Assessment of risk to human health associated with outbreaks of highly pathogenic H5N1 avian influenza in poultry

14 May 2004

Background

In late January 2004, WHO activated its influenza pandemic preparedness plan in response to confirmation, in Viet Nam and Thailand, 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 linked to outbreaks of highly pathogenic H5N1 avian influenza in the poultry populations of these two countries.

From mid-December 2003 through February 2004, outbreaks of H5N1 infection in poultry were detected in six additional Asian countries (Cambodia, China, Indonesia, Japan, Laos, and the Republic of Korea). The outbreaks have been historically unprecedented in their geographical scope, international spread, and economic consequences for the agricultural sector. More than half of the affected countries experienced highly pathogenic avian influenza for the first time in their histories. The challenges for control have been considerable.

The implications of these events for human health are two-fold. First, the H5N1 strain has demonstrated its capacity to infect humans and cause severe disease, with high fatality, on three separate occasions beginning in 1997. 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 are invariably associated with high morbidity and mortality and great social and economic disruption.

Risk assessment

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 highly pathogenic avian influenza 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 in poultry populations

Control efforts, which have necessitated the culling of more than 100 million birds, have reduced the risk to humans. However, it cannot be assumed, because of difficulties in the collection of surveillance data, that the virus has been eliminated in poultry populations and that the risk to human health no longer exists.

In April, some countries on the verge of declaring their outbreaks over have either detected a recurrence of infection in areas thought to have been freed of the disease or reported a continuing spread of the disease to new areas. Rumours of new outbreaks in May are awaiting confirmation. WHO has repeatedly warned about the dangers of premature declarations of disease-free status.

The need to apply strong surveillance, control and biosecurity measures has been stressed in order to ensure that new outbreaks are detected and contained immediately, and to prevent re-introduction into disease-free areas. Without such measures, further outbreaks, and further spread within and between countries are likely to occur.

Proposed immediate actions

Because of uncertainties about the disease situation in poultry and the potentially severe consequences for human health, WHO is calling to:

• continue to work towards elimination of H5N1 in all poultry populations, including small holdings as well as commercial flocks;
• promptly report to relevant authorities and organizations new outbreaks in poultry;
• put in place mechanisms to verify control progress and eventually monitor freedom from the disease;
• ensure close collaboration between public health and agricultural sectors and veterinary services;
• strengthen communicable disease surveillance in humans and collect and provide the data needed for an accurate assessment of risks to human health;
• share viruses with laboratories in the WHO Global Influenza Surveillance Network.