Avian Influenza (H7N9) Viruses Co-circulating among Chickens, Southern China

Genetic characterization and pathogenicity of H7N9 highly pathogenic avian influenza viruses isolated from South China in 2017

Since 2017, the new H7N9 highly pathogenic avian influenza viruses (HPAIVs) have been responsible for more than 200,000 cases of chicken infection and more than 120,000 chicken deaths in China. Our previous study found that the Q26 was chicken-origin H7N9 HPAIV. In this study, we analyzed the genetic characterization of Q24, Q65, Q66, Q85, and Q102 H7N9 avian influenza viruses isolated from Guangdong, China in 2017. Our results showed that these viruses were highly pathogenic and belonged to two different genotypes, which suggested they occurred genetic reassortant. To investigate the pathogenicity, transmission, and host immune responses of H7N9 virus in chickens, we selected Q24 and Q26 viruses to inoculate chickens. The Q24 and Q26 viruses killed all inoculated chickens within 3 days and replicated effectively in all tested tissues. They were efficiently transmitted to contact chickens and killed them within 4 days through direct contact. Furthermore, we found that the expressions of several immune-related genes (e.g., TLR3, TLR7, MDA5, MAVS, IFN-β, IL-6, IL-8, OAS, Mx1, MHC I, and MHC II) were upregulated obviously in the lungs and spleen of chickens inoculated with the two H7N9 viruses at 24 h post-inoculation (HPI). Among these, IL-6 and IFN-β in lungs were the most upregulated (by 341.02-381.48-fold and 472.50-500.56-fold, respectively). These results suggest that the new H7N9 viruses isolated in 2017, can replicate and transmit effectively and trigger strong immune responses in chickens, which helps us understand the genetic and pathogenic variations of H7N9 HPAIVs in China.

Multiple basic amino acids in the cleavage site of H7N9 hemagglutinin contribute to high virulence in mice

Background

Avian influenza A (H7N9) virus has caused more than 1,500 cases of human infection since its emergence in early 2013. Displaying little or no pathogenicity in poultry, but a 40% case-fatality rate in humans, five waves of H7N9 human infections occurred in China during 2013–2017, caused solely by a low pathogenicity strain. However, avian isolates possessing a polybasic connecting peptide in the hemagglutinin (HA) protein were detected in mid-2016, indicating that a highly pathogenic virus had emerged and was co-circulating with the low pathogenicity strains.

Methods

Here we characterize the pathogenicity of a newly emerged human H7N9 variant with a PEVPKRKRTAR/GLF insertion motif at the cleavage site of the HA protein in vitro and in vivo.

Results

This variant replicates in MDCK cells independently of TPCK-trypsin, which is indicative of high pathogenicity in chickens. The 50% mouse lethal dose (MLD₅₀) of this novel isolate was less than 10 plaque forming units (PFU), compared with 3.16×10⁴ for an identical virus lacking the polybasic insertion, indicating a high virulence phenotype.

Conclusions

Our results demonstrate that the multiple basic amino acid insertion in the HA protein of the H7N9 variant confers high virulence in mammals, highlighting a potential risk to humans. Continuous viral surveillance is therefore necessary in the China region to improve pandemic preparedness.

H7N9 Influenza Virus in China

In early 2013, human infections caused by a novel H7N9 avian influenza virus (AIV) were first reported in China; these infections caused severe disease and death. The virus was initially low pathogenic to poultry, enabling it to spread widely in different provinces, especially in live poultry markets. Importantly, the H7N9 low pathogenic AIVs (LPAIVs) evolved into highly pathogenic AIVs (HPAIVs) in the beginning of 2017, causing a greater threat to human health and devastating losses to the poultry industry. Fortunately, nationwide vaccination of chickens with an H5/H7 bivalent inactivated avian influenza vaccine since September 2017 has successfully controlled H7N9 avian influenza infections in poultry and, importantly, has also prevented human infections. In this review, we summarize the biological properties of the H7N9 viruses, specifically their genetic evolution, adaptation, pathogenesis, receptor binding, transmission, drug resistance, and antigenic variation, as well as the prevention and control measures. The information obtained from investigating and managing the H7N9 viruses could improve our ability to understand other novel AIVs and formulate effective measures to control their threat to humans and animals.

