Background information

Avian influenza in birds

Avian influenza naturally circulates in wild waterfowl such as ducks and geese often causing little or no symptoms. Many other bird species are susceptible to infection with these influenza viruses and in many of these species it may cause severe disease associated with high mortality.

Outbreaks associated with high bird mortality are called Highly Pathogenic Avian Influenza (HPAI) to distinguish them from less pathogenic influenza.

Avian influenza viruses like other influenza viruses are described according to properties of two surface proteins: haemagglutinin (H) and neuraminidase (N). Large outbreaks in poultry have, for example, been described in Pennsylvania in 1982 (H5N2), Mexico in 1993 (H5N2), Hong Kong in 1997 (H5N1) and The Netherlands in 2003 (H7N7). In January 2004 avian influenza H5N1in poultry was confirmed in Vietnam. Subsequently, there have been very substantial outbreaks of avian influenza associated with high mortality affecting poultry in a number of countries ( list of currently affected countries). These outbreaks are caused by the H5N1 subtype of influenza A virus, the same subtype (but not identical to the virus) that caused the outbreak in Hong Kong in 1997.

Human cases

Very infrequently, avian influenza viruses are transmissible to humans. In the outbreak in Hong Kong in 1997 there were 18 confirmed human cases with 6 deaths. After thorough investigation all these cases all were linked to exposure to live infected poultry.

In the Netherlands in 2003 there were 89 confirmed mild human cases including 1 death. Again there were no reported cases of person-to-person transmission.

In the current outbreak of avian influenza human cases and deaths due to H5N1 have been reported from a number of countries ( List of countries reporting human cases of influenza A (H5N1)) who have reported outbreaks of influenza H5N1 in poultry stocks. The World Health Organization provides regular updates on confirmed case numbers. Given the scale of the outbreaks in poultry the virus does not appear to infect humans easily. All human cases are investigated to determine the likely source of the infection.

Influenza Pandemics

Outbreaks of influenza affecting many thousands and sometimes millions of people with high mortality occurred in 1918 ('Spanish'), 1957 (Asian), 1968 (Hong Kong) and 1977 (Russian). New subtypes of influenza caused these pandemics. These were probably formed by combination of genes from both avian and human influenza viruses. Emergence of new highly pathogenic avian influenza with the capacity to infect humans is a concern because it may lead to circumstances where a new subtype of influenza can develop that both causes serious disease and can spread from person to person.

HPA Pandemic Influenza Homepage

Travel and travellers returning

Up to date travel advice may be obtained from the Department of Health or the National Travel Health Network and Centre (NaTHNaC). The currently available influenza vaccine for the UK probably provides little or no protection against H5N1 influenza. An up-to-date list of countries affected by outbreaks of H5N1 influenza in poultry can be found on the HPA website or on the World Organisation for Animal Health website.

The possibility of H5N1 influenza in travellers returning from countries currently experiencing outbreaks of avian influenza is low. Guidelines for reporting and investigation of possible cases are available on the HPA website.

Last reviewed: 26 November 2008