Emergence and Adaptation of a Novel Highly Pathogenic H7N9 Influenza Virus in Birds and Humans from a 2013 Human-Infecting Low-Pathogenic Ancestor

Abstract

Since its emergence in 2013, the H7N9 low-pathogenic avian influenza virus (LPAIV) has been circulating in domestic poultry in China, causing five waves of human infections. A novel H7N9 highly pathogenic avian influenza virus (HPAIV) variant possessing multiple basic amino acids at the cleavage site of the hemagglutinin (HA) protein was first reported in two cases of human infection in January 2017. More seriously, those novel H7N9 HPAIV variants have been transmitted and caused outbreaks on poultry farms in eight provinces in China. Herein, we demonstrate the presence of three different amino acid motifs at the cleavage sites of these HPAIV variants which were isolated from chickens and humans and likely evolved from the preexisting LPAIVs. Animal experiments showed that these novel H7N9 HPAIV variants are both highly pathogenic in chickens and lethal to mice. Notably, human-origin viruses were more pathogenic in mice than avian viruses, and the mutations in the PB2 gene associated with adaptation to mammals (E627K, A588V, and D701N) were identified by next-generation sequencing (NGS) and Sanger sequencing of the isolates from infected mice. No polymorphisms in the key amino acid substitutions of PB2 and HA in isolates from infected chicken lungs were detected by NGS. In sum, these results highlight the high degree of pathogenicity and the valid transmissibility of this new H7N9 variant in chickens and the quick adaptation of this new H7N9 variant to mammals, so the risk should be evaluated and more attention should be paid to this variant. Due to the recent increased numbers of zoonotic infections in poultry and persistent human infections in China, influenza A(H7N9) virus has remained a public health threat. Most of the influenza A(H7N9) viruses reported previously have been of low pathogenicity. Now, these novel H7N9 HPAIV variants have caused human infections in three provinces and outbreaks on poultry farms in eight provinces in China. We analyzed the molecular features and compared the relative characteristics of one H7N9 LPAIV and two H7N9 HPAIVs isolated from chickens and two human-origin H7N9 HPAIVs in chicken and mouse models. We found that all HPAIVs both are highly pathogenic and have valid transmissibility in chickens. Strikingly, the human-origin viruses were more highly pathogenic than the avian-origin viruses in mice, and dynamic mutations were confirmed by NGS and Sanger sequencing. Our findings offer important insight into the origin, adaptation, pathogenicity, and transmissibility of these viruses to both poultry and mammals.

Rapid Evolution of H7N9 Highly Pathogenic Viruses that Emerged in China in 2017

“…We previously reported that the H7N9 highly pathogenic virus was not lethal in mice (Shi et al, 2017), but other groups reported that some chicken H7N9 highly pathogenic viruses had killed certain mice after inoculation with 10 6 50% egg infectious dose (EID 50 ) of virus (Liu et al, 2018; Qi et al, 2018). H7N9 viruses with four different motifs in their HA cleavage sites were all highly lethal in chickens (Shi et al, 2017); however, it is not known whether differences in these motifs affect the virulence of the viruses in mammals.…”

“…Avian and human H7N9 viruses were able to bind to both avian-type and human-type receptors (Belser et al, 2013; Richard et al, 2013; Watanabe et al, 2013; Zhou et al, 2013), but the human isolates were more lethal in mice and more transmissible in ferrets than the avian isolates (Zhang et al, 2013). In early 2017, a few H7N9 HA mutants were detected from samples collected in the live poultry markets in Guangdong province, and animal studies indicated that these mutants were highly pathogenic for chickens (Qi et al, 2018; Shi et al, 2017). Although the index H7N9 HA mutant was not lethal in mice or ferrets, our previous study (Shi et al, 2017) and a study performed by Imai et al (Imai et al, 2017) revealed that the H7N9 HA mutants could acquire additional mutations during their replication in ferrets or humans, and then become highly lethal in mammals and transmissible in ferrets by respiratory droplet.…”

The Protective Effects of the A/ZJU01/ PR8/2013 Split H7N9 Avian Influenza Vaccine Against Highly Pathogenic H7N9 in BALB/c Mice

“…Notably, viruses of human origin were more pathogenic in mice than avian viruses. Now, these novel HPAI H7N9 strains have caused human infection in 3 provinces and poultry farm outbreaks in 8 provinces [6]. Considering the rapid geographical expansion of the HPAI H7N9 viruses, effective control measures are urgently needed [7].…”

Emergence of waterfowl‐originated gene cassettes in HPAI H7N9 viruses caused severe human infection in Fujian, China

“…To date, most evidence supports the notion that the insertion event probably occurred in poultry in the Pearl River Delta region and that the Guangdong Province was the original location of the HPAI H7N9 virus . The HPAI H7N9 virus might have emerged in mid‐2016, according to a molecular clock model .…”

“…In early 2017, human infections with a highly pathogenic avian influenza (HPAI) A(H7N9) virus were reported in Taiwan and Guangdong Provinces of China . Phylogenic analyses showed that the HPAI H7N9 virus originated from the LPAI H7N9 virus . The signature genetic difference between HPAI and LPAI H7N9 virus was an insertion of four amino acids at the cleavage site of the HA protein .…”