The Pandemic Threat of Emerging H5 and H7 Avian Influenza Viruses.

Troy Sutton

doi:10.3390/v10090461

Abstract

The 1918 H1N1 Spanish Influenza pandemic was the most severe pandemic in modern history. Unlike more recent pandemics, most of the 1918 H1N1 virus’ genome was derived directly from an avian influenza virus. Recent avian-origin H5 A/goose/Guangdong/1/1996 (GsGd) and Asian H7N9 viruses have caused several hundred human infections with high mortality rates. While these viruses have not spread beyond infected individuals, if they evolve the ability to transmit efficiently from person-to-person, specifically via the airborne route, they will initiate a pandemic. Therefore, this review examines H5 GsGd and Asian H7N9 viruses that have caused recent zoonotic infections with a focus on viral properties that support airborne transmission. Several GsGd H5 and Asian H7N9 viruses display molecular changes that potentiate transmission and/or exhibit ability for limited transmission between ferrets. However, the hemagglutinin of these viruses is unstable; this likely represents the most significant obstacle to the emergence of a virus capable of efficient airborne transmission. Given the global disease burden of an influenza pandemic, continued surveillance and pandemic preparedness efforts against H5 GsGd and Asian lineage H7N9 viruses are warranted.

Highlights

Recent analysis using a host-specific molecular clock to evaluate the evolution of the 1918 H1N1 virus strongly indicates that the polymerase basic 2 (PB2), polymerase basic 1 (PB1), polymerase acidic (PA), nucleoprotein (NP), neuraminidase (NA), matrix (M), and non-structural (NS) gene segments were derived directly from an avian host, while the H1 hemagglutinin (HA) may have been contributed via reassortment with a human virus that emerged between 1915 and 1917 [10,11]
As all subsequent pandemic influenza viruses have contained gene segments derived from the 1918 H1N1 virus, this virus has earned the designation as the “mother”
While the number of zoonotic infections with these viruses has decreased in the past year, H5 viruses remain endemic in poultry in the Middle East, and H7N9 viruses continue to circulate and cause poultry outbreaks in China

Summary

Avian Origin of 1918 H1N1 “Spanish Influenza” Pandemic

This year marks 100 years since the 1918 H1N1 “Spanish Influenza” pandemic. This was the most severe pandemic in modern history and is estimated to have infected 500 million people worldwide (one-third of the global population) [1], resulting in 50–100 million deaths [2,3]. 1918 H1N1 viral RNA sequences indicated that the virus crossed directly from birds into humans [4,5,6]. This analysis was controversial due to limited contemporary avian influenza gene sequences and conflicting phylogenic analyses [7,8,9]. In 1977, an H1N1 virus again initiated a pandemic This H1N1 virus originated in China or Russia and shared high sequence identity with the H1N1 viruses that circulated prior to 1957 [18,19,20]. There is significant concern that viruses from these subtypes could evolve and initiate the pandemic

Properties of Pandemic Influenza Viruses

Pathogenicity and Lineages of Avian H5 and H7 Avian Influenza Viruses

Asian Lineage Low Pathogenic H7N9 Influenza Viruses

Asian Lineage Highly Pathogenic H7N9 Influenza Viruses

Findings

Conclusions