Aquatic wild bird populations form the natural reservoir for a diverse pool of influenza A strains – avian influenzas. They contribute to the most significant impacts of influenza viruses on humans. These viruses are divided into those of high and low pathogenicity, according to their severity in the avian species they usually infect. HPAI epizootics continue to be documented worldwide due to the A(H5) and A(H7) virus groups.
Most but not all cases of animal influenza in humans have been due to transmission from birds. Many millions of birds have died in huge epizootics either directly from the infection or from culling undertaken to control the infection. Outbreaks amongst humans have rarely been due to the introduction of HPAI from wild birds, but do occur.
There are two forms of risk to the health of humans from avian influenza viruses, from infection by the native form of the avian influenza virus and from the potential for the emergence of new pandemic strains either directly from avian viruses, or from their recombination with human or other animal viruses.
The ‘high’ and ‘low’ pathogenicity is not related to the disease in humans. Some avian viruses do not cause disease in humans, or are known to only cause mild disease. Some, however, are known to cause severe disease in humans, for example, A(H5N1). The classification of avian influenza viruses as ‘low pathogenic’ or ‘highly pathogenic’ is defined either by the composition of the cleavage site in the haemagglutinin (HA) gene or by the intravenous pathogenicity index in six-week old chickens in accordance with the criteria listed in Council Directive 2005/94/EC and the OIE International Health Standards.
Any detection of avian influenza viruses of the H5 and H7 subtype in poultry holdings during regular surveillance needs to be notified. Precautionary measures are then applied to prevent potential avian-to-human infection.
The avian influenza virus A(H5N1) emerged in 1996. It was first identified in Southern China and Hong Kong. The A(H5N1) virus kills a high proportion of the poultry that it infects and is therefore known as a highly pathogenic avian influenza virus. It remains poorly adapted to humans. Transmission from birds to humans is infrequent and no sustained human-to-human transmission has been observed, however it can cause severe disease in humans.
The A(H5N1) virus has only sporadically been detected in European poultry or wild birds.
A novel influenza A avian influenza virus, A(H7N9), was identified in China in March 2013, causing severe illness in humans. It was the first time that a low pathogenic avian influenza A virus had been associated with fatal outcomes for people.
Influenza virus detections in poultry in EU/EEA
In 2014 and 2015, outbreaks of highly pathogenic avian influenza virus A(H5N8) in poultry were notified by several European countries. Previously, this virus has been detected among wild birds and domestic poultry in Asia.
In 2015, highly pathogenic avian influenza virus A(H5N1) was detected in poultry and wild birds in Bulgaria and Romania.
In 2015 and 2016, outbreaks of highly pathogenic avian influenza viruses A(H5N1), A(H5N2), and A(H5N9) as well as detections of low pathogenic avian influenza viruses A(H5N2) and A(H5N3) have been reported.
No human infection with these viruses have been reported in EU/EEA countries. The risk of transmission to the general public is extremely low. Persons at risk are mainly those in direct contact/handling diseased birds or poultry, or their carcasses (e.g. farmers, veterinarians and labourers involved in the culling and rendering).
The latest updates and rapid risk assessments on avian influenza virus are available in the news and epidemiological updates and risk assessments sections.
Ongoing monitoring and testing of wild birds and domestic poultry in the EU is important to detect and prevent further spread of this highly pathogenic virus to poultry and other farmed birds.