Previous outbreaks of highly pathogenic avian influenza worldwide
1959 Scotland, chicken, H5N1
1963 England, turkey, H7N3
1966 Ontario (Canada), turkey, H5N9
1976 Victoria (Australia), chicken, H7N7
1979 Germany, chicken, H7N7
1979 England, turkey, H7N7
1983-1985 Pennsylvania (USA)*, chicken, turkey, H5N2
1983 Ireland, turkey, H5N8
1985 Victoria (Australia), chicken, H7N7
1991 England, turkey, H5N1
1992 Victoria (Australia), chicken, H7N3
1994 Queensland (Australia), chicken, H7N3
1994-1995 Mexico*, chicken, H5N2
1994 Pakistan*, chicken, H7N3
1997 New South Wales (Australia), chicken, H7N4
1997 Hong Kong (China)*, chicken, H5N1
1997 Italy, chicken, H5N2
1999-2000 Italy*, turkey, H7N1
2002Hong Kong (China), chicken, H5N1
2002 Chile, chicken, H7N3
2003 Netherlands*, chicken, H7N7
*Outbreaks with significant spread to numerous farms, resulting in great economic losses. Most other outbreaks involved little or no spread from the initially infected farms.
Observations from previous outbreaks (1959-2003)
Outbreaks of highly pathogenic avian influenza can be extremely difficult to control, even under favourable conditions (concentration of infected birds in well-maintained commercial production facilities, limited geographical occurrence).
- The 1983 Pennsylvania (USA) outbreak took two years to control. Some 17 million birds were destroyed at a direct cost of US$62 million. Indirect costs have been estimated at more than US$250 million.
- The 2003 outbreak in the Netherlands spread to Belgium and Germany. In the Netherlands, more than 30 million birds – a quarter of the country’s poultry stock – were destroyed. Some 2.7 million were destroyed in Belgium, and around 400,000 in Germany.
In the Netherlands, 89 humans were infected, of whom one (a veterinarian) died. In that outbreak, measures needed to protect the health of poultry workers, farmers, and persons visiting farms included wearing of protective clothing, masks to cover the mouth and nose, eye protection, vaccination against normal seasonal human influenza, and administration of prophylactic antiviral drugs.
Control is even more difficult in countries with dense poultry populations.
- The Italian outbreak of 1999-2000 caused infection in 413 flocks, including 25 backyard flocks, and resulted in the destruction of around 14 million birds. Control was complicated by the occurrence of cases in areas with extremely dense poultry populations. Compensation to farmers amounted to US$63 million. Costs for the poultry and associated industry have been estimated at US$620 million. Four months after the last outbreak ended, the virus returned in a low-pathogenic form, rapidly causing a further 52 outbreaks.
- Although the last outbreak of highly pathogenic avian influenza in Mexico occurred in 1995, the causative agent – the H5N2 strain – has never been entirely eliminated from the country, in its present low-pathogenicity form, despite years of intense efforts, including the administration of more than 2 billion doses of vaccines of varying efficacy. Similarly, the vaccination policy pursued in Pakistan does not appear to have resulted in eradication of the causative agent.
Avoidance of contact between poultry and wild birds, especially ducks and other waterfowl, can help prevent the introduction of a low-pathogenicity virus into domestic flocks. Though no evidence to date has conclusively linked the current outbreaks with wild migratory birds in Asia:
- Several of these outbreaks have been linked to contact between free-ranging flocks and wild birds, including the shared use of water sources. Faecal contamination of water supplies is considered a very efficient way for waterfowl to transmit the virus. Virus (low-pathogenicity) has been readily recovered from lakes and ponds where migratory birds congregate.
- An especially risky practice is the raising of small numbers of domestic ducks on a pond in proximity to domestic chicken and turkey flocks. Domestic ducks attract wild ducks, and provide a significant link in the chain of transmission from wild birds to domestic flocks.
Aggressive control measures, including culling of infected and exposed poultry, are recommended for avian influenza virus subtypes H5 and H7 even when the virus initially shows low pathogenicity. (H5 and H7 are the only subtypes implicated in outbreaks of highly pathogenic disease.)
- Several of the largest outbreaks (Pennsylvania, Mexico, Italy) initially began with mild illness in poultry. When the virus was allowed to continue circulating in poultry, it eventually mutated (within 6 to 9 months) into a highly pathogenic form with a mortality ratio approaching 100%. Moreover, the initial presence of low-pathogenicity virus in these outbreaks complicated diagnosis of the highly pathogenic form. Continue reading