Avian flu

Alternative names

Bird flu; Influenza virulent in birds; Avian influenza

Definition

Avian influenza (also known as bird flu) is a type of influenza virulent in birds. It was first identified in Italy in the early 1900s and is now known to exist worldwide.

Infection

The causative agent is the avian influenza (AI) virus. AI viruses all belong to the influenza virus A genus of the Orthomyxoviridae family and are negative-stranded, segmented RNA viruses.

Avian influenza spreads in the air and in manure. Wild fowl often act as resistant carriers, spreading it to more susceptible domestic stocks. It can also be transmitted by contaminated feed, water, equipment and clothing; however, there is no evidence that the virus can survive in well cooked meat.

Cats are also thought to be possible infection vectors for H5N1 strains of avian flu (Kuiken et al, 2004).

The incubation period is 3 to 5 days. Symptoms in animals vary, but virulent strains can cause death within a few days.

Avian influenza in humans

Of the 15 subtypes known, only subtypes H5, H7 and H9 are known to be capable of crossing the species barrier from birds to humans. 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 (though a variety of sources quote average figures even higher, up to 100 million in some cases). Many health experts are concerned that a virus that mutates to the point where it can cross the species barrier (i.e. from birds to humans) will inevitably mutate to the point where it can be transmitted from human to human. It is at that point that a pandemic becomes likely.

The first cases of avian influenza passed from birds to humans was H5N1 in 1997 in Hong Kong. 16 people were infected, of whom 4 died. The outbreak was limited to Hong Kong. All chickens in the territory were slaughtered.

In January 2004, a major new outbreak of H5N1 avian influenza surfaced in Vietnam and Thailand's poultry industry, and within weeks spread to ten countries and regions in Asia, including Indonesia, South Korea, Japan and mainland 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. In February 2004, avian influenza virus was detected in pigs in Vietnam, increasing fears of the emergence of new variant strains.