The novel H7N9 bird flu virus may be human transmissible through direct contact as well as through airborne exposure, according to a new study.
Scientists came up with this conclusion after discovering that the virus, which has already killed 36 people in China, can spread between ferrets.
However, H7N9 has not yet shown compellingly that it is human transmissible, the experts, from China, Canada and the U.S., pointed out in the journal Science.
A comprehensive genetic analysis of H7N9 was recently carried out by scientists in China, revealing the origin and evolutionary history of the virus.
For the study, six ferrets and four pigs were inoculated with H7N9 isolated from a deadly human case in Shanghai. The virus infected all of the animals, according to the researchers.
This means that the H7N9 virus can infect three mammalian species, including humans. In order to get a better idea of the host range of the virus, studies should be conducted on other species, including pet animals like cats and dogs.
The H7N9 virus can infect mammals, including people, easier than the associated H5N1 avian flu because of certain mutations that it carries.
Although there have not been any new cases since May 7th, scientists believe that this unaccountable lull will pass.
The authors said:
Infections in other mammal species could provide the virus with opportunities to mutate and to adapt further. Pigs can be co-infected with avian and human flu, allowing the viruses to swap genes to create new strains, although extensive sampling in China has found no pigs harbouring H7N9."
H7N9 spreads through direct contact & airborne exposureSix ferrets infected with the virus were divided into pairs and each pair was placed in a cage with one uninfected ferret so that the experts could determine whether H7N9 could be spread by direct contact. All of the uninfected ferrets became infected with the H7N9 bird flu virus strain. In order to determine whether the virus could be passed through airborne exposure, three other flu-free ferrets were put in cages 10 centimeters away from the infected animals.
The virus infected one of these ferrets, while another had some H7N9 antibodies - an indication of exposure.
The researchers explained:
"The relative ease of spread between ferrets is in sharp contrast to what has been observed so far in the human outbreaks that were first reported in March. It is possible that person-to-person spread occurred in three family clusters of H7N9 cases, but so far there is no firm evidence of this."
Ferrets are an "imperfect model" for flu virulence in peopleAll flu viruses that can easily transmit between people can also spread through the air between ferrets, according to flu virologist Malik Peiris, from the University of Hong Kong and a co-author of the report.
Flu infections that do not easily transmit between humans, like the H5N1 virus, do not spread through the air between ferrets.
The new study indicates, Peiris said, "that this virus is closer to acquiring human-to-human transmission than other pandemic candidates out there, but not as transmissible as true seasonal [flu] viruses or pandemic 2009 H1N1".
Compared to the serious disease experienced by humans, the ferrets' flu symptoms were restricted to:
- runny noses
- mild lethargy
"It's self-evident that the best study of human disease is the study of humans with the disease," Peiris added. However, the reports on ferrets do complement the studies on humans.
Peiris pointed out that autopsy data from H7N9 cases in humans are insufficient and usually reveal late-stage infections, and therefore, the data do not cover the early stages of the disease.
Data from infected people, from cultured human cells, and from animal studies "all provide important and different dimensions of the questions we want to address," said Peiris.
However, all of the answers will not be found in just one approach, he concluded.
A recent report revealed that a vaccine that provides wider protection against multiple strains of the bird flu virus, H7N9, is currently being worked on.
Written by Sarah Glynn