu viruses, and these migrating birds are also the most resistant to virus infection. They can carry the Bird Flu virus over great distances, and excrete it in their droppings, yet the migrating birds themselves develop only mild and short-lived illness. Domestic ducks, however, are very susceptible to lethal infections, as are chickens, turkeys, geese, and all other species of poultry raised on commercial or small backyard farms.
