ANALYSIS - Outbreaks of highly pathogenic avian influenza (HPAI) have tended to be more severe and prolonged in less developed countries with a high poultry density, according to avian flu expert, Dr David Swayne. Stamping out is the best method for control but vaccination can be an effective tool, he said. Senior editor, Jackie Linden, reports on his presentation to the World Poultry Congress.
At the World Poultry Congress in Salvador, Brazil, in August, Dr David Swayne of the US Southeastern Poultry Research Laboratory in the US outlined the findings of a study he made while on secondment to the World Organisation for Animal Health (OIE) to study the development of H5N1 influenza A in the global poultry population. His study focussed particularly on the role of vaccination to control the disease in poultry.
As predictors of the severity of outbreaks, he found that transmission occurred more easily and outbreaks were more prolonged in areas of high poultry population density in less developed countries.
There was no significant association between economic indicators such as gross domestic product (GDP) or agricultural GDP on the successful control of HPAI. However, in countries that are members of the Organisation for Economic Co-operation and Development (OECD), there were fewer outbreaks, quicker eradication, lower bird mortality and higher culling rates. Dr Swayne attributed the differences to greater transparency in both animal and human health in OECD members as well as to more open democracy and thus better governance over how money was used.
His study revealed that where there is a better level of veterinary services, there was also better disease control at federal, regional and local levels.
Both low-pathogenic and highly pathogenic forms of influenza of the H5 and H7 subtypes are notifiable and most countries had similar control systems in place regarding quarantine rules, traceability and compensation for affected farmers, said Dr Swayne. Some countries have additional components in place, such as a crisis management framework, rapid diagnostic capabilities and capacity for early processing of at-risk flocks. Culling policies differed, with some countries opting to extend culling to neighbouring flocks or those with 'dangerous contacts' - in other words, those with potential links to already-infected birds.
Dr Swayne stressed that while controls often looked similar on paper, he observed marked qualitative differences between countries in these respects.
The main focus of his study was on the use of vaccination in the control of avian flu.
In some countries, vaccination was permitted but was not used, either because there was no disease and there was no immediate risk or because stamping out had previously been successful. In some cases, there was a lack of resources for vaccination.
Other countries where there had been outbreaks have used vaccination. Reasons given were that stamping out had not previously been effective or there were particular reasons for using vaccination, such as to protect hobby, zoo or genetically valuable breeder flocks or because of persistent infection in another sector. In the latter case, he cited the example of persistent infection that can occur in mobile flocks of ducks in Asia.
Dr Swayne stressed that vaccination can increase resistance to infection, reduce replication of the virus and prevent illness or death. However, vaccination cannot eradicate the disease.
His study shows that vaccination did not create enzootic HPAI infection although it may be helping to maintain it.
Looking at vaccine use over the period 2002 to 2010, more than 113 billion doses have been administered to poultry worldwide, with 25.7 million doses in 2010, according to Dr Swayne. Coverage averaged around 40 per cent, or 11 per cent of global production. The great majority of the vaccines were of the traditional inactivated whole virus type, with just 4.5 per cent of the recombinant type. Fourteen countries have now used vaccination, most notably China, Egypt, Indonesia and Viet Nam.
For viral transmission to be reduced, Dr Swayne said that between 60 and 80 per cent of birds need to be vaccinated; only in Hong Kong and Egypt had those levels been reached. He did point out the great difficulties of reaching and effectively vaccinating village and family flocks. Several countries have opted to focus on one type of bird for vaccination, such as meat birds in Egypt or ducks in Viet Nam, or on flocks in one region, as in Pakistan.
Finally, Dr Swayne addressed the topic of antigenic drift. He said that, over time, the flu viruses have been shown to drift and therefore that vaccine seed strains need to be updated every two or three years. Historically, H5 strains showed similar antigenicity, he said, but the newer ones are drifting from the root so this is likely to becoming a more significant issue over time.
Further ReadingYou can visit the Avian Flu page by clicking here.