WASHINGTON, July 18 (Xinhua) -- The H7N9 bird flu virus may be highly transmissible among ferrets, a common animal model for studying how flu might spread in humans, Chinese researchers reported Thursday.
Though H7N9 appears to have been brought under control, the researchers warned in a study published online in the U.S. journal Science that the character of the virus, including its pandemic potential, "remains largely unknown" and that it's possible the virus can efficiently spread between humans eventually.
"We must have material and technical reserves, including policies and measures for possible reemergence of the H7N9 virus in the future, otherwise the virus could hit the world hard," Chen Hualan, director of China's National Avian Influenza Reference Laboratory at Harbin Veterinary Research Institute, who led the study, told Xinhua by phone.
To investigate the possible origins of the H7N9 viruses that caused human infections, Chen's team collected more than 10,000 samples from poultry markets, poultry farms, wild bird habitats, and poultry and swine slaughterhouses across China from March to May this year.
All samples were inoculated individually into 10-day-old embryonated chicken eggs for virus isolation, and 238 influenza viruses were isolated from these samples, of which 52 viruses were confirmed as the H7N9 subtype, Chen said.
To understand the genetic relationship of these viruses, the researchers then sequenced the genome of 37 representative H7N9 viruses that had been isolated from various bird species, the majority of which originated in live poultry markets, and compared them with those of the five human isolates that have been reported.
According to the researchers, the viruses were genetically closely related and bound to human airway receptors, and some also maintained the ability to bind to avian airway receptors.
They also found all of the H7N9 strains isolated from birds replicated easily in chickens, ducks and mice without causing any disease but human strains caused mice to shed up to 30 percent of their body weight.
Most importantly, one virus isolated from humans is able to transmit efficiently between ferrets by respiratory droplet, raising the possibility of eventual airborne transmission between humans.
"We found great similarities among these human and avian strains, except for dozens of amino acid differences, but surprisingly they showed large differences in biological characteristics," Chen said.
"The findings suggest that only a few amino acid changes would be needed to make the avian H7N9 viruses highly transmissible," she said.
The researchers also warned that its nonpathogenic nature in poultry enables the H7N9 virus to "replicate silently" in avian species and to transmit to humans, providing further opportunities for the virus to acquire more mutations and become more virulent and transmissible in the human population.
The H7N9 virus has caused 132 human infections with 43 deaths in China since February. Implementation of compulsory control measures in H7N9 virus-positive live poultry markets seems to have successfully prevented further human infections.
Chen however warned that the virus still exists in animals and continues to evolve and the elimination of the virus from nature is a huge and long-term challenge.