Avian influenza (AI) is a highly contagious viral disease caused by subtypes of the influenza A virus (IAV) specifically adapted to affect various species of domestic and wild birds. Occasionally, mammals such as rats, cats, dogs, horses, pigs and humans may also be affected. It represents a significant threat to animal health in the face of major losses in the poultry industry, fatally compromising wild bird groups. The emergence of AI cases is notifiable to the World Organization for Animal Health (OIE).
Therefore, IAVs have a great capacity for mutation ("drift" and antigenic "shift") and, consequently, for adaptation to new hosts. The exact mechanism of bird-to-bird transmission has not yet been fully elucidated. However, it is known that infected birds release the virus in large quantities through their feces and respiratory tract. In avian species, avian influenza strains are characteristically classified into highly pathogenic avian influenza (HPAI) and low pathogenic avian influenza (LPAI). LPAI strains are therefore more common and usually develop limited disease in birds. In humans, in contrast, HPAI and LPAI strains can cause fatal outbreaks of avian influenza, although HPAI strains do so more frequently. Adaptation of the viruses to humans has been responsible for a high case fatality rate and the possibility of human-to-human transmission could pose a high risk to the global population.
The spread and outbreaks of avian influenza have increased due to global trade, poultry production, climate change, bird migration, human movements and global population growth. Direct exposure to infected wild birds is the main transmission factor, as avian species act as natural hosts and reservoirs of the virus, playing an important role in the evolution, maintenance and spread of IAVs. Birds are often asymptomatically infected, allowing them to carry the virus long distances along migratory routes. The main wild species involved are usually waterfowl and shorebirds. In connection with markets and fairs where live birds are sold, close contact between different avian species, other animals and humans is easy to occur, favoring transmission and increasing the possibility of genetic recombination between different types of influenza viruses. Finally, the intense flow of people around the world, as well as goods, caused by globalization and international trade, has greatly increased the risk of spreading diseases, including AI.
Approximately 75% of zoonotic diseases and 60% of emerging infectious diseases worldwide are of animal origin. Several emerging and re-emerging zoonoses significantly affect the lives of people and animals, resulting in significant economic and social development difficulties. Therefore, the "One Health" principle is widely used for the prevention and control of zoonotic diseases. However, the control of avian influenza continues to pose problems. Currently, biosecurity and culling are the main recognized control strategies for the prevention of avian influenza, while vaccination is an optional measure for some countries.
In addition, ecosystem health is the key factor influencing avian influenza infections and outbreaks. Disrupting the balance between humans and nature by invading the migratory routes of wild birds can accelerate viral recombination and lead to the emergence of new IAVs. From a "One Health" perspective, the interconnectedness of human, animal and ecosystem health is recognized, which includes coordinated, collaborative, multidisciplinary and intersectoral approaches to address potential risks at the animal-human-environment interface. In light of the above, it should be noted that the "One Health" approach includes suggestions relevant to the prevention and control of avian influenza. First, comprehensive surveillance of AI infections in animals and humans is essential to assess risk and guide the use of control measures. Information and factors influencing avian influenza outbreaks in humans and birds can be collected through a variety of effective and sensitive surveillance systems, which serve as useful tools for analyzing the occurrence and development of outbreaks, thus detecting infections early. Second, scientific dissemination of virus sequences is vital, as they may be the progenitors of the next human pandemic virus. Therefore, the full support of the international and scientific community is essential for a complete understanding of the spread, evolution, characteristics and pathogenesis of AI viruses.
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