Two Outbreak Sources of Influenza A (H7N9) Viruses Have Been Established in China

Abstract

Due to enzootic infections in poultry and persistent human infections in China, influenza A (H7N9) virus has remained a public health threat. The Yangtze River Delta region, which is located in eastern China, is well recognized as the original source for H7N9 outbreaks. Based on the evolutionary analysis of H7N9 viruses from all three outbreak waves since 2013, we identified the Pearl River Delta region as an additional H7N9 outbreak source. H7N9 viruses are repeatedly introduced from these two sources to the other areas, and the persistent circulation of H7N9 viruses occurs in poultry, causing continuous outbreak waves. Poultry movements may contribute to the geographic expansion of the virus. In addition, the AnH1 genotype, which was predominant during wave 1, was replaced by JS537, JS18828, and AnH1887 genotypes during waves 2 and 3. The establishment of a new source and the continuous evolution of the virus hamper the elimination of H7N9 viruses, thus posing a long-term threat of H7N9 infection in humans. Therefore, both surveillance of H7N9 viruses in humans and poultry and supervision of poultry movements should be strengthened. IMPORTANCESince its occurrence in humans in eastern China in spring 2013, the avian H7N9 viruses have been demonstrating the continuing pandemic threat posed by the current influenza ecosystem in China. As the viruses are silently circulated in poultry, with potentially severe outcomes in humans, H7N9 virus activity in humans in China is very important to understand. In this study, we identified a newly emerged H7N9 outbreak source in the Pearl River Delta region. Both sources in the Yangtze River Delta region and the Pearl River Delta region have been established and found to be responsible for the H7N9 outbreaks in mainland China.

Sudden increase in human infection with avian influenza A(H7N9) virus in China, September–December 2016

“…All 33 viruses had completed full genetic analyses, and the genetic markers of mammalian adaptation and antiviral resistance of virus strains that were isolated in the fifth epidemics remained similar (Dr Yuelong Shu in China CDC, unpublished data) to earlier epidemics. ( 9 ) The genetic sequences of these viruses will be shared with the international community through the usual channels.…”

Assessing Change in Avian Influenza A(H7N9) Virus Infections During the Fourth Epidemic — China, September 2015–August 2016

“…Since the emergence of A(H7N9) virus, the majority of viruses from both humans and poultry have contained two hemagglutinin (HA) amino acid residues, 186V and 226L/I in H3 numbering (177 and 217 in H7 numbering), which are likely to increase human receptor binding (5). During the first three epidemics, the number of A(H7N9) viruses identified in humans retaining the avian receptor binding residues decreased (5).…”

Avian influenza A (H7N9) virus infections in humans across five epidemics in mainland China, 2013–2017

“…There have been repeated introductions of A (H7N9) viruses from these two sources to the other areas and the persistent circulation of A (H7N9) viruses in poultry has led to continuous epidemic waves. The AnH1 genotype was predominant during the first epidemic wave, but was replaced by JS537, JS18828, and AnH1887 genotypes during the second and third epidemic waves (11). Among 166 A (H7N9) virus HA gene sequences from the fifth epidemic, 159 were from the Yangtze River Delta lineage versus 7 from the Pearl River Delta lineage.…”