The number of cases of avian influenza in birds and in people globally this season is higher than usual and particularly concerning due to the associated risks to poultry industries and human health. Outbreaks were reported in birds in 41 countries across Europe and Asia within six months between May and November 2021, some of which were of subtypes that have zoonotic potential (OIE, 2021a). And the season is still ongoing. The World Organisation for Animal Health (OIE) calls the genetic variability of subtypes "unprecedented” that creates “an epidemiologically challenging landscape" (OIE, 2021a).
Avian influenza, commonly known as bird flu, is a highly contagious viral disease affecting the respiratory, digestive and/or nervous system of many species of birds. For example, in a rare event, a fox tested positive for H5N1 in the Netherlands in January this year (Wageningen, 2022). Some subtypes can infect people and other animal species in certain circumstances (WHO, 2018). While spillover into other species is occasional, such events can have devastating effects, such as the H1N1 (also known as the “swine flu”) pandemic which was caused by spillover from birds into pigs before becoming transmissible between people in 2009.
Situation in People
Avian influenza subtypes that currently cause infections in people include subtypes H5N1, H5N6, H5N8, H7N9, and H9N2 (WHO, 2018). There are further subtypes that have zoonotic potential, including other H5 subtypes, other H7 subtypes, H6N1, H10N7, and H10N8 (FAO EMPRES, 2021).
Along with a steep increase in H5N1 cases in birds in the UK, the first person in UK was confirmed positive for H5N1 in January this year (Grierson, 2022).
Rises in subtype H5N6 infections in people in China is causing particular concern globally. China reported thirty-five human infections with H5N6 in 2021 compared with only five in 2020 (WHO, n.d.; NewsDesk,2021). This increase in human infections with H5N6 is cause for concern, as a previously circulating strain appears to have changed and could be more infectious to people (Patton, 2021). The rise in numbers of reported human cases of H5N6 infection may reflect not only the continued circulation of the virus in birds but also the enhanced surveillance system and diagnostic capacity in response to the COVID-19 pandemic (WHO, 2021).
Situation in Birds
At the same time, the number of avian influenza outbreaks in birds is on the rise and the season is lasting longer than usual, especially in Europe and Asia (OIE, 2021b). The last season of avian influenza persisted into the summer months, which is atypical. The current season is considered to be one of the largest HPAI epidemics that has ever occurred in Europe (EFSA et al., 2021).
Over three thousand birds were reported as infected, and tens of millions of birds were culled due to avian influenza mostly in Europe and Asia in 2021 (FAO EMPRES-i, n.d.; Fast spreading, 2021). The largest number of premises ever are infected with H5N1 in the UK, resulting in the culling of hundreds of thousands of birds (Marshall, 2021; Kirka, 2021). The UK is dealing with possibly the most serious form of avian influenza yet to be encountered there (The Newsroom, 2021).
An H5N1 outbreak occurred in domestic birds in Newfoundland, Canada in December 2021 resulting in the death of 360 of 409 mixed species of birds (FAO EMPRES, 2021). This is the first time that such a virus has been detected in the Americas since 2015 (OFFLU, 2021). Genetic analyses confirmed the virus is related to those currently circulating in Europe.
The current avian influenza season in birds has been dominated by H5N1 (Figure 1A) while last year’s is dominated by H5N8 (Figure 1B) (FAO EMPRES, 2020-2021). Cases of H5N1 are on the rise in birds (Figure 2), which is unusual as compared to last year (FAO EMPRES, 2020-2021). In the last month of 2021, 651 cases of avian influenza with zoonotic potential were reported in birds, of which 594 cases (91 %) were of the H5N1 strain (FAO EMPRES, 2021). During the same period in 2020, 401 cases of avian influenza with zoonotic potential were reported, of which only five cases (1.2 %) were H5N1. Further increases and spread of avian influenza is expected in the coming months (OIE, 2021b).
The high incidence of subtype H5N1 infection indicates a possible introduction and spread of the virus through wild bird migration (OIE, 2021b). Because of the risk of introduction to poultry from wild birds, some countries mandate farmers to keep poultry indoors (DEFRA, 2021; Dutch order, 2021). Such regulations have an impact on animal welfare and on the labelling of products, such as eggs from free-range chickens. Poultry farmers are thus faced with the challenge of complying with conflicting labeling requirements and control measures for avian influenza.
Influence of climate change
Evidence has shown that climate change can influence avian influenza outbreaks in many ways including impacts on the survivability of the virus and on wild bird migration.
Higher temperatures and lower humidity have been shown to favor avian influenza viral activity (Elsobky et al., 2020). The longer the virus can survive in the environment, the higher the potential for introduction and spread in birds.
At the same time, warming temperature scan change the timing and patterns of bird migration, creating new opportunities for viral transport and reassortment among different bird species (Morin et al., 2018). One study found that Richard's pipits are migrating on an east to west axis instead of heading south towards warmer latitudes (Dufour et al., 2021). A separate study found that some trans-Saharan migratory birds spend less time in their winter retreats in Africa and more time in their breeding grounds in Europe (Lawrence et al., 2021).
Longer and hotter summers can result in longer breeding seasons, and thus can increase opportunities for virus transfer at summer breeding sites globally (Lawrence et al., 2021). Birds may no longer need overwinter in southern regions if they can find food and habitat in northern regions all year round.
Increases in avian influenza outbreaks in people and birds may be symptoms of much larger issues at hand. Climate change coupled with changes in globalization and intensification of livestock production are significant factors that influence the epidemiology of the disease. At the same time, the COVID-19 pandemic has resulted in both increased pandemic preparedness activities as well as the diversion of resources and attention away from other diseases. A One Health approach to surveillance and control that considers the interactions of human, animal, and environmental factors is needed to fully understand and thus inform decisions on global prevention and response to avian influenza.
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