Although we keep watch on a large number of influenza subtypes in the wild, the H3N8 virus gets extra attention because it is believed to have sparked human pandemic at the turn of the last century, variants of it have become endemic in horses and dogs over the past few decades, and it persists in the wild as an avian virus.
It doesn't preclude seeing our seeing an H7N9 or H5N1 pandemic, but over the past 130 years we've only seen human influenza epidemics caused by H1, H2, and H3 subtypes, leading some scientists to wonder:
While the jury is still out on that, it is worth noting that H3N8 has also been detected in camels and in November of 2011, we saw a die off of seals in New England that was eventually determined to be due to a Mammalian Adapted avian H3N8 virus.
Just last month we saw PLoS One: Evidence of Subtype H3N8 Influenza Virus Infection among Pet Dogs in China, although it wasn't clear whether North American canine H3N8 had been imported into China, or if avian (or possibly equine) H3N8 jumped species there, as we saw with avian H3N2 virus in Korea a decade ago.
Since we don't get a lot of surveillance data out of China, the following report in the Journal of Agricultural Biotechnology where researchers characterized two H3N8 viruses collected from ducks in Guizhou province, is of interest.
First the abstract, then I'll return with a bit more.
Biological Characteristic of Two H3N8 Subtype Avian influenza viruses(AIV)
CUI Peng-Fei;LU Kun-Peng;CHEN Si;XIAO Li;GUAN Li-Zheng;DENG Guo-Hua;CHEN Hua-Lan;
H3N8 influenza viruses have a broad host range, which are commonly found in wild birds and domestic poultry, and have been isolated from some mammals. Moreover, H3N8 influenza viruses have the ability to occasionally infect and transmit among other species.
In recent years, avian influenza surveillance data indicates that the number of isolated H3N8 subtype Avian influenza virus(AIV) has a tendency to increase year by year.
To understand the biological characteristics of the H3N8 AIV, two H3N8 viruses(A/duck/Guizhou/S1092/2013(H3N8)(DK/GZ/S1092/2013) and A/duck/Guizhou/S1145/2013(H3N8)(DK/GZ/S1145/2013)) were isolated from Guizhou province at 2013 for phylogenetic analysis, infectious experiment in mice (Mus musculus) and receptor-binding specificity analysis.
The phylogenetic analysis results indicated that the 2 viruses displayed obviously genetic diversity, and all genome of the viruses had different origins except for hemagglutinin(HA) gene.
The mice study suggested that the 2 viruses could effectively replicate in the lung and nasal turbinate of mice without pre-adaptation. The virus titers of DK/GZ/S1092/2013 in lung and nasal turbinate were 4.00 and 5.50 log10EID50/m L(EID50: Fifty percent embryo infection doses), respectively, and the virus titers of DK/GZ/S1145/2013 in lung and nasal turbinate were 4.25 and 3.58 log10EID50/m L, respectively.
In addition, the 2 H3N8 influenza viruses displayed low virulence to mice, and mice did not display obvious clinical symptoms after infection with the viruses.
In the observation period, DK/GZ/S1092/2013 only caused1.8% bodyweight loss and DK/GZ/S1145/2013 only caused 0.8% bodyweight loss. The receptor- binding specificity analysis demonstrated that the 2 viruses had the ability to bind both sialic acid(SA) α2,3- Gal receptor from poultry and SA α2,6- Gal receptor from human, which indicated that the 2 H3N8 subtype AIV posed potential threat to infect human.
In conclusion, this study systematically researched the biological characteristic of the 2 H3N8 subtype AIV, and the results indicated that the H3N8 influenza viruses posed potential risk to infect mammals. This study plays an important role in the prevention and control of H3N8 subtype AIV.
While both viruses replicated (with only low virulence) in mice, they also showed the ability to bind to both avian (α2,3) and mammalian (α2,6) receptor cells.
This is significant because purely Avian adapted flu viruses bind preferentially to the alpha 2,3 receptor cells found in the gastrointestinal tract of birds.
The ability to bind to human α2-6 receptor cells - which line the human upper respiratory tract - is considered the single biggest obstacle that an avian flu virus must overcome in order to successfully jump to humans.
But it isn’t the only one.
Avian viruses also typically replicate at the higher temperatures found in birds, and would need to adapt to the lower (roughly 33C) temps found in the upper human respiratory tract.
There are undoubtedly other obstacles – some we know about, others we don’t – that must come in proper sync to allow an avian virus to become a `humanized' virus.
Today's report suggests that the avian H3N8 viruses these researchers examined have gained at least one of the traits that could someday make them a potential human health risk.