What is avian influenza?
Avian influenza is a contagious disease of animals caused by type A strains of influenza viruses that normally infect only birds, and sometimes pigs. Avian influenza has two forms; one that causes mild illness in birds, and another, known as "highly pathogenic avian influenza" that is extremely contagious and rapidly fatal for infected birds. It is an important disease economically for poultry farmers because of losses in poultry flocks. Transmission to humans in close contact with poultry or
other birds occurs rarely and only with some strains of avian influenza. The potential for transformation of avian influenza into a form that both causes severe disease in humans and spreads easily from person to person is a great concern for world health.
Avian influenza viruses (of which there are 15 types) infect wild bird populations, particularly water birds, typically without causing symptoms. The virus spreads through bird faeces and contaminated water or dust. When avian influenza spreads to poultry or other birds, it can cause more severe disease. Outbreaks of avian influenza have been recognised in poultry flocks in most countries of the world for many years.
There are several types of avian influenza. The strain that causes the greatest number of deaths is called highly pathogenic avian influenza (HPAI). The HPAI strain involved in the current outbreak is called H5N1. It was first recognised in 1997 in Hong Kong. At that time millions of chickens were slaughtered after the virus was found to cause disease in people exposed to infected birds. 18 people were known to affected, with six deaths. Fortunately, the virus was not able to spread from person to person, and the outbreak was halted in Hong Kong by slaughter of the chickens.
Of the 15 subtypes known, only subtypes H5 and H7 are known to be capable of crossing the species barrier from birds to humans. The symptoms of avian influenza in humans are akin to those of human influenza, ie. fever, sore throat, cough and in severe cases pneumonia.
In January 2004, a major new outbreak of H5N1 avian influenza surfaced again in Vietnam and Thailand's poultry industry, and within weeks spread to ten countries and regions in Asia, including Indonesia, South Korea, Japan and China. Intensive efforts were undertaken to slaughter chickens, ducks and geese (over 40 million chickens alone were slaughtered in high-infection areas), and the outbreak was contained by March, but the total human death toll in Vietnam and Thailand was 23 people.
It is feared that if the avian influenza virus undergoes antigenic shift with a human influenza virus, the new subtype created could be both highly contagious and highly lethal in humans. Such a subtype could cause a global influenza pandemic, similar to the Spanish Flu that killed over 20 million people in 1918. In February 2004, avian influenza virus was detected in pigs in Vietnam, increasing fears of the emergence of new variant strains.