Transcriptome Profiles of Highly Pathogenic Pure Avian H7N9 Virus-Infected Lungs of BALB/c Mice

Avian influenza A (H7N9) viruses emerged in China in 2013 and caused a zoonotic disease associated with a high case-fatality ratio of more than 30%. Transcriptional profiles obtained using animal models reveal host responses to the disease, thereby providing insights into disease pathogenesis. Therefore, we aimed to characterize the host responses of the H7N9 virus infected-mouse lungs in this study. First, we isolated an avian-originated H7N9 strain, which was shown to be highly pathogenic to both chickens and mice. Genomic analysis results suggested that a 12-nucleotide-insertion was present at the hemagglutinin cleavage site, and both the hemagglutinin and neuraminidase genes belonged to the Yangtze River Delta lineage. RNA sequencing results revealed 566 differentially expressed genes in the H7N9-infected lungs. Moreover, transcriptome analysis revealed that over-activated antiviral signals and intense interferon-stimulated gene products possibly contributed to the high virulence of the virus in mice. Importantly, lung concentrations of inflammatory cytokines, including interleukin-1β and interleukin-6, interferon-β, and tumor necrosis factor-α, were upregulated in response to H7N9 virus infection. Overall, the present study provided a comprehensive understanding of H7N9 virus pathogenicity and correlated host immune responses.

Pathogenicity of different H5N6 highly pathogenic avian influenza virus strains and host immune responses in chickens

The H5N6 highly pathogenic avian influenza virus (HPAIV) has been circulating in China since 2013. In this report, we describe our recent chicken experimental studies investigating the pathogenicity and transmission of four H5N6 HPAIV field strains of different origins (GS39, CK44, DK47 and CK74) and the host immune responses. Four-week-old specific-pathogen-free chickens were inoculated intranasally with one of the four H5N6 HPAIV strains (one strain per group). Among the contact chickens, the GS39 and CK74 strains caused 100 % mortality, the CK44 strain caused 80 % mortality, and the DK47 strain caused 40 % mortality. The viruses were effectively replicated in multiple tissues of the inoculated chickens, in which high viral titers were detected in virus-infected tissues, and significantly upregulated expression of immune-related genes was found in the infected chickens at 24 hpi. The chicken serum antibody levels increased from 5log2 at 7 dpe to 7.67-8log2 at 14 dpe. The major histocompatibility complex molecules were upregulated 21.22- to 32.98-fold in lungs and 5.10- to 18.47-fold in spleens. In summary, H5N6 viruses can replicate within chickens and be effectively transmitted between chickens. Our study contributes to further understanding the pathogenesis of clade 2.3.4.4 H5N6 avian influenza viruses in chickens.

Antigenic Variant of Highly Pathogenic Avian Influenza A(H7N9) Virus, China, 2019

In China, influenza A(H7N9) virus appeared in 2013, then mutated into a highly pathogenic virus, causing outbreaks among poultry and cases in humans. Since September 2017, extensive use of the corresponding vaccine, H7-Re1, successfully reduced virus prevalence. However, in 2019, a novel antigenic variant emerged, posing considerable economic and public health threats.A.

