Source identification of H7N9 influenza virus causing human infections


In March 2013, a novel H7N9 influenza virus was identified in China as the etiological agent of a flu-like disease in humans, resulting in some deaths. A group of scientists, led by Professor Chen Hualan (National Avian Influenza Reference Laboratory, State Key Laboratory of Veterinary Biotechnology, Harbin Veterinary Research Institute, Chinese Academy of Agricultural Sciences) have investigated the origins of this novel H7N9 influenza virus (Shi et al., 2013). Following analysis of H7N9 influenza viruses collected from live poultry markets, it was found that these viruses circulating among birds were responsible for human infections. These novel H7N9 viruses are reassortants in which the six internal genes were derived from avian H9N2 viruses; however the origins of their hemagglutinin (HA) and neuraminidase (NA) genes were unclear.

A total of 970 samples were collected from live poultry markets and poultry farms located in Shanghai and Anhui Province. Samples analyzed included drinking water, feces, contaminated soil, and cloacal and tracheal swabs. Of these samples, 20 were positive for the presence of H7N9 influenza viruses. All of the positive samples originated from live poultry markets in Shanghai. Of these 20 positive samples, 10 were isolated from chickens, 3 from pigeons, and 7 were from environmental samples.

The complete genome of three H7N9 isolates, from a chicken, pigeon, and environmental sample, was sequenced and deposited into the GISAID database. Genetic analysis of these isolates revealed high homology across all eight gene segments. Phylogenetic analysis of these novel H7N9 influenza virus isolates showed that that the six internal genes were derived from avian H9N2 viruses, but the ancestor of their HA and NA genes is unknown. According to the GenBank database, the HA genes of the novel isolated viruses were most similar to those from duck H7N3 influenza viruses, sharing 95.2 to 95.8% homology at the nucleotide level. The NA gene of the novel H7N9 virus isolates shared highest homology (97.3 to 97.9%) with NA genes from H4N9 or H11N9 influenza viruses isolated from ducks, and environmental samples from duck farms, located in the Dongting Lake region. It is clear that the novel H7N9 viruses are the product of gene reassortment, with the internal genes from one donor, and HA and NA genes from one or several other donors.

HA receptor-binding specificity is a major molecular determinant for the host range of influenza viruses. Amino acids at positions 226 and 228 of HA are critical for specificity of receptor-binding in influenza viruses. Within the HA protein of novel H7N9 viruses, there was a leucine residue at position 226, which is characteristic of the HA gene in human influenza viruses. This finding implies that H7N9 viruses have partially acquired human receptor-binding specificity. All of the H7N9 human isolates examined contained a lysine residue at position 627 in the PB2 protein. It is well known that this lysine residue contributes to the replication and transmission of avian influenza viruses in mammalian hosts. It is likely that the acquisition of this lysine in H7N9 viruses during their replication in human hosts has significantly contributed to their virulence and lethality in humans.

Tracing the source of these novel H7N9 influenza viruses, and their subsequent characterization, was a collaborative effort involving researchers from the National Avian Influenza Reference Laboratory at the Harbin Veterinary Research Institute, and Shanghai Animal Disease Control Center. This research project was partially supported by the National Basic Research Program of China (2011CB505000), the China Agriculture Research System (CARS-42-G08), and the National Science and Technology Major Project (2012ZX10004214). We suggest that strong measures, such as continued surveillance of avian and human hosts, control of animal movement, shutdown of live poultry markets, and culling of poultry in affected areas, should be taken during this initial stage of virus prevalence to prevent a possible pandemic. Additionally, it is also imperative to evaluate the pathogenicity and transmissibility of these H7N9 viruses, and to develop effective vaccines and antiviral drugs against so as to reduce their adverse effects upon human health.

 

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