https://pubmed.ncbi.nlm.nih.gov/36458831
Alarming situation of emerging H5 and H7 avian influenza and effective control strategies Affiliations
Avian influenza viruses continue to present challenges to animal and human health. Viruses bearing the hemagglutinin (HA) gene of the H5 subtype and H7 subtype have caused 2634 human cases around the world, including more than 1000 deaths. These viruses have caused numerous disease outbreaks in wild birds and domestic poultry, and are responsible for the loss of at least 422 million domestic birds since 2005. The H5 influenza viruses are spread by migratory wild birds and have caused three waves of influenza outbreaks across multiple continents, and the third wave that started in 2020 is ongoing. Many countries in Europe and North America control highly pathogenic avian influenza (HPAI) by culling alone, whereas some countries, including China, have adopted a "cull plus vaccination" strategy. As the largest poultry-producing country in the world, China lost relatively few poultry during the three waves of global H5 avian influenza outbreaks, and nearly eliminated the pervasive H7N9 viruses that emerged in 2013. In this review, we briefly summarize the damages the H5 and H7 influenza viruses have caused to the global poultry industry and public health, analyze the origin, evolution, and spread of the H5 viruses that caused the waves, and discuss how and why the vaccination strategy in China has been a success. Given that the H5N1 viruses are widely circulating in wild birds and causing problems in domestic poultry around the world, we recommend that any unnecessary obstacles to vaccination strategies should be removed immediately and forever.
Keywords: H5 and H7 avian influenza; Review; evolution; spread; vaccination.
Conflict of interest statementHuman infections caused by H5 and H7 viruses
In 1997, H5N1 avian influenza viruses transmitted from birds to humans in Hong Kong causing the deaths of 6 of 18 infected persons [Citation3]; this was the first report of human infection with lethal H5N1 virus and attracted wide attention. Since 2003, 865 human cases of H5N1 virus infection have been reported in more than 20 countries across Asia, Africa, Europe, and North America (
). Seventy-five human cases of H5N6 virus infection have been reported in China and Lao, whereas seven human cases of H5N8 virus infection were reported in Russia (). Among the 947 human cases involving different viruses reported from 2003 to April 2022, 488 were fatal (). Studies have identified several key amino acids in the HA of H5N1 viruses that increase the affinity of these viruses for human-type receptors [Citation10,Citation11,Citation32], and several research groups have demonstrated that H5N1 virus can become transmissible via respiratory droplets in ferrets or guinea pigs after obtaining certain mutations or reassorting with human influenza viruses [Citation10–12].
Table 2. Human infections caused by H5 viruses around the world from January 2003 to April 2022*.
As described above, different subtypes of H7 highly pathogenic influenza viruses have caused disease outbreaks in poultry around the world. Historically, both low and highly pathogenic H7 influenza viruses caused human infections, and a total of 1687 human cases were documented in eight countries between 1959 and 2019 (
). The cases reported in Australia, Canada, Italy, Mexico, the UK, and the US ranged from one to 10, and all of the infected individuals survived the infection () [Citation5,Citation7,Citation8,Citation33–43]. Eighty-nine human cases infected with highly pathogenic H7N7 virus were reported in The Netherlands in 2003, and one veterinarian died from the infection [Citation6]. One human case infected with H7N4 virus and 1568 human cases infected with H7N9 viruses were reported in China [Citation4]; 616 of the H7N9 virus infections were fatal (Table 3. Human infections caused by H7 viruses around the world since 1959.
The thousands of human cases of infection with H5 or H7 viruses indicate that humans are highly susceptible to these viruses. Epidemiology studies have shown that humans become infected mainly through exposure to virus-infected poultry or a contaminated environment [Citation44]; human-to-human transmission has been very limited. Therefore, before the H5 and H7 viruses acquire the ability to transmit from human to human, control of these viruses in animals is essential and effective to prevent them from infecting humans.
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