The reasons for the genetic variability of AI viruses are complex, but are mainly the result of high mutation rates, rapid replication kinetics and the large population sizes of RNA viruses. Alterations in the genome of AI viruses that are selected and become ‘fixed’ in the viral population can therefore result in changes to the epidemiology of the disease. The genome of influenza A viruses are prone to the accumulation of point mutations, which if occurring in major epitopes can lead to phenotypic changes (genetic drift) and facilitate virus escape from the host immune response, and these may contribute to the reduced efficacy of vaccines. The genomes of AI viruses can also evolve and diversify through other important mechanisms. Genetic reassortment (so-called genetic shift) may occur during the infection cycle, leading to the emergence of novel virus strains with unpredictable phenotypes. This genetic variability can also have a negative impact on virus detection and disease diagnosis, including by molecular tests (e.g. PCR) that target the viral genome, as well as by conventional diagnostic methods, or flock monitoring using serological assays. Advanced genome sequencing methods are also now capable of rapidly and accurately studying the genetic diversity and evolution of AI viruses. Together with veterinary investigations in the field, such laboratory tools may be used to better understand the epidemiology of these infections, including their source and spread. The network of international reference laboratories also enables study of AI viruses at a range of spatial scales, from local to global, providing greater insights into viral diversity and patterns of disease.

The epidemiology of the disease has become more complicated in recent years, and is related to the virus strain and the host population affected. Conventional control measures for avian influenza in poultry have included early detection and stamping out, ideally supported through compensation. These methods can result in successful control of outbreaks. Unfortunately, however, in some parts of the world over recent years uncontrolled spread has led to a situation where some HPAI viruses - such as Eurasian lineage H5N1 HPAI strains derived from the common A/goose/Guangdong/1/96 ancestor - have become endemic and beyond likely immediate control. H9N2 LPAI strains are also recognised to be endemic in many regions of the world, especially in the Middle East and North Africa. These infections although classed as low pathogenicity can on occasions cause high mortality, usually as a result of secondary bacterial infections, suboptimal husbandry and/or extreme climatic conditions. In response there has been extensive use of vaccination to limit the spread, clinical signs of and flock losses and economic consequences due to disease caused by both HPAI and LPAI viruses. However, success in control or eradication of both infections has been inconsistent.

The epidemiology of avian influenza continues to evolve worldwide, and AI viruses remain a moving target with changes to key viral properties and mechanisms relating to virulence, transmission and pathogenesis in birds and other host species, including people. In turn this presents considerable challenges to national and international veterinary and public health authorities to ensure timely detection and effective control of the disease at the animal source, and to minimise the socio-economic impacts of outbreaks when they occur, especially in countries where there is a reliance on poultry as a main source of protein. However, only by continued surveillance for and study of the epidemiology of avian influenza can we gain fresh insights and a fuller understanding to enable the successful diagnosis, prevention and control of this disease of global importance.

Richard IRVINE
Animal and Plant Health Agency (APHA), U.K.

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^{Information from the Avian Flu Forum hosted by Boehringer on April 2017}