Control measures undertaken in Asia during the crisis

The control of the current AI epidemic is a real concern, as it is considered an emerging disease with Asia as its epicentre. An FAO/OIE/WHO Technical Consultation on the Control of Avian Influenza was held on 3-4 February in Rome, followed by an FAO/OIE Emergency Regional Meeting on Avian Influenza Control in Animals in Asia held on 26-28 February 2003 in Bangkok. During these meetings, conclusions were drawn and recommendations were made to assist the countries affected by AI in responding to this emerging disease. In addition, during the 72nd Annual General Session of the World Organisation for Animal Health (OIE) held on 23-28 May 2004 in Paris, the OIE proposed new articles in the Terrestrial Animal Health Code to improve control of AI and provide greater protection for importing countries while eliminating unjustified barriers to trade. Recommendations were also made following the FAO missions in the different Asian countries.

Epidemiological considerations and implications of disease control issues

Control of the disease is difficult because of the ubiquitous presence of many viruses and their constant evolution antigenically and characteristics of virulence. The spread of the disease and its persistence are exacerbated by the fact that the AI virus wild reservoir is represented by wild waterfowl and migrating bird populations and that AI viruses’ hosts include many species, such as birds, pigs, horses and humans (Figure 1). Virus mutations and genetic reassortments of AI viruses increase during a widespread AI epidemic in domestic birds, such as the one that came to international attention in January 2004. Genetic reassortments can also occur in pigs and other hosts. However, during the current crisis, investigation of pigs living in close contact with infected poultry, which could play a key role in influenza epidemics, has produced no evidence of transmission between pigs and avian species.

The hypothesis that Asia is an influenza epicentre has been put forward on several occasions. This theory reflects the emergence of the Asian/57 (H2N2) and Hong Kong/68 (H3N2) human pandemics from this region and the recent outbreaks in China, Hong Kong Special Administrative Region in 1997 (H5N1) and 1999 (H9N2). The conditions necessary for influenza viruses in avian species to be transmitted directly to humans are present: most of the poultry sector is represented by smallholders, which are at high risk of infection, considering the common production and marketing methods, substandard sanitary conditions and environment where several species of poultry are sold. Markets can act as “breeding sites” for new influenza viruses.

Approaches and strategies adopted to control avian influenza

Once the disease was identified in Asia, affected countries began to implement a series of control measures, including vaccination in the cases of China, Indonesia and Pakistan. Through FAO assistance, several international experts in epidemiology, laboratory diagnosis and vaccine production visited the region and had the opportunity to review the strategies adopted in different economic, social and political contexts.

Some control measures have been adopted by all the countries affected and have commonly been considered successful to some degree in decreasing the load on the environment and lowering (but not eliminating) the risk of infection to other animals, including humans. All countries adopted stamping out measures on “infected” premises, and to limit the spread of the disease by sequestration of the infected premises, implementing a buffer zone, with movement control measures and surveillance. These restrictions in the infected premises were to be kept in place for at least 21 days, which represents several incubation periods for HPAI (Figure 2). These control options contributed, to some extent, to the successful control of the disease and kept it from becoming a wider epidemic.

However, there is some variation within the strategies adopted by the different countries, such as the size of the areas to be culled, the vaccination strategies used and the duration of the restriction measures. A general summary of control options implemented in the region is presented in Figure 2, showing the implementation phases in time (from “suspected” to “free”) and space (zoning policies).

As shown in Figure 3, the sizes of the culling areas and the buffer zones varied among countries, even though most adopted a 3 km radius for the culling area and a 10 km radius for the buffer zone.

In other parts of the world (and therefore in different ecological and epidemiological situations), other approaches have been used and implemented successfully. As an example, the European Union legislation defines a protection zone as having a minimum radius of 3 km and a surveillance zone as having a minimum radius of 10 km. In Australia, three different zones are defined: the infected farm, a restricted area from 1 to 5 km around the infected farm and a controlled area from 2 to 10 km around the restricted area.

In emergency situations, it must be highlighted that strategies implemented to curb the spread of the disease must reflect local conditions in the field (epidemiological, ecological, socio-economic and cultural) and be based on risk assessment studies in order to define control, buffer zone or surveillance areas. Furthermore, GIS integrating various spatial risk parameters could be a valuable tool to carry out risk assessment studies and assist decision-makers in defining sound control strategies.

Stamping out and vaccination issues

The use of either stamping out or vaccination has been discussed on several occasions and in several fora during the current crisis. Mass culling was undertaken by most of the countries sometimes combined with pre-emptive culling in places with a suspected risk (the Republic of Korea and Thailand). China and Indonesia undertook large-scale vaccination to minimize the spread of disease.

Arguments against the use of vaccines have been advanced on the basis of economic and social consequences of AI outbreaks. The agricultural community remains divided on the effectiveness of a large vaccination programme. As recalled during the FAO/OIE/WHO Technical Consultation on the Control of Avian Influenza, stamping out and vaccination are two valuable tools for the control of the disease, and both have been used successfully on previous occasions.

Vaccine is often used when stamping out policies are unsuccessful in an attempt to reduce the amount of virus shed into the environment and avoid depopulation of millions of poultry. Nevertheless, antigenic variants can emerge, particularly with a poorly immunogenic vaccine that induces low levels of immunity, or when vaccination is used for a long time. For this reason the quality of vaccine must be assured in accordance with OIE guidelines and, to detect and eradicate persistence of infection, vaccination has to be combined with stamping out, surveillance and strict biosecurity measures.

Direct control measures including quarantine, management of animal movement, surveillance and biosecurity are vital even with the use of quality vaccines.

Difficulties and consequences of the implementation of control measures

Several countries where AI has occurred have had to face difficulties in order to implement control strategies, in part because of lack of preparation. Apart from Japan and the Republic of Korea, which already had contingency plans of action and rapidly and efficiently controlled the disease, the affected countries were not prepared to face the HPAI outbreaks; no plan of action was in place, and they had a limited knowledge of the poultry sector, including the location of sites of production and trade.

Many countries in the region have weak veterinary public services and low financial resources. However, once AI was acknowledged and the extent of the problem known, the authorities established emergency task forces at the national and local levels to implement control strategies. Countries with poor veterinary infrastructure including communications and training had difficulties implementing control measures against AI.

The current crisis demonstrates the need for much improved early warning and early response capabilities to prevent, and if diagnosed control, future HPAI epizootics. Emergency preparedness and contingency plans adapted to local and regional situations should be instituted not only for AI but also for other animal health emergencies. These plans of action should be reviewed regularly.

At the international level, early warning systems should be strengthened to allow a timely detection of known and emerging pathogens and foster a concerted, coordinated and sustained response to disease emergency situations.

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