Circulation of avian Influenza (AI) viruses is not a new phenomenon. There are many descriptions of historical outbreaks of avian influenza disseminating within domestic poultry flocks in the literature. AI occurs worldwide and different strains are more prevalent in certain areas of the world than others.
AI is a highly contagious viral disease that affects both domestic and wild birds. AI viruses have also been isolated, although less frequently, from mammalian species, including rats, mice, weasels, ferrets, pigs, cats, tigers, dogs and horses, as well as from humans.
Influenza viruses are grouped into three types; A, B, and C. Only type A is known to infect animals and is zoonotic, meaning it can infect animals and also humans. Type B and C mostly infect humans and typically cause mild disease.
Avian influenza viruses are extremely variable and are widespread among birds. In those, they all belong to the Orthomyxoviridae family and are placed in the genus influenzavirus A.
Influenza A viruses are classified into subtypes based on two surface proteins, the hemagglutinin (HA) and neuraminidase (NA). For example, a virus that has HA 7 protein and NA 9 protein is designated as subtype H7N9.
At least 16 hemagglutinins (H1 to H16), and 9 neuraminidases (N1 to N9) subtypes have been found in viruses from birds, while two additional HA and NA types have been identified, to date, only in bats.
AI virus strains are usually classified into two categories according to the severity of the disease in poultry:
To date, naturally occurring highly pathogenic influenza A viruses that produce acute clinical disease in chickens, turkeys and other birds of economic importance have been associated only with the H5 and H7 subtypes.
Molecular epidemiology and characterisation of the genotypes of AI virus in poultry and wild birds is important to understand the distribution of different viral strains in various hosts. National and OIE Reference Laboratories use molecular diagnostic techniques in the surveillance and detection of highly pathogenic strains and emergence of novel subtypes from unknown hosts or hosts that have not been previously reported.
Differentiation between low and high pathogenicity AI viruses is based on the results of laboratory tests, which are described in the OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals.This characterisation of AI viruses as low or high pathogenicity is specific to poultry and other birds and does not necessarily apply to other species that can be susceptible to AI viruses, including humans.
The viral HA, and to a lesser extent the NA, are major targets for the immune response. Influenza A viruses are very diverse, and two viruses that share a subtype may be only distantly related. The high variability is the result of two processes: mutation and genetic reassortment. Once these proteins have changed enough, immune responses against the former HA and NA may no longer be protective and this can provide the virus with the ability to rapidly adapt to new hosts.
Consequently the OIE Terrestrial Animal Health Code (Terrestrial Code) defines “avian influenza” as an infection of poultry and other birds, including wild birds, caused by any influenza A virus with high pathogenicity (HPAI), and by H5 and H7 subtypes with low pathogenicity, as there is a risk for them to become highly pathogenic by mutation.
Several factors can contribute to the spread of AI viruses, such as:
In birds, AI viruses are shed in the faeces and respiratory secretions. They can all be spread through direct contact with secretions from infected birds, especially through faeces or through contaminated feed and water. Because of the resistant nature of AI viruses, including their ability to survive for long periods when temperatures are low, they can also be carried on farm equipment and spread easily from farm to farm.
Wild birds can normally carry AI viruses in their respiratory or intestinal tracts but they do not commonly get sick, which allow them to carry the viruses on long distances along their migration flyways.
Wild birds, as natural host and reservoir for all types of avian influenza viruses, play a major role in the evolution, maintenance, and spread of these viruses. The main wild species involved have proved to be waterfowls, gulls, and shorebirds, however the virus seem to pass easily between different bird species.
The incidence of infection appears to be seasonal, with the highest isolation rate being in juvenile birds in the fall of the year.
Several routes of exposure of wild bird viruses to poultry have been documented or suspected of being the origins of outbreaks. Direct exposure to wild birds is the most likely transmission factor.
Therefore, limiting exposure of poultry to wild birds through confinement rearing and other biosecurity measures provides an opportunity to reduce the risk of introduction of avian influenza virus from wild birds, and consequently is key to decrease the risk of evolution into highly pathogenic forms, exposure and infection of humans, and recombination with human viruses components to form viruses that can not only infect humans but readily transmit among humans.
Avian Influenza outbreaks can lead to devastating consequences for the poultry industry as well as at national level. Experience has shown that:
People who are in close contact with infected birds are at risk for acquiring avian influenza.
While many human cases are limited to conjunctivitis or mild respiratory disease, some viruses tend to cause severe illness.
However, there is no evidence to suggest that the consumption of poultry or eggs fit for human consumption could transmit the AI virus to humans. As a precautionary and regulatory measure, animals that have been culled as a result of measures to control an AI outbreak should not be allowed to enter the human food and animal feed chain, and precautionary measures for the cleaning and cooking process should be respected.
More information are available on the World Health Organization website: Influenza at the human-animal interface