Ever since it first declared itself in October 2021, Britain’s largest-ever outbreak of avian influenza (AI), better known as bird flu, continues to be responsible for a high death toll in captive birds, poultry, and wild birds. On 29 November 2022, members of the Parliamentary Environment, Food and Rural Affairs Committee thus heard Richard Griffiths, Chief Executive of the British Poultry Council, confirm that about a half of Britain’s free-range turkeys earmarked for Christmas, amounting to 600,000 out of a total of between 1.2 to 1.3 million, have either been culled or died of AI. In an attempt to minimise further losses of turkeys, it has been suggested that the birds be culled wherever possible before Christmas under a ‘freeze and thaw’ scheme, so that numbers can be guaranteed for the festive period.
The current outbreak of AI has been caused by H5N1, a virulent and highly infectious strain of influenza A virus first detected in dead geese in southern China’s Guangdong Province in 1996. The first human case of AI was confirmed soon thereafter, in May 1997, in a boy from Hong Kong who succumbed to his illness. This was followed by another seventeen cases from the same region during November-December that year, including a further five fatalities. From its epicentre in southern China, H5N1 has since made its way across Asia, Africa, Europe, and North America, affecting diverse bird populations and the occasional human. Apart from viruses of the H5 subtype, other strains of avian influenza virus known to infect humans include those belonging to H7, H9, and H10 subtypes. Despite sporadic reports of human cases across the world, AI has remained largely confined to birds, although other mammalian species have been affected at times.
Influenza viruses belong to a family of RNA viruses known as Orthomyxoviruses and can be classified into one of four types: A, B, C, and D. Type A strains are distinguished by two surface proteins: haemagglutinin (HA), which has 16 H subtypes, and neuraminidase (NA), which has nine N subtypes. The H and N subtypes determine the virus’s identity and are used to name individual strains. Influenza viruses adapt to environmental circumstances, in a case of ‘survival of the fitness’, through mutations, causing small variations or antigenic drifts, and by reassortments of genetic material between strains, causing large variations or antigenic shifts.
Wild birds, such as waterfowl (ducks, geese, swans) and gulls are the natural host reservoir species for avian flu viruses and are also less likely to display signs of infection. These viruses may adapt themselves to survival in gallinaceous birds or land fowl (chickens and turkeys), which are not their natural hosts. Avian influenza viruses in domesticated birds may be either low pathogenic strains, which cause few if any symptoms, or highly pathogenic viruses, which spread rapidly and cause severe and often fatal infections. Although traceable to wild birds, much of the devastating spread of AI can be related to intensive poultry farming and to the movement of poultry and poultry products over large distances.
Viruses responsible for bird flu mostly spread by direct contact, with faeces (droppings), feathers, or flesh (carcasses) of infected birds, rather than by airborne spread, although feather dust may be conveyed in aerosols. Contamination of food, water, bedding, or equipment can facilitate the unrestricted spread of virus within flocks of domesticated birds. Humans can be infected by direct contact with infected poultry or surfaces contaminated by bird droppings or their bodily fluids (saliva, nasal and respiratory secretions). It is thus important not to touch any sick or dead birds. Direct human-to-human transmission is distinctly unusual.
Avian flu often announces its presence within a flock by a sudden increase in the number of dead birds. Infected birds may refuse food and water, stop laying eggs, and may display a wide variety of signs, such as fever, a swollen head, runny eyes, lethargy, lack of coordination, drooping of the wings, dragging of the legs, swollen and discoloured comb and wattles, and breathing difficulties, among others.
As part of disease surveillance, members of the public in Great Britain are advised to report to the Defra (Department for Environment, Food and Rural Affairs) Helpline (03459 335577) a single dead bird of prey or owl, three or more dead waterfowl (ducks, geese, swans) or gulls, or five or more dead birds of any other species.
Avian flu is a notifiable disease in animals, and suspected cases have to be reported to the Animal and Plant Health Agency in the UK for further investigation. If cases of avian flu are confirmed in a poultry farm, a concentric 3 km protection zone and a 10 km surveillance zone have to be set up around the premises. Since 7 November 2022, all captive birds and poultry in England have been subject to ‘housing orders’-a form of lockdown- keeping them indoors to prevent exposure to H5N1 that may be carried by wild birds.
The UK Government has most recently deemed the risk posed by H5N1 to the general public’s health as “very low”. But the threat of new mutations that readily jump species and spread from birds to humans has also prompted the development of a human vaccine against AI, particularly important in high-risk professions at the animal-human interface, such as poultry farmers and veterinarians. The unpredictability of viral behaviour and challenges to bringing bird flu under control mean the battle continues. Meanwhile, just as a shortage of free-range eggs and poultry (geese and turkeys) seems inevitable and farmers can be expected to seek compensation for their financial losses, the bigger question remains about how humans can better redefine their by no means synergistic relationship with the world of poultry.
Ashis Banerjee