Where does this virus come from?
The causative agent is a virus of avian origin. It comes from a reassortment – rearrangement of genes – from different donor viruses. Its genome includes elements from three different sources:
- a gene that may come from viruses circulating in domestic ducks in the relevant Chinese provinces;
- a gene that may come from viruses circulating in wild birds or domestic ducks;
- genes from viruses very frequently detected in poultry in the provinces concerned by the human cases.
This combination of genes has never before been reported either in birds or in humans. This is also the first time that a virus of the H7N9 subtype has been reported in humans. Finally, it is also the first time that an infection by an avian influenza virus, lethal in humans, is not virulent for birds.
What is the situation in China and around the world?
After the first three cases reported on March 31 in Shanghai and Anhui Province (west of Shanghai), a total of 132 cases of human infection with the H7N9 virus (including 37 deaths) have been reported as of May 21. . Apart from one case described in Taiwan, but imported from China, all cases have been reported in 8 provinces and 2 cities (Shanghai and Beijing). 80% of cases are concentrated in Shanghai and the two neighboring provinces Zhejiang and Jiangsu (south and north of Shanghai respectively).
Cases are usually isolated and no sustained human-to-human transmission has been observed to date. After peaking in the first half of April, when almost half of the cases were reported, the number of cases has been declining rapidly.
How do humans become infected?
Given the avian origin of the emerging H7N9 virus and despite the absence of disorders observed in birds, investigations were carried out on poultry and wild birds. The geographical origin of the birds in which the virus was detected overlaps with the provinces affected by human cases, with the exception of the province of Guangdong (South China, close to Hong Kong) which has not yet reported no human cases. No positive sample was found on the farm, which suggests that poultry become infected after leaving the farm, during transit through multiple retailers and/or in live poultry markets.
An OIE expert mission carried out in conjunction with the WHO and the FAO in April concludes that live bird markets play a key role in the infection of humans and poultry by the H7N9 virus. The drastic halt in the appearance of new cases after the closure of live poultry markets goes in the same direction.
However, the genetic analysis of the virus reveals that it has markers of adaptation to mammals, which explains the cases observed and raises fears of an evolution of the virus allowing it to be transmitted easily from man to man by air. Another potential source of human contamination is thus still sought.
What is ANSES doing about this virus?
Since the announcement of the first cases, ANSES has carried out, together with the Institute for Public Health Surveillance (InVS), daily international monitoring and established weekly situation updates. In addition, from the first cases, the national reference laboratory for avian influenza (ANSES Ploufragan laboratory) immediately checked the validity of the detection and characterization tools it already had, in consultation with the global network of reference laboratories OIE/FAO, and informed the diagnostic laboratories approved for the detection of avian influenza viruses. The laboratory is ready to contribute to any investigations that may prove necessary.