H5N1 avian influenza: update on the global situation

Type A/H5N1 influenza virus, a highly pathogenic avian influenza (HPAI) was isolated in Hong Kong and Southern China in 1996 and, in 1997 when outbreaks in poultry and human cases were reported in Hong Kong [1]. No cases are known to have occurred between 1998 and 2002, but outbreaks of highly pathogenic avian influenza type H5N1 began to be reported in wild birds and poultry in east Asia in 2003, and in Europe in 2005 [1, 2]. Human cases of H5N1 have so far only been detected in five east Asian countries [3].

Human cases of avian influenza H5N1 infection

Sporadic human cases continue to be reported in east Asia. There is no evidence of any increase in clustering (which could imply substantial person to person transmission) although such transmissions have taken place occasionally [4]. From January 2003 to 14 December 2005, there have been 138 human cases of avian influenza H5N1 infection reported and accepted by the World Health Organization (WHO) as having been laboratory confirmed. Seventy one patients (51%) are known to have died. The cases have occurred in five countries: Cambodia (4 cases, resulting in 4 deaths), China (5 cases, including 2 deaths), Indonesia (14 cases, including 9 deaths), Thailand (22 cases, including 14 deaths) and Vietnam (93 cases, including 42 deaths) [3].

Both Vietnam and Thailand reported human cases in 2004 and 2005. In 2005, Cambodia, China and Indonesia have also reported human cases.

The Chinese authorities reported the first Chinese cases of human infection occurring outside of Hong Kong in November 2005 [1,5]. Five of the 2005 cases are considered by the WHO to have been laboratory confirmed, and two of these patients have died. Cases have been reported from four provinces: Anhui (two female farmers who have died), Hunan (9 year old boy who has recovered)*, Guangxi (10 year old girl currently admitted to hospital) and Liaoning (31 year old female farmer who has recovered). This report of human cases is not unexpected as China has reported outbreaks of H5N1 since 2004. It is, however, a cause for concern, because it indicates a considerable opportunity for the mixing of human and H5N1 influenza in the large rural populations of China, who make up approximately 70% of the 1.25 billion population.

Location of H5N1 avian influenza outbreaks in birds

Table. Countries reporting H5N1 avian influenza outbreaks to December 2005

Country or region	Month of most recent outbreak, according to information received by the World Organisation for Animal Health (OIE) [6]
Cambodia	March 2005
China*	November 2005
Croatia	October 2005
Hong Kong (1 bird)	January 2005
Indonesia*	July 2005
Japan	March 2004
Kazakhstan	August 2005
Malaysia (Peninsular)	November 2004
Mongolia	August 2005
Romania	December 2005
Russia	October 2005
South Korea	March 2004
Thailand	November 2005
Turkey	October 2005
Ukraine	November 2005
Vietnam*	November 2005

* Numerous outbreaks are known to be currently occurring in the countries marked with an asterisk.

In the WHO European Region, outbreaks of H5N1 avian influenza in birds have been confirmed in Croatia, Kazakhstan, Romania, Russia and Turkey [2,6]. The mode of spread seems to be through migratory birds occasionally infecting domestic flocks. In Ukraine, where outbreaks were detected on 25 November 2005, the agent has been identified as influenza virus subtype H5. Samples have been sent to a European Union (EU) reference laboratory to confirm the diagnosis [7].

The H5N1 virus was also isolated in pooled samples of dead mesia birds from the quarantine centre in Essex (United Kingdom), where a parrot from South America had been reported as testing positive for H5N1 previously. Results of the virological examination conclude that most likely the H5N1 infection was introduced into the centre by the mesias, which had been imported from Taiwan [8]. As a consequence the European Union banned the importation of live captive birds in October 2005 [9].

There are some worrying aspects in the management of avian influenza in birds. International networks of influenza laboratories (notably WHO’s FluNet and OIE’s OFFLU) do not yet incorporate some key countries, such as Russia. In Indonesia, where veterinary public health services are recognised to be weak and decentralised, immunisation is being used as the only method of control in some areas. Some countries appear to be employing widespread use of avian influenza vaccination of poultry without a clear exit strategy [10]. There also appears to be large scale production of avian influenza vaccine without the benefit of independent oversight bodies responsible for quality assurance and health and safety. Finally, systems of human surveillance and response need strengthening in all five affected countries so that if clusters emerge, they can be detected and responded to as recommended by WHO in its global plan.

Risk to human health in Europe from the H5N1 virus

The confirmation of H5N1 in poultry and humans in China and Indonesia is of concern, as there are very large rural human and domestic bird populations (880 million and 14 billion respectively in China) in close cohabitation, which could facilitate mixing of animal and human influenza viruses and lead to the emergence of a pandemic strain. It is also unclear whether poultry vaccination will increase the likelihood of human exposure to H5N1, or reduce the level of H5N1 circulating in the bird population; there are currently few field data to resolve this important question.

Whether or not any of the variants of H5N1 has pandemic potential remains unclear. However at present the viruses remain poorly adapted to humans and so the risk to human health in Europe itself from H5N1 in its current form birds and animals remains low and is focused in certain groups. These are people living on farms with infected poultry, and workers involved with culling (killing) and disposing infected birds and animals [11,12]. The disease has a 50% mortality rate for the few people who do become infected, which poses problems for risk communication and infection. The appearance of H5N1 in the imported bird in the United Kingdom demonstrates that another potential mode of spread is through legally (or illegally) imported exotic birds. Hence a further risk group are those working in quarantine centres and those coming into contact with sick birds. The European Centre for Disease Prevention and Control (ECDC) has published interim guidance for the protection of poultry workers [13]. These guidelines on the protection of poultry workers proposed by the ECDC were discussed and endorsed at a meeting at the European Commission on 12th December 2005 between European Chief Medical Officers (CMO) and Chief Veterinary Officers (CVO) [14].

Other than the few high risk groups, most Europeans have little direct contact with birds and so are at little or no danger of being exposed to the virus. Precautions for the general public in Europe include good personal hygiene (handwashing, especially after contact with birds) and in particular, not touching birds found sick or dead, and following pre-existing advice to cook eggs and poultry thoroughly (to prevent infection by salmonella and other organisms, as well as the almost non-existent risk of avian influenza) [11]. The ECDC has also issued advice for travellers to affected areas [15]. Some Europeans do however live closely with birds in areas where H5N1 has been reported in wild birds, and so sporadic human cases may occur, as they have in Asia

* The Chinese authorities and an international mission considered that the boy’s sister had also been infected, but she died shortly before to his diagnosis and her body was cremated, making her final diagnosis impossible.

Acknowledgements: The authors are grateful to Johanna Takkinen for comments on this manuscript.