Up until January of this year, and for the first four years of its existence, H7N9 has been exclusively a low pathogenic virus (in birds). Unlike its HPAI H5 cousins, which are known to cause high mortality in poultry and some types of wild birds, LPAI H7N9 had produced few if any symptoms in poultry.
It was, however, extremely pathogenic in humans, and its ability to spread silently in poultry and other birds made it very difficult to track, with our first clue of an outbreak often coming only after humans in contact with birds fell ill.H5 and H7 LPAI viruses have the ability to mutate into HPAI viruses, and that is precisely what happened in January of this year in southern Guangdong Province, where at least three human infections, and several poultry outbreaks were caused by a newly emerged HPAI version of H7N9.
The big questions at the time were:
- Just how `biologically fit' was this new HPAI variant?
- Would this virus spread via migratory birds beyond Guangdong Province?
- How does it compare in human infectivity with the LPAI version?
After China announced two human infections with HPAI H7N9, and Taiwan announced one (imported) case, last January - we've heard of no additional human HPAI infections.
Either there haven't been any, or China simply isn't differentiating between LPAI and HPAI human infections in their reporting.We have, however, continued to see reports of heavy poultry losses due to H7N9 - first in neighboring Hunan Province, then further north in April and May - a pretty good sign that the HPAI version is spreading.
Some of these outbreaks have been confirmed as HPAI H7N9, while we await official word on others (see CHINA: HPAI H7N9 Expands Its Range).As far as the biological `fitness' of the virus, three weeks ago in Eurosurveillance: Biological Characterisation Of (HPAI) A(H7N9) Viruses In Humans, we learned that HPAI H7N9 virus retains the ability to infect both birds and humans, and in fact, appears to be potentially slightly more infectious in both than the LPAI version. The authors write:
Our data show that the HPAI H7N9 viruses retained dual receptor binding properties, with slightly increased binding preference for both receptors compared with LPAI H7N9 (AnH1) viruses. . . . . The persisting preference for both avian- and human-type receptors of HPAI H7N9 viruses may result in their circulation in poultry and possible transmission among humans.
They also found that his HPAI H7N9 virus can quickly mutate into an antiviral resistant virus early during treatment. This HPAI virus is also antigenically different from the older LPAI H7N9 virus, and so existing vaccines are unlikely to be effective.
On paper, at least, HPAI H7N9 looks very fit.It is not alone, of course. It circulates concurrently across China with two distinct LPAI H7N9 lineages (Pearl River Delta and Yangtze River Delta), encompassing dozens of genotypes and variants, all of which are looking for an evolutionary advantage.
And new genotypes and variants are formed practically on a daily bases. Some will be evolutionary failures, while others may prove competitive enough to thrive. The point being, the H7N9 threat is constantly evolving.
All of which brings us to one of many reports in the Chinese media this weekend of a very large and sudden die off in poultry in northern Shaanxi Province. Reportedly 22,000 birds died over a matter of a few days, and while test results are pending, H7N9 is suspected.
This (translated) report from Hong Kong's Oriental Daily.
Shaanxi farm suspected H7 bird flu burst
WASHINGTON roundup both sides re-transmission of bird flu, Yuyang District, Yulin City, Shaanxi has a chicken farm suspected outbreak of H7 avian influenza, caused by a large number of chickens died.
Network file transfer Yulin City Animal Husbandry and Veterinary Bureau issued, referring to the local chicken farm located in the Green Cheung ecological cattle home town of the beam, since there are chickens on Sunday onset; as of last Thursday, a total of forty-five thousand chicken farm , there are more than twenty-two thousand chickens died; another report showed that chickens suspected of being infected H7 virus, authorities have sent samples of animal disease prevention and control center of the provincial tests and is awaiting test results.
Assuming this report - and last week's report from Tianjing - are confirmed as HPAI H7N9 (other HPAI viruses, including H5N6 and H5N8 are also possible), we'll have additional evidence that this HPAI virus is moving steadily north - likely carried by migratory birds.
Already, parts of Eastern Russia are on alert for the possible arrival this summer of H7N9 from China (see here), and other countries, from Vietnam to the south to Mongolia to the north are on watch as well.While the good news remains that H7N9 (LPAI or HPAI) has yet to achieve the ability to spread easily from human-to-human, the virus continues to evolve, leaving many of the world's best bird flu experts seriously concerned (see NPR: A Pessimistic Guan Yi On H7N9's Evolution).