Avian Influenza (Bird Flu)

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.