While we keep a watchful eye on a number of avian H5 and H7 viruses around the globe, the progression of human influenza pandemics over the past 130 years is believed to have been H2, H3, H1, H2, H3, H1, H1 . . . .
The oldest pandemic we can be certain of was the 1918 `Spanish Flu’, which killed anywhere between 40 million and 100 million people, and was caused by an H1N1 virus, followed by H2N2 in 1957, H3N2 in 1968, and H1N1 in 2009.
But before that, the 1890-93 `Russian flu’ pandemic has generally been attributed to the H2N2 virus, while the H3N8 virus has been tentatively pegged as causing the 1900 epidemic (see Transmissibility and geographic spread of the 1889 influenza pandemic).
Given this repeating pattern (and with H1N1 and and H3N2 monopolizing the global stage for the past four decades) it wouldn't be a complete shock were an H2 virus to reemerge after an absence of nearly 50 years (circa 1968).
In 2007 the flu world was abuzz (see The Reassortment Tango) over the discovery of a mammalian-adapted H2N3 virus, isolated in swine at two separate Missouri farms in 2006.
Previously, the H2N3 virus had only been known to infect birds. This was the first detection of an H2 influenza virus in a mammalian host since 1968, when the human H2N2 virus was supplanted by the the H3N2 pandemic virus.
In 2012, a PLoS One article (Recently Emerged Swine Influenza A Virus (H2N3) Causes Severe Pneumonia in Cynomolgus Macaques) found this swine H2N3 produced greater pathogenicity in a non-human primate than did the human H2N2 virus.
This is important as those born after 1968 are likely to have little immunity against an emerging H2 virus.
Also in 2012, in H2N2: What Went Around, Could Come Around Again, we looked at a study conducted by scientists at St. Jude Children's Research Hospital who surveyed H2 viruses circulating in the wild, and concluded that this virus could well pose a threat to humanity again.
Two years ago, the journal Biomed Environ Sci. carried a report on `A novel reassortant H2N3 influenza virus isolated from China’ from a live poultry market in Guangdong province in 2009, demonstrating the H2 viruses are alive and reassorting well with a variety of influenza viruses in the wild.
The genes of this virus belong to Eurasian-lineage avian viruses. The virus is a reassortant with the HA gene from an H2N2 virus and the NA gene from an H5N3 virus. The PB1, PB2, and NP genes were from an H4N6 virus, the PA was from an H3N8 virus, the M gene was from an H1N3 virus, and the NS gene was from an H10N6 virus.
A novel avian-origin reassortant H2N3 influenza virus was detected in a live poultry market. Its potential impacts and evolution should be closely monitored.
The threat posed by a returning H2 virus is taken seriously enough that in 2011, in Nature: A Preemptive H2N2 Vaccine Strike?, we looked at a bold proposal by Gary J. Nabel, Chih-Jen Wei & Julie E. Ledgerwood to Vaccinate for the next H2N2 pandemic now.
While that proposal failed to garner broad support, we continue to monitor H2 viruses, and their evolution, around the globe.
Today we have a new report - this time of a reassorted H2N8 virus - isolated from a duck from Zhejiang Province, China.
While not as concerning as it would be had this been detected in pigs, Eastern China is viewed as a prime location for the creation of new flu viruses (see EID Journal: Predicting Hotspots for Influenza Virus Reassortment), and so we watch viral evolution there closely.
Virus Genes. 2016 Jul 5. [Epub ahead of print]
Genetic and molecular characterization of a novel reassortant H2N8 subtype avian influenza virus isolated from a domestic duck in Zhejiang Province in China.
Wu H1, Peng X1, Peng X1, Cheng L1, Wu N2.
The circulation of the H2 subtype influenza viruses in domestic animals increases the risk of human exposure to these viruses.
An H2N8 avian influenza virus (AIV) was isolated from a domestic duck during AIV surveillance of poultry in live poultry markets (LPMs) in Zhejiang Province, Eastern China, in 2013.
The phylogenetic trees suggested that this strain is a novel reassortant virus derived from multiple AIV subtypes from aquatic birds and poultry in Eastern Asia.
Although this reassortant strain exhibited low pathogenicity in mice, it was able to replicate in the lungs of the mice without prior adaptation. Continued surveillance of domestic ducks in LPMs is required for early detection of AIV outbreaks in poultry and humans.