Scientists who closely analyzed and compared virus samples from several poultry species in China found more clues about how the new H7N9 virus, along with a previously unknown H7N7 virus that has the capacity to infect mammals, evolved.
The findings shed new light on the threat H7 viruses might pose beyond China's latest outbreak, according to the findings reported today by an international research team led by Yi Guan, MD, PhD, a virologist at Hong Kong University. The group's report appears in the latest issue of Nature.
Soon after news of the first human infections with the new H7N9 virus emerged last spring, investigators started sampling birds in three Chinese cities: two (Wenzhou and Rizhao) that bordered the main outbreak area in Zhejiang and Shandong provinces, respectively, and one (Shenzhen) in Guangdong province, which had not reported a human case.
They collected 1,341 pairs of oropharyngeal and cloacal samples from chickens, ducks, geese, pigeons, partridges, and quail. The team collected 1,006 more fecal and water samples from live poultry markets, farms, and wetlands.
Evidence of hemagglutinin was found in 388 samples, including 60 H7 and 85 H9 influenza A viruses. All H9 isolates were H9N2, and most were from live poultry markets.
The H7 viruses were found only in the samples from live poultry markets in Wenzhou and Rizhao; all from Rizhao were H7N9 and all from Wenzhou were H7N7, except for two from ducks that were H7N2 and H7N3.
In Wenzhou's poultry markets, H7 viruses were most prevalent in chickens, followed by ducks and pigeons. In Rizhao's poultry markets, the H7N9 virus was found only in chickens.
When researchers looked at the chicken isolates, most of the H7N9 and H9N2 viruses were from oropharyngeal swabs, as were more than half (65%) of H7N7 viruses, which hints that H7N9 and H7N7 might replicate in poultry upper airways, similar to H9N2 viruses that circulate in the region's poultry.
Evolution, clues to infectious potential
To explore how those viruses evolved, the researchers sequenced the H7N9, H7N7, and H9N2 isolates and compared them with other strains that had been collected in southern China since 2000.
The analysis revealed that H7 viruses likely passed from domestic ducks to chickens in China at least two separate times. Also, the group found that the H7 viruses reassorted with H9N2 viruses circulating in poultry to produce the H7N9 outbreak strain along with the related and previously undetected H7N7 strain.
An analysis of the new H7N7 virus found that it has only some of the molecular markers seen in human H7N9 isolates, but the group thought it might still have the ability to infect humans or mammals.
Experiments in which ferrets were exposed to H7N7 showed that the virus caused significant infection, but virus shedding was lower than seen for the 2009 H1N1 virus or H7N9.
The group concluded that domestic ducks seem to be a key mixing vessel for a variety of avian influenza viruses from migratory birds and can transmit the different combinations to chickens. They further concluded that reassortment with H9N2 circulating in poultry probably led to the H7N9 and H7N7 viruses they found in chickens.
The new H7N9 virus rapidly spread though poultry markets, leading to spillover into humans. The group wrote that the drop in human cases after poultry market closures, similar to what occurred in Hong Kong in 1997 after H5N1 emerged, supports the theory.
Their detection in Wenzhou chickens of the new H7N7 virus with the potential to infect mammals suggests that the current pandemic threat extends beyond the H7N9 virus, the team wrote.
It's too soon to know if H7N9 has been eradicated from poultry in the region, and it's possible that the H7N9 and H7N7 are still circulating in poultry, according to the investigators. They state that controlling the viruses will hinge on the management of live poultry markets in urban areas.
Expert weighs study implications
Les Sims, BVSc, an animal health consultant in Australia who has extensive experience with avian influenza in Asia, told CIDRAP News that overall, the study is a good summary of the state of play on the probable origins of the genes in the recently emerged H7N9 viruses. The extent of the H7N7 risk to humans is unknown, but it's important to identify where the virus is circulating, he said.
Animal health experts have known for some time that H7 viruses are present in both wild and domestic ducks, Sims said. "The production and marketing systems in China prior to the H7N9 outbreak afforded ample opportunities for these viruses to infect terrestrial poultry and undergo reassortment with H9N2 viruses, which are also widespread in farms and markets.
"So it is not surprising that the viruses in terrestrial poultry are related to and probably derived from those ducks or that there has been more than one introduction," he said.
The findings in Rizhao in Shandong province were more than 100 kilometers from the nearest human and avian cases in the province, which may mean the virus was more widespread in poultry than official surveillance and disease investigations suggest, he said.
The absence of the virus from Shenzhen tracks with test results from Hong Kong on mainland and local poultry, Sims indicated, but the virus must have been circulating recently in southern Guangdong province, based on the announcement of a human case in a poultry market worker earlier this month.
The prevalence of H7N9 in the Rizhao samples was low at 0.7% of samples, Sims noted, adding that surveillance activities in markets can miss positive birds if only a small number of samples are collected—"which was not the case in this study."
Meanwhile, the high prevalence of H7N7 in the Wenzhou market and the fact that it wasn't detected anywhere else is intriguing, according to Sims. If the virus is circulating widely in terrestrial poultry, it will complicate H7 serology to some extent,he said. "However as all H7 viruses fall under the category of notifiable avian influenza, it will be necessary to investigate any positive results through additional virological and serological testing of the positive flocks and other flocks on the same farm."
Lam T T-Y, Wang J, Shen Y, et al. The genesis and source of the H7N9 influenza virus causing human infections in China. Nature 2013 (published online Aug 21) [Abstract]
Aug 21 NIH press release