Influenza is an infectious disease that primarily presents with respiratory signs and symptoms, caused by RNA viruses. The most significant impacts of influenza viruses on humans are those arising from the influenza A strains. The natural reservoir of influenza A strains is a diverse pool of viruses among aquatic wild bird populations, the avian influenza (AI) viruses. These viruses are divided into those of high and low pathogenicity (hence HPAI and LPAI), according to their severity in the avian species they usually infect. HPAI epizootics continue to be documented worldwide all due to the A/H5 and A/H7 virus groups.
Most but not all of 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 have rarely been due to the introduction of HPAI from wild birds, but these do occur. Since 2000 there have been more and larger outbreaks of HPAI in poultry. The reason for this is unknown. Both large and small outbreaks have taken place in Europe.
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.
Human Infections due to Avian Influenza - Zoonotic Transmission
Apart from the significant epidemics of A(H5N1) and A(H7N9), only a limited number of human cases associated with HPAI outbreaks in poultry have been reported since 1959, when such cases were first identified. There have been single cases of human disease in Europe arising from infection acquired from domestic poultry which mixed with wild birds. Low pathogenicity strains (LPAI) only seem to cause minor self-limiting illnesses in humans, except the A(H7N9) virus which is low pathogenic to birds but can cause severe disease in humans.
Avian influenza A(H7N9) in China
A novel influenza A avian influenza virus, A(H7N9), was identified in China in March 2013, causing severe illness in humans. This is the first time that human infection with a low pathogenic avian influenza A virus has been associated with fatal outcome in humans. Since then, cases continue to be reported in China.
Highly pathogenic avian influenza A(H5N1)
Since 1997 a new and more lethal strain of HPAI viruses – A(H5N1) have affected poultry, initially in the Far East and later in Europe, Middle East and Africa. After 2003 it appeared in many Asian countries causing very large outbreaks in birds and a small number of severe human infections.
Occasionally, A(H5N1) infects humans in contact with infected poultry, and causes severe disease in humans. The A(H5N1) virus has not established in poultry in Europe because of high levels of biosafety. The A(H5N1) virus has not adapted to be able to infect humans easily or to transmit from person to person efficiently.
Pandemic Potential of avian influenza viruses: A(H5N1) as an example
Each novel influenza has to be evaluated for its potential to cause severe disease, a pandemic or both.
There is a possibility of an avian influenza virus, for example, the A(H5N1), adapting to humans and causing a pandemic. To date that adaptation has not been observed. Particularly, risk of a pandemic could arise through recombination with circulating well-adapted (human) viruses through dual infections in humans and other mammals that could produce a pandemic strain.
In relation to humans, A(H5N1) can be characterized as follows:
• Influenza virus type A(H5N1) is a group of avian (bird) viruses that are highly pathogenic and very infectious for a number of bird species including most poultry species kept domestically by humans.
• As an influenza virus A(H5N1) to date is poorly adapted to the human species so that it only rarely infects those who become infected.
• For those few people that do become infected the virus can be highly pathogenic. The mortality rate is significant.
• Also reflecting its poor adaptation to humans, H5N1 viruses generally do not transmit on easily from one infected person to another human. Though human to human transmission has occurred it has been inefficient.
• The above points all have the caveat that H5N1 viruses are mutating and evolving constantly. Like all influenza viruses they can also change abruptly by recombination - exchanging genetic material with other influenza viruses. Though there have been no obvious changes in the behaviour of H5N1 viruses in humans or animals since they emerged in the 1990s there remains the possibility that they could change, and suddenly, for example, be transmitting more efficiently among humans or becoming less pathogenic for humans or birds.
Avian Influenza A(H5N8) in poultry
In November 2014, outbreaks of highly pathogenic avian influenza virus A(H5N8) in poultry were announced by several European countries. Previously, this virus has been detected among wild birds in south-east Asia and has caused several outbreaks on poultry farms there.
No human infection with this virus has ever been reported world-wide. The risk of transmission of the virus to the general public is extremely low.
Ongoing monitoring and testing of wild birds and domestic poultry in the EU is important to detect and prevent further spread of this virus that is highly pathogenic to poultry and other farmed birds.