Comparative Pathogenicity and Transmissibility of the H7N9 Highly Pathogenic Avian Influenza Virus and the H7N9 Low Pathogenic Avian Influenza Virus in Chickens

There were five outbreaks of H7N9 influenza virus in humans in China since it emerged in 2013, infecting >1000 people. The H7N9 low pathogenic influenza virus was inserted into four amino acids in the HA protein cleavage site to mutate into the H7N9 highly pathogenic virus. This emerging virus caused 15 outbreaks in chickens from the end of 2016 to date. Two H7N9 avian influenza virus (AIV) strains, A/chicken/Guangdong/A46/2013 (LPAIV) and A/chicken/Guangdong/Q29/2017 (HPAIV), were selected to compare the pathogenicity and transmissibility between H7N9 LPAIVs and HPAIVs in chickens. We inoculated 3- to 4-week-old specific-pathogen-free (SPF) chickens with 6 log₁₀EID₅₀/0.1 mL viruses via the ocular-nasal route and co-housed four chickens in each group. The inoculated chicken mortality rate in the A46 and Q29 groups was 1/5 and 5/5, respectively. Q29 virus replication was more efficient compared to the A46 virus in inoculated chickens. Infected chickens initiated viral shedding to naïve contact chickens through respiratory and digestive routes. Both viruses transmitted between chickens by naïve contact, but the Q29 virus had a higher pathogenicity in contact chickens than the A46 virus. Compared with early H7N9 LPAIVs, the pathogenicity and transmissibility of the emerging H7N9 HPAIV was stronger in chickens, indicating that H7N9 influenza virus may continue to threaten human and poultry health.

Genetic characteristics, pathogenicity and transmission of H5N6 highly pathogenic avian influenza viruses in Southern China

Since 2014, H5 highly pathogenic avian influenza viruses (HPAIVs) from clade 2.3.4.4 have been persistently circulating in Southern China. This has caused huge losses in the poultry industry. In this study, we analysed the genetic characteristics of seven H5N6 HPAIVs of clade 2.3.4.4 that infected birds in Southern China in 2016. Phylogenetic analysis grouped the HA, PB2, PA, M and NS genes as MIX-like, and the NA genes grouped into the Eurasian lineage. The PB1 genes of the GS24, GS25, CK46 and GS74 strains belonged to the VN 2014-like group and the others were grouped as MIX-like. The NP genes of GS24 and GS25 strains belonged to the ZJ-like group, but the others were MIX-like. Thus, these viruses came from different genotypes, and the GS24, GS25, CK46 and GS74 strains displayed genotype recombination. Additionally, our results showed that the mean death time of all chickens inoculated with 10⁵ EID₅₀ of CK46 or GS74 viruses was 3 and 3.38 days, respectively. The viruses replicated at high titers in all tested tissues of the inoculated chickens. They also replicated in all tested tissues of naive contact chickens, but their replication titers in some tissues were significantly different (p < 0.05). Thus, the viruses displayed high pathogenicity and variable transmission in chickens. Therefore, it is necessary to focus on the pathogenic variation and molecular evolution of H5N6 HPAIVs in order to prevent and control avian influenza in China.

Prevalence of H7N9 subtype avian influenza viruses in poultry in China, 2013-2018

H7N9 subtype avian influenza viruses (AIV) circulating in China over recent years have had an enormous impact on public health and economy. During the period between November 2016 and April 2017, an increase in human infections caused by these viruses was reported, with rapid emergence and spread of variants in China. Consequently, the government of China implemented a controversial vaccination strategy in September 2017. Here, we provide evidence of the prevalence of H7N9 AIVs in China based on systematic large-scale surveillance in poultry during 2013-2018. Emerging variants were confirmed as highly pathogenic in chickens using the intravenous pathogenicity index (IVPI) test. The currently available vaccine provided complete protection against the H7N9 HPAIV challenge in chickens. The collective findings clearly indicate that the vaccination strategy implemented not only significantly decreases the prevalence of H7N9 AIVs in poultry but also effectively prevents human infection with H7N9 viruses.

