In late January 2004, the World Health Organization (WHO) activated its Influenza Pandemic Preparedness Plan in response to confirmation, in Thailand and Viet Nam, of human cases of severe disease caused by an H5N1 strain of avian influenza. These and subsequent human cases, reported through mid-March, were directly related to outbreaks of HPAI, subtype H5N1, in the poultry populations of these two countries.
The 2004 outbreak in humans, which is the largest recorded, resulted in 34 officially reported cases, of which 23 were fatal. The disease in humans has no vaccine to confer protection and no specific treatment once illness becomes severe.
A second and greater concern is the possibility that a new virus subtype with pandemic potential could emerge. Pandemics recur periodically yet unpredictably, and they are invariably associated with high morbidity and mortality and great social and economic disruption.
In the present situation, the risks to human health remain so long as H5N1 continues to circulate in domestic poultry. In several affected countries, surveillance and reporting systems for both human and animal disease are weak. The absence of reported human cases cannot be taken as assurance that the threat to human health no longer exists. Assessment of the risk to humans needs to be based on a risk assessment of the disease situation in poultry that considers the prevalence of HPAI and the adequacy of the surveillance system. A reliable system of review and verification is needed to ascertain that poultry are disease-free in an area or country. Equally important is a robust surveillance system for human respiratory illnesses that might signal transmission of avian H5N1 infection to humans.
Control efforts, which have necessitated the culling of more than 100 million birds, have reduced the risk to humans. However, because of difficulties in the collection of surveillance data, it cannot be assumed that the virus has been eliminated in poultry populations and that the risk to human health no longer exists.
World Health Organization (WHO) Web site (http://www.who.int/en/).
