Avian influenza, more commonly known as bird flu is a contagious viral infection which can affect all species of birds. This infectious disease is caused by type A strain of the influenza virus. There are about fifteen subtypes of influenza viruses. Influenza A (H5N1) is a subtype of the Type A influenza virus. Wild waterfowl are the main carriers of this virus and can be responsible for the primary introduction of infection into domestic poultry. When avian influenza spreads to poultry or other birds, it can cause more severe diseases. Avian influenza viruses that cause no obvious disease in waterfowl can be highly pathogenic in domestic poultry. Among domestic poultry species, turkeys are more commonly infected than chickens. All birds are thought to be susceptible to infection with avian influenza, though some species are more resistant to infection.

The virus was first isolated from birds (terns) in South Africa in 1961. Outbreaks of avian influenza have been recognised in poultry flocks in most countries of the world for many years. If the disease is severe, outbreaks are eradicated by slaughtering all birds in affected flocks. Avian influenza A (H5N1) was first recognised in 1997 in Hong Kong; millions of chickens were slaughtered after the virus was found to cause disease in people exposed to infected birds. This was the first time that the avian influenza virus had ever been found to transmit directly from birds to humans. During this outbreak, 18 people were affected, with six deaths and the outbreak was halted in Hong Kong by slaughter of the chickens. Prior to the above case, avian influenza usually caused only mild symptoms, such as pink eye. In 2003, H7N7 avian influenza affected poultry flocks in the Netherlands, leading to one human death amongst 83 affected people. The outbreak was halted by culling affected flocks.

H5N1 has recently re-emerged in many Asian countries in slightly altered form. The disease have been confirmed among poultry in Cambodia, China, Hong Kong, Indonesia, Japan, Laos, South Korea, Thailand and Vietnam. Millions of poultry have been slaughtered. The disease has also infected humans in Vietnam and Thailand and deaths have been reported.

Symptoms

Signs of the disease range from a mild infection with no symptoms to a severe epidemic that kills up to 100 per cent of infected birds. The symptoms can vary from a mild disease with little or no mortality to a highly fatal, rapidly spreading epidemic (highly pathogenic avian influenza) depending on the infecting virus strain, host factors, and environmental stressors. Symptoms include coughing, sneezing, ruffled feathers, swelling of the skin under the eyes, blisters on the combs, swollen heads, nervous signs like depression, and diarrhea. Decreased food consumption and drops in egg production are among some of the earliest and most predictable signs of disease. In some cases, birds die rapidly without clinical signs of disease.

Avian influenza A viruses do not usually infect humans; only people who come into contact with birds are at risk. Subtypes of the influenza A virus known as (H5N1) and (H9N2) have been known to infect humans. The symptoms of avian influenza in humans are akin to those of human influenza, fever, fatigue, malaise, myalgia, sore throat, cough and in severe cases pneumonia. Conjunctivitis is seen in some patients.

Spreading

Certain water birds act as hosts of avian influenza virus by carrying the virus in their intestinal tract and shedding it in their feces. Infected birds shed virus in saliva, nasal secretions and feces. Avian influenza viruses are transmitted to susceptible birds through inhalation of influenza particles in nasal, respiratory and fecal material from infected birds; however, fecal-to-oral transmission is the most common mode of spread. Live bird markets have also played an important role in the spread of epidemics.

The avian influenza virus can remain viable for long periods of time at moderate temperatures, and can survive indefinitely in frozen material. As a result, the disease can be spread through improper disposal of infected carcasses, manure, or poultry by-products.

Transmission of Avian influenza A from birds to humans is a rare event; but it may spread to humans, when they come into contact with the droppings of infected birds. Although it was not possible to establish how every case became infected, it is most likely that the individuals who contracted avian flu did so directly through exposure to chickens. However, if the virus were to mutate or combine with a human flu virus, it could start spreading from person to person. The human flu virus can spread more easily than the SARS virus. There is no evidence that the virus can survive in well cooked meat.

Prevention

Infected birds shed virus in the first two weeks of infection. Four weeks after infection, virus can no longer be detected. Hence, prevention is best accomplished by preventing contact between newly infected and susceptible birds. Preventing direct contact with free-flying birds and protecting domestic poultry from contact with the feces of wild birds is an important way to prevent avian influenza. Countries across Asia are battling to stop the spread of the disease by culling poultry.

The quarantining of infected farms and destruction of infected or potentially exposed flocks are standard control measures aimed at preventing the spread of the virus in a country’s poultry population. Apart from being highly contagious, avian influenza viruses are readily transmitted from farm to farm by mechanical means, such as by contaminated equipment, vehicles, feed, cages, or clothing. Highly pathogenic viruses can survive for long periods in the environment, especially when temperatures are low. In the absence of prompt control measures backed by good surveillance, the epidemic can last for years. Any object located on an infected poultry farm must be considered contaminated and should be completely cleaned and disinfected before it is moved from that premises.

Influenza viruses are very sensitive to most detergents and disinfectants. They are readily inactivated by heating and drying. However, flu viruses are well-protected from inactivation by organic material and infectious virus can be recovered from manure for up to 105 days. Complete removal of all organic material is part of any effective disinfection procedure. All buildings should be cleaned and disinfected after an infected flock is removed. The poultry litter should be composted before being used as manure to cultivated lands.

Contaminated houses are to be heated for several days to inactivate virus. Organic material should be removed followed by complete cleaning and disinfection of all surfaces. Contaminated litter and manure is problematic and should be composted to ensure that it does not spread infectious virus.

Treatment

There is no effective treatment for avian influenza. Good husbandry, proper nutrition and broad spectrum antibiotics may reduce secondary infections. It must be remembered that recovered flocks continue to intermittently shed the virus.

Diagnosis of avian influenza may be made on the basis of symptoms and events leading to the disease. However, since the symptoms and course of avian influenza are similar to other diseases, laboratory diagnosis is essential.

Avian influenza in humans can be detected reliably with standard influenza tests. Antiviral drugs are clinically effective in both preventing and treating the disease.

Recently discovered anti-viral drug, Tamiflue is considered as the only possible defense against an outbreak of human to human avian flue. A chemical compound called shikimic acid is the basic material for making Tamiflue drug. The acid is extracted from a spice named star anise, which is harvested in China, North Vietnam and neighbouring countries. The drug has proved effective against the lethal H5N1 strain of the bird flu. The drug will not prevent the Avian flue but it can reduce the severity of the disease.

Vaccines, however, take at least four months to be made and must be prepared for each subtype. Once the disease has been detected and reported, stringent disease control measures must be taken.

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