Decreased Frequencies of Th17 and Tc17 Cells in Patients Infected with Avian Influenza A (H7N9) Virus

The outbreak of avian influenza A (H7N9) virus infection, with a high mortality rate, has caused concern worldwide. Although interleukin-17- (IL-17-) secreting CD4⁺ T (Th17) and CD8⁺ T (Tc17) cells have been proven to play crucial roles in influenza virus infection, the changes and roles of Th17 and Tc17 cells in immune responses to H7N9 infection remain controversial. In this study, we found that the frequencies of Th17 and Tc17 cells among human peripheral blood mononuclear cells (PBMCs) as well as IL-17A protein and mRNA levels were markedly decreased in patients with acute H7N9 virus infection. A positive correlation was found between the serum IL-17A level and the frequency of these two cell groups. In vitro infection experiments revealed decreased Th17 and Tc17 cell frequency and IL-17A levels at various time points postinfection. In addition, Th17 cells were the predominant sources of IL-17A in PBMCs of patients infected with H7N9 virus. Taken together, our results indicate immune disorder in acute H7N9 infection and a restored Th17 and Tc17 cell frequency might serve as a biomarker for predicting recovery in patients infected with this virus.

Multiple amino acid substitutions involved in the adaption of three avian-origin H7N9 influenza viruses in mice

Background

Avian influenza A H7N9 virus has caused five outbreak waves of human infections in China since 2013 and posed a dual challenge to public health and poultry industry. The number of reported H7N9 virus human cases confirmed by laboratory has surpassed that of H5N1 virus. However, the mechanism for how H7N9 influenza virus overcomes host range barrier has not been clearly understood.

Methods

To generate mouse-adapted H7N9 influenza viruses, we passaged three avian-origin H7N9 viruses in mice by lung-to-lung passages independently. Then, the characteristics between the parental and mouse-adapted H7N9 viruses was compared in the following aspects, including virulence in mice, tropism of different tissues, replication in MDCK cells and molecular mutations.

Results

After ten passages in mice, MLD₅₀ of the H7N9 viruses reduced >750-3,160,000 folds, and virus titers in MDCK cells increased 10-200 folds at 48 hours post-inoculation. Moreover, the mouse-adapted H7N9 viruses showed more expanded tissue tropism and more serious lung pathological lesions in mice. Further analysis of the amino acids changes revealed 10 amino acid substitutions located in PB2 (E627K), PB1 (W215R and D638G), PA (T97I), HA (H3 numbering: R220G, L226S, G279R and G493R) and NA (P3Q and R134I) proteins. Moreover, PB2 E627K substitution was shared by the three mouse-adapted viruses (two viruses belong to YRD lineage and one virus belongs to PRD lineage), and PA T97A substitution was shared by two mouse-adapted viruses (belong to YRD lineage).

Conclusions

Our result indicated that the virulence in mice and virus titer in MDCK cells of H7N9 viruses significantly increased after adapted in mouse model. PB2 E627K and PA T97A substitutions are vital in mouse adaption and should be monitored during epidemiological study of H7N9 virus.

Electronic supplementary material

The online version of this article (10.1186/s12985-018-1109-1) contains supplementary material, which is available to authorized users.

Protective efficacy of an inactivated chimeric H7/H5 avian influenza vaccine against highly pathogenic avian influenza H7N9 and clade 2.3.4.4 H5 viruses

The highly pathogenic avian influenza (HPAI) H5 and H7N9 viruses pose a serious challenge to public health and the poultry industry in China. In this study, we generated a chimeric H7/H5 recombinant virus that expressed the entire HA1 region of the HPAI A/chicken/Guangdong/RZ/2017(H7N9) virus and the HA2 region of the HPAI A/chicken/Fujian/5/2016(H5N6) viruses. The resulting chimeric PR8-H7/H5 virus exhibited similar growth kinetics as the parental PR8-H5 and PR8-H7 viruses in vitro. The inactivated chimeric PR8-H7/H5 vaccine induced specific, cross-reactive hemagglutination inhibition antibodies against the H7 virus only but induced serum-neutralizing antibodies against both H7 and H5 viruses. Furthermore, the inactivated chimeric PR8-H7/H5 vaccine significantly reduced virus shedding and protected chickens from challenge with the HPAI H5N6 and H7N9 viruses. Our results suggested that the inactivated chimeric PR8-H7/H5 vaccine was effective against HPAI H5 and H7N9 viruses in chickens.

