Epidemiological update of 18 April: avian influenza A(H7N9) virus in China

Since 31 March 2013, eighty-seven (87) cases of human infection with influenza A(H7N9) have been reported from six provinces in eastern China with a combined population of about 330 million.

Onset of disease has been between 19 February and 14 April 2013 in: Shanghai (32), Jiangsu (21), Zhejiang (27), Anhui (3), Henan (3) and Beijing (1). The date of disease onset is currently unknown for sixteen patients. Most cases have developed severe respiratory disease. Seventeen patients died (case-fatality ratio=20%). The median age is 64 years with a range between 4 and 89 years; 25 out of 82 patients are female.

The Chinese health authorities are responding to this public health event by enhanced surveillance, epidemiological and laboratory investigation and contact tracing. The animal health sector has intensified investigations into the possible sources and reservoirs of the virus. The authorities reported to the World Organisation for Animal Health (OIE) that avian influenza A(H7N9) was detected in samples from pigeons, chickens and ducks, and in environmental samples from live bird markets ('wet markets') in Shanghai, Jiangsu, Anhui and Zhejiang provinces. Authorities have closed markets and culled poultry in affected areas.

An official statement from the Chinese Ministry of Agriculture reports that 47 801 tests samples have been taken from live animal markets, farms and slaughter houses. Of those, only 39 samples were positive for the H7N9 avian influenza virus, of which 38 were from Shanghai, Anhui, Zhejiang and Jiangsu provinces. One positive sample was from a wild pigeon in Jiangsu.

The source and mode of transmission have not yet been confirmed. The outbreak is caused by a reassortant avian influenza virus with low pathogenicity for birds, hence it does not cause the signal 'die-offs' in poultry associated with highly pathogenic strains of avian influenza viruses. Genetic analyses of the isolates have shown changes which suggest that the H7N9 virus may have greater ability to infect mammalian species, including humans, than most other avian influenza viruses.

Pathogenicity for humans appears to be high and higher age appears to be a risk factor for disease.

The most likely scenario is that the influenza A(H7N9) virus is spreading undetected in poultry populations and occasionally infecting humans who have close contact with poultry or poultry products but this will have to be validated as further data become available.

At the present time there is no evidence of sustained human-to-human transmission. More than 1000 close contacts of confirmed cases are reported to have been followed up. There is one family cluster with two confirmed cases for which human-to-human transmission cannot be ruled out but where common exposure is the most likely explanation. In addition, the virus has been detected in one asymptomatic 4 year old boy during surveillance of risk groups. The father of this boy is reported to have purchased poultry from the father of the first case in Beijing.

The rapid geographic spread and the increase of confirmed cases is likely to be the result of strengthened case finding and increased testing. Influenza A(H7N9) test kits have been distributed to over 400 laboratories across China and this increased ascertainment is expected to provide important epidemiological information.

An increasing incidence of sporadic cases and expansion of geographic spread in China and possibly neighbouring countries are expected over the coming weeks. Individual imported human cases to Europe cannot be ruled out and countries need to prepare for detecting and diagnosing such cases. Critical developments that would change this assessment would be evidence of sustained human-to-human transmission and detection of avian influenza A(H7N9) in bird populations in Europe.

ECDC is closely monitoring developments and is continuously re-assessing the situation in collaboration with WHO, US CDC, China CDC and other partners.

This epidemiological update does not change the conclusions and recommendations of the updated risk assessment published on April 12.

Figure 1: Distribution of influenza A(H7N9) cases by province, China, as of 18 April 2013

Figure 2: Distribution of influenza A(H7N9) cases by week of onset of symptoms, China, as of 18 April 2013

*Date of onset is currently unknown for sixteen patients.

Figure 3: Distribution of cumulative number of influenza A(H7N9) cases by province, China, 19 February – 18 April 2013

Related links:

• ECDC Rapid Risk Assessment on avian influenza A(H7N9) virus in China, 12 April 2013
• More on avian influenza on ECDC website