AVIAN INFLUENZA
CAUSE OF DISEASE
Avian influenza is a disease of birds caused by type A strains of the influenza virus. All birds are susceptible to infection with the virus, although some species are more resistant to infection than others. There are different forms of the disease: Highly Pathogenic Avian Influenza (HPAI) can cause a 100% mortality in a flock, while other strains of the virus (Low Pathogenic Avian Influenza = LPAI) may cause no signs of disease at all. To date, all outbreaks of the highly pathogenic form have been caused by influenza A viruses of the H5 and H7 subtypes.
The current outbreak of HPAI in the Far East is caused by the H5N1 subtype. It
was first diagnosed in North Korea and subsequently Japan, Vietnam and
Thailand also reported outbreaks of the disease to the International Animal
Health Organisation (OIE).
TRANSMISSION
Migratory waterfowl (especially wild ducks) are natural reservoirs of the virus and these birds are mostly resistant to infection. Domestic poultry, including chickens and turkeys are very susceptible to HPAI. The virus is transmitted in poultry through direct contact or indirect contact with anything that has been contaminated with faecal material from an infected bird.
VIRUS BEHAVIOUR
Influenza viruses are genetically unstable and frequent changes in their genetic material (re-assortment) could result in a virus of low pathogenicity becoming highly pathogenic. It is also possible for subtypes from different species to 'swap' or reassort genetic materials and merge. If a person is simultaneously infected with a human influenza virus and an avian influenza virus, there is a possibility that a new, highly fatal virus emerges that can be transmitted between people.
CONTROL
The control of an outbreak of HPAI involves quarantine measures, movement control and the slaughtering of chickens as a precautionary measure to prevent the spread of the disease. The control measures have two aims, namely to:
(a) limit damage to the poultry industry and
(b) prevent exposure of humans to the virus, thus limiting the risk of the virus changing into a human disease.
HUMAN DISEASE
Close contact between birds and humans is required for the avian influenza virus to cause disease in humans. There has never been any evidence that humans could become infected via the consumption of poultry meat.
Human-to-human transmission is theoretically possible in this epidemic. It will however, only become of major concern if the virus succeeds in changing to a human disease virus.
SOUTH AFRICAN SITUATION
South Africa is free from highly pathogenic avian influenza. (Sporadic cases of low pathogenic avian influenza (LPAI) have occurred in a few provinces in 2002/2003 and were controlled by vaccination with an inactivated vaccine). South Africa's freedom from HPAI is ensured by not importing any live birds poultry meat or eggs from a country that has the disease.
The likelihood of transmitting the disease through poultry products is very low and the risk to the poultry industry can be controlled easily because the distance from the Far East prevents direct contact with infected birds. The main concern to the South African population is the possible change of the virus into a disease transmitted by humans. This risk is receiving international attention through the World Health Organisation (WHO).
issued by the Department of Agriculture
Date 27 January 2004