Influenza Viral Vectors Expressing Two Kinds of HA Proteins as Bivalent Vaccine Against Highly Pathogenic Avian Influenza Viruses of Clade 2.3.4.4 H5 and H7N9

The H5 and H7N9 subtypes of highly pathogenic avian influenza viruses (HPAIVs) in China pose a serious challenge to public health and the poultry industry. In this study, a replication competent recombinant influenza A virus of the Í5N1 subtype expressing the H7 HA1 protein from a tri-cistronic NS segment was constructed. A heterologous dimerization domain was used to combine with the truncated NS1 protein of 73 amino acids to increase protein stability. H7 HA1, nuclear export protein coding region, and the truncated NS1 were fused in-frame into a single open reading frame via 2A self-cleaving peptides. The resulting PR8-H5-NS1(73)H7 stably expressed the H5 HA and H7 HA1 proteins, and exhibited similar growth kinetics as the parental PR8-H5 virus in vitro. PR8-H5-NS1(73)H7 induced specific hemagglutination inhibition (HI) antibody against H5, which was comparable to that of the combination vaccine of PR8-H5 and PR8-H7. The HI antibody titers against H7 virus were significantly lower than that by the combination vaccine. PR8-H5-NS1(73)H7 completely protected chickens from challenge with both H5 and H7 HPAIVs. These results suggest that PR8-H5-NS1(73)H7 is highly immunogenic and efficacious against both H5 and H7N9 HPAIVs in chickens.

Highlights:

- PR8-H5-NS1(73)H7 simultaneously expressed two HA proteins of different avian influenza virus subtypes.

- PR8-H5-NS1(73)H7 was highly immunogenic in chickens.

- PR8-H5-NS1(73)H7 provided complete protection against challenge with both H5 and H7N9 HPAIVs.

Avian influenza overview November 2017 - February 2018

Between 16 November 2017 and 15 February 2018, one highly pathogenic avian influenza (HPAI) A(H5N6) and five HPAI A(H5N8) outbreaks in poultry holdings, two HPAI A(H5N6) outbreaks in captive birds and 22 HPAI A(H5N6) wild bird events were reported within Europe. There is a lower incursion of HPAI A(H5N6) in poultry compared to HPAI A(H5N8). There is no evidence to date that HPAI A(H5N6) viruses circulating in Europe are associated with clades infecting humans. Clinical signs in ducks infected with HPAI A(H5N8) seemed to be decreasing, based on reports from Bulgaria. However, HPAI A(H5N8) is still present in Europe and is widespread in neighbouring areas. The majority of mortality events of wild birds from HPAIV A(H5) in this three‐month period involved single birds. This indicates that the investigation of events involving single dead birds of target species is important for comprehensive passive surveillance for HPAI A(H5). Moreover, 20 low pathogenic avian influenza (LPAI) outbreaks were reported in three Member States. The risk of zoonotic transmission to the general public in Europe is considered to be very low. The first human case due to avian influenza A(H7N4) was notified in China underlining the threat that newly emerging avian influenza viruses pose for transmission to humans. Close monitoring is required of the situation in Africa and the Middle East with regards to HPAI A(H5N1) and A(H5N8). Uncontrolled spread of virus and subsequent further genetic evolution in regions geographically connected to Europe may increase uncertainty and risk for further dissemination of virus. The risk of HPAI introduction from Third countries via migratory wild birds to Europe is still considered much lower for wild birds crossing the southern borders compared to birds crossing the north‐eastern borders, whereas the introduction via trade is still very to extremely unlikely.