While H5N1 pretty much hogged the avian flu limelight in the ten years from 2003 until the spring of 2013, the past 18 months have seen a sharp increase in the number of emerging novel avian flu subtypes that have infected humans and/or poultry.
Since then we’ve seen an Expanding Array Of Novel Flu Strains, including the H7N9 epidemic in China, the 1st Known Human Infection With H5N6, Taiwan reported Infection With Avian H6N1, and last winter no fewert han three cases of H10N8 infection were reported in China.
Thus far, out of this new group of viruses, only H7N9 has shown genuine signs of pandemic potential. But as we discussed last month (see EID Journal: Mutations Of A(H10N8) Virus in Chicken Eggs and MDCK Cells) there are hints that this new virus `might be undergoing rapid adaptation to mammals and developing antiviral drug resistance’.
Admittedly, last May we saw reassuring media reports suggesting that the H10N8 virus – while worthy of watching – wasn’t `currently’ considered a big pandemic threat (see H10N8 bird flu unlikely to threaten public health), based on receptor binding testing done by the MRC National Institute for Medical Research (MRC-NIMR).
But the only real constant with influenza viruses is that they are constantly changing. What can be said about a virus today may not hold true tomorrow, or a year from now.
Over the past year we’ve become increasingly aware of two factors that may help drive the evolution of novel avian flu strains.
- The first being the ongoing genetic reassortments with the ubiquitous and relatively stable LPAI H9N2 virus, which provided its internal genes to the H5N1, H7N9, and H10N8 viruses (see The Lancet: H9N2’s Role In Evolution Of Novel Avian Influenzas) and continues to aid and abet the creation of new clades.
- The second piece of the puzzle involves the genetic mixing that goes on in LPMs (Live Poultry Markets) – combined with enhanced human-poultry interaction – that appears to be exacerbating both the evolution of avian viruses, and their jumping to humans.
Last month, in CDC: Risk Factors Involved With H7N9 Infection, we looked at a case-control study done on the H7N9 epidemic in China that pretty much nailed LPMs as the predominant risk factor for infection.
While even casual exposure to poultry in live bird markets was cited as the primary risk factor, people who owned, raised, or slaughtered birds at home, on farms, or in the wild were not found to be at any increased risk.
Tao Zhang, Yuhai Bi, Huaiyu Tian, Xiaowen Li, Di Liu, Ying Wu, Tao Jin, Yong Wang, Quanjiao Chen, Ze Chen, Jianyu Chang, George F. Gao, and Bing Xu
Human infection with avian influenza virus A(H10N8) was initially reported in China in December 2013. We characterized H10N8 strains from a human patient and from poultry in live markets that infected persons had visited. Results of genome sequencing and virus characterization suggest that the virus strains that infected humans originated from these markets.
Avian influenza virus (AIV) is classified into 16 subtypes on the basis of hemagglutinin (HA) and 9 subtypes on the basis of neuraminidase (NA); additional bat-derived influenza-like genomes, H17N10 and H18N11, have recently been reported (1). Birds can be infected with AIV through direct contact with infected hosts or through contact with contaminated surfaces or materials, including water and food. In China, H10N8 virus was isolated from the environment of Dongting Lake in Hunan Province in 2007 (2) and from a duck in a live poultry market (LPM) in Guangdong Province in 2012 (3). This AIV was not then known to directly infect humans or other mammals.
In December 2013, H10N8 virus infection in a person was reported in Nanchang, Jiangxi Province, China (4); 2 more human cases followed. The initial reported case was in a 73-year-old woman who visited a local LPM 4 days before the onset of her illness (4). Because genetic information on AIV is essential for understanding of the biology of these viruses, their spread among avian species, and their potential transmission to humans, in January 2014, we conducted surveillance of several LPMs in Nanchang, including those visited by the 3 reported case-patients, to determine the source of these infections.
Our results provide evidence that the novel avian influenza virus A(H10N8) that infected humans in Nanchang, Jiangxi Province, China, could have derived from strains circulating in LPMs. In the LPMs, the sale of freshly slaughtered poultry, live poultry transportation, and mixed trading of different domestic animals provide environments conducive to genome segment reassortment, gene mutation, and interspecies transmission of AIVs (8,9). Human-infecting H7N9 virus strains are believed to be directly related to those found in the live poultry traded in LPMs (10,11); closure of LPMs has been shown to partly control the spread of these infections (8). Moreover, serologic evidence recently confirmed the infection of dogs with an H10 subtype influenza virus in close proximity to LPMs in Guangdong Province (12). Other recent research has shown that the internal genes of the H5N1, H7N9, and H10N8 viruses are constantly reacquired from poultry H9N2 viruses (9,13,14). Taken together, these data suggest that LPMs act as gene sources, facilitating reassortment of AIV genome segments (15).
In summary, exposure to infected and/or virus-carrying poultry or to contaminated environments in LPMs and the emergence of mammal-adapted and drug-resistant viruses puts humans at high risk for infection with novel influenza viruses. Measures to improve poultry farming practices must be enforced, including strict biosecurity measures for the trade and transport of live birds, proper disposal of diseased and dead birds, and even closure of LPMs.
While the jury is still out as to whether an H5, H7, or H10 avian flu virus could actually adapt to humans well enough to spark a pandemic (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?) – given the likely lack of immunity mankind has to these avian strains - few scientists are willing to ignore the possibility.
All of which makes enhanced surveillance and biosecurity measures (such as LPM closures) in influenza hotspots like Eastern China all the more important, particularly considering the ease with which viruses can now spread globally via international travel and trade.