Poultry outbreaks and human cases of infection with highly pathogenic avian influenza (HPAI) virus A(H5N1) continue to be documented in Asia, with several new human infections being reported since December 2004, following a period of 2 months when no cases were reported.

As of 19 January 2005, the total number of confirmed human cases of avian influenza (AI) A(H5N1) in Vietnam and Thailand since 28 January 2004 was 51 (17 in Thailand and 34 in Vietnam) and 37 of these were fatal (12 in Thailand and 25 in Vietnam) [1]. Human cases in the last two months serve as a reminder that the cause of outbreaks in poultry is by no means under control and the continued full attention of human and animal influenza surveillance networks in the region and the rest of the world is warranted.

The most recently confirmed human A(H5N1)infection was in a man from Hanoi, and is the first human case of A(H5N1) in the northern region of Vietnam [2]. As the A(H5N1) virus had already spread to domestic ducks and chickens in Vietnam’s central region, the virus has now spread throughout the whole country. Attempts by the Vietnamese government to control these new outbreaks include culling infected birds, prevention of transport of poultry to and from infected areas, control over the slaughter and transport of poultry, and a temporary stop to imports of fowl and fowl products from neighbouring countries [3]. Until an A(H5N1) vaccine becomes available, the success of these measures will be crucial .

The sudden recurrence of AI A(H5N1) in humans in August of last year prompted WHO to warn of a potential pandemic. Since the first outbreaks of HPAI A(H5N1) and infection of humans in Hong Kong in 1997, it has been postulated that not only pigs, but also humans, may serve as the mixing vessel for the emergence of the next pandemic influenza virus. In addition, adaptation of the A(H5N1) virus to mammals as demonstrated by transmission to and between felines suggests that mixing is probably not necessary for the virus to become pandemic [4].

As avian influenza viruses become more active at cooler temperatures, further poultry outbreaks, possibly accompanied by sporadic human cases, are expected. At this time of year, poultry marketing, transportation, and consumption increase in Vietnam with the approach of the Lunar New Year in early February. These activities create conditions favouring the spread of poultry outbreaks.

After Vietnam, Thailand has had the second largest number of human cases of AI A(H5N1) in the past year. Following the reports of the new cases in Vietnam at the beginning of 2005, health workers have been put on alert and ordered to conduct stringent surveillance measures.

While it remains to be seen whether the measures taken by the Vietnam and Thai authorities will succeed in controlling outbreaks among poultry with HPAI A(H5N1), examples where outbreaks of HPAI involving human cases have been successfully controlled can be found in Europe (the Netherlands; H7N7 outbreak in 2003; [5,6]) and Canada (H7N3 outbreak in 2004; [7,8]); in both these countries, poultry farming is intensive and concentrated in certain regions, and live bird markets (‘wet markets’) are scarce compared with South East Asia, so there is less mixing of bird species, which will have limited the ability of HPAI to evolve. These factors probably contributed to the successful eradication of the HPAI viruses in the poultry outbreaks described here. However, in both the Netherlands and Canada, human infections with HPAI were documented and one Dutch patient died. In the Netherlands outbreak, the movement of infected persons was not restricted, and thereby increasing the potential for human transmission [9]. If the separate systems for the surveillance and control of human and animal infections had been coordinated, this might have been prevented. Moreover, prolonging the period of circulation of HPAI in the human population increases the chance of a pandemic strain emerging.

Human influenza surveillance in Europe is organised by the European Influenza Surveillance Scheme (http://www.eiss.org) where the Community Network of Reference Laboratories (CNRL) performs influenza diagnostics on clinical samples collected by a sentinel network of general practioners from patients presenting with influenza-like illness. Detailed characterisations of virus strains are performed by most of the CNRL members with the aim of providing a timely warning of emerging virulent or pandemic strains across the continent. Furthermore, the CNRL prepares itself for the detection of AI A(H5N1) by continually updating protocols and control materials to match currently circulating strains in South East Asia. Inventories were performed in 2003 upon the re-emergence of AI A(H5N1) in Vietnam and Thailand and in October 2004 when birds infected with the A(H5N1) virus were smuggled into Belgium [10], when no human infections with the A(H5N1) virus were detected. At the beginning of December 2004, the EISS network was alerted to a possible human case of AI A(H5N1) in France [11]. On the same day, information was exchanged between the members of the CNRL and although the patient proved negative for AI A(H5N1), it demonstrated the alertness of the EISS network to the speed with which infection with AI A(H5N1) could be introduced into Europe in the age of extensive air travel.