Influenza A virus subtype H7N9

Virology

Main article: Influenza A virus

H7N9 is a subtype of Influenza A virus. Like all subtypes it is an enveloped negative-sense RNA virus, with a segmented genome. Influenza viruses have a relatively high mutation rate that is characteristic of RNA viruses. The segmentation of its genome facilitates genetic recombination by reassortment in hosts infected with two different strains of influenza viruses at the same time.

Figure 1 shows a diagramatic representation of the genetic relatedness of Asian H5N1 hemagglutinin genes from various isolates of the virus Through a combination of mutation and genetic reassortment the virus can evolve to acquire new characteristics, enabling it to evade host immunity and occasionally to jump from one species of host to another.

Highly pathogenic avian influenza

Epidemiology

Some species of wild aquatic birds act as natural asymptomatic carriers of a large variety of influenza A viruses, which they can spread over large distances in their annual migration. Symptoms of avian influenza vary according to both the strain of virus underlying the infection, and on the species of bird affected. Symptoms of influenza in birds may include swollen head, watery eyes, unresponsiveness, lack of coordination, respiratory distress such as sneezing or gurgling.

Humans and other mammals can only become infected with avian influenza, including A/H7N9, after prolonged close contact with infected birds or contaminated environments. In mammals including humans, infection with avian influenza (whether LPAI or HPAI) is rare. Symptoms of infection vary from mild to severe, including fever, diarrhoea, and cough.

As of February, 2024 there have been very few instances of human-to-human transmission, and each outbreak has been limited to a few people. All subtypes of avian Influenza A have potential to cross the species barrier into humans, with H5N1 and H7N9 considered the biggest threats.

History

Outbreak in China, 2013-2019

Prior to 2013, A/H7N9 had previously been isolated only in birds, with outbreaks reported in the Netherlands, Japan, and the United States. Until the 2013 outbreak in China, no human infections with A/H7N9 had been reported.

A significant outbreak of Influenza A virus subtype H7N9 (A/H7N9) started in March 2013 when severe influenza affected 18 humans in China; six subsequently died. It was discovered that a low pathogenic strain of A/H7N9 was circulating among chickens, and that all the affected people had been exposed in poultry markets. Further cases among humans and poultry in mainland China continued to be identified sporadically throughout the year, followed by a peak around the festival season of Chinese New Year (January and February) in early 2014 which was attributed to the seasonal surge in poultry production.

Infections among humans and poultry continued during the next few years, again with peaks around the new year. In 2016 a virus strain emerged which was highly pathogenic to chickens. In order to contain the HPAI outbreak, the Chinese authorities in 2017 initiated a large scale vaccination campaign against avian influenza in poultry. Since then, the number of outbreaks in poultry, as well as the number of human cases, dropped significantly. In humans, symptoms and mortality for both LPAI and HPAI strains have been similar. Although no human H7N9 infections have been reported since February 2019, the virus is still circulating in poultry, particularly in laying hens. It has demonstrated antigenic drift to evade vaccines, and remains a potential threat to the poultry industry and public health.

As of May 2022, a total of 1568 confirmed A(H7N9) human infections with 616 deaths have been reported, with a case fatality ratio (CFR) of 39%.

Genetic characterisation of the "Asian lineage" strain of avian influenza A/H7N9 shows that it was not related to A/H7N9 strains previously identified in Europe and North America. This new strain resulted from the recombination of genes between several parent viruses noted in poultry and wild birds in Asia. The H7 gene is most closely related to sequences found in samples from ducks in Zhejiang province in 2011.The N9 gene was closely related to isolated wild ducks in South Korea in 2011. Other genes resembled samples collected in Beijing and Shanghai in 2012. The genes would have been carried along the East Asian flyway by wild birds during their annual migration.

The genetic characteristics of Asian lineage A/H7N9 virus are of particular concern because of their pandemic potential. The virus is able to infect humans relatively easily, but does not sustain human-to-human transmission. If the virus were to acquire the ability to transmit easily between humans (either through mutation or genetic reassortment) there is potential for a severe epidemic or pandemic.

Other occurrences

During early 2017, outbreaks of avian influenza A(H7N9) occurred in poultry in the USA. The strain in these outbreaks was of North American origin and is unrelated to the Asian lineage H7N9 which is associated with human infections in China.

In May 2024, an HPAI A/H7N9 was detected on a poultry farm with 151,894 birds in Terang, Australia. There were 14,000 clinically affected birds. It is presumed that wild birds were the source of the outbreak.

An H7N9 outbreak in the US was confirmed on March 13, 2025, on a commercial broiler breeder chicken farm in Noxubee, Mississippi, housing 47,654 chickens. All 47,654 chickens were euthanized as a precautionary measure to prevent further spread of the virus.

Vaccine

The continuing evolution and antigenic drift of the A/H7N9 virus makes further development difficult as any significant outbreak could involve a completely new strain. A number of human vaccines targeting A/H7N9 have been trialled in relatively small groups of subjects; in the event of any outbreak, a candidate vaccine virus would be distributed to manufacturers

References

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