Avian influenza A/H5N1 is currently spreading through domestic poultry and a variety of other birds in Asia. This epidemic is unprecedented, both for its immense geographical scale and for its human health implications. Since its onset late in 2003, eight Asian countries have been affected. Human cases with a high fatality rate have been reported in Vietnam and Thailand, both countries with widespread outbreaks in poultry.

The avian influenza A/H5N1 2004 viruses associated with this epidemic are believed to be very volatile. They are antigenically and genetically distinct from the 1997 viruses, which caused the ‘Chicken Flu’ outbreak in Hong Kong and are thought to have evolved from the A/Goose/Guangdong/96 virus through both a number of reassortment events with other avian viruses, and through the accumulation of stepwise single mutations.1

Although 34 human H5N1 cases have been confirmed, there is no direct evidence of human-to-human transmission. One family cluster of four cases in Vietnam has been extensively investigated for this possibility; however, if person-to-person transmission was occurring, larger clusters of disease might be expected. The sequence analyses of the H5N1 viruses recovered from this family have shown them to be similar to the avian virus, providing no evidence of any genetic change.

The emergence of a novel influenza A virus able to be spread efficiently from person to person has occurred three times in the last 100 years (in 1918, 1957, and 1968) and the resulting pandemics have been associated with high morbidity and mortality. The most recent estimates of mortality during the 1918 Spanish (H1N1) Pandemic suggest between 50–100 million people died—2–5% of the world’s population. With the 1957 Asian (H2N2), and 1968 Hong Kong (H3N2) Pandemics, the pig is believed to have acted as a ‘mixing vessel’ for the reassortment of the genes from an avian virus and animal influenza A virus.2 H3N2 viruses are present in pigs throughout Asia, so infection of pigs with the H5N1 2004 virus is another possible route for the emergence of a genetically modified virus with pandemic potential.

Human influenza A viruses are endemic in tropical and sub-tropical Asian countries, so the risk of a co-infection with H5N1 2004 in a human also exists. Workers engaged in culling operations (which can involve intensive exposure to infected poultry) are at considerable risk, and the use of personal protective equipment and vaccination (with the current trivalent human influenza vaccine) are recommended procedures to decrease the risk of possible dual infections.3 However, as long as this virus continues to circulate in the poultry population, the possibility of further human H5 infections, the risk of a double infection, and the emergence of a new virus strain through genetic reassortment (with the capacity to spread easily among humans) remains.

Little is known about the origin of this current epidemic and the mechanism for its rapid spread, however initial seeding of the H5N1 2004 virus through Asia may have been by wild migratory bird species in which most avian influenza viruses cause mild or asymptomatic infections. Some of the recent H5N1 viruses have been highly pathogenic for a wide range of avian species, so other dissemination mechanisms may be implicated. Since the 1980s, the poultry industry has undergone substantial expansion, and the commercial and other movement of live poultry between Asian countries (along with the live bird and wet market practices) may have provided the fertile environment for virus amplification and dissemination in this epidemic.4

The control of this avian epidemic is essential to prevent continuing human outbreaks and avert a human influenza pandemic. Control through culling (of infected or potentially exposed flocks), quarantine, and movement restriction are standard World Organisation for Animal Health (OIE) recommended measures aimed at preventing the spread of highly pathogenic avian influenza viruses. To date, in excess of 100 million chickens have died or been slaughtered. Some countries are using strategic vaccination of poultry, however this practice is controversial.

Unlike human influenza viruses, avian influenza viruses can survive in the environment for long periods. Prolonged survival in faeces in the poultry house environment has been shown for up to 5 weeks. However, the lessons learned following the 1997 Hong Kong outbreak clearly show that rapid culling, followed by the introduction of biosecurity measures (including basic hygiene measures) were pivotal to the control of that outbreak and subsequent virus circulation.

The initial control measures of an outbreak are very reliant on disease surveillance. Rapid reporting is required for both national response interventions and for international collaborative support. As this epidemic continues, of considerable concern is the insidious spread through small groups of poultry in widespread rural communities. In China, it is estimated that 80% of its 13 billion chicken population occurs in backyard farms. Outbreaks in rural communities are extremely difficult to identify and difficult to control.

Virological surveillance is as equally important. Virus isolates are required for the confirmation of outbreaks, for understanding the epidemiology of the virus, and for prototype vaccine strain selection. The sharing of viruses and information within the WHO global influenza network has immense benefits to the World community; however, commercial and political considerations in the current epidemic have often taken priority—leading to the poor international transparency of some countries.

As this epidemic continues to evolve in Asia, leadership must be taken from the Food and Agriculture Organisation (FAO), OIE, and the World Health Organisation (WHO) for the delivery of control strategies to the region, and from WHO for preparation for the worst case scenario, the emergence of a virus with human pandemic potential.

New Zealand must continue to be aware of these international efforts and place an increased emphasis on its national influenza awareness programme and pandemic planning activities.

As we enter the New Zealand 2004 influenza season, we can be confident that there is an increased awareness of the seriousness of influenza amongst the public and healthcare professionals. Through the efforts of the National Influenza Strategy Group (NIISG), influenza vaccine coverage has increased to an estimated 63% of the general population aged 65 years and older.5

New Zealand has an advanced Pandemic Action Plan. It is one of only five countries in the Asian-Pacific region to have a plan, and one of only four countries globally to have their plans formalised and endorsed by Government. This level of preparedness has been achieved through initiatives such as Exercise Virex6 and the use of the Pandemic Plan as the framework for the Ministry of Health’s 2003 SARS Outbreak Response. The lessons being learned from the current Asian avian influenza outbreak must be used to strengthen New Zealand’s public health disaster preparedness.

Author information: Lance C. Jennings, Virologist, Canterbury Health Laboratories, Canterbury District Health Board—and the Christchurch School of Medicine and Health Sciences, University of Otago, Christchurch

Correspondence: Dr Lance Jennings, PO Box 151, Christchurch. Fax: (03) 364 0750; email: lance.jennings@chmeds.ac.nz