Experts investigate China's human H5N1 cases
Authors: Margaret Harris Cheng
Journal: The Lancet (2009)

China's human cases of H5N1 avian influenza have concerned experts because few outbreaks of the virus have been reported in domestic bird populations. Margaret Harris Cheng reports. After dead ducks and chickens infected with the H5N1 virus washed-up on a beach in Hong Kong, questions are being asked about why China has accumulated more human cases of avian influenza in the first month of this year than for all of 2008, but has reported few outbreaks of the disease in domestic poultry flocks. China's public-health authorities are confident they are not yet facing a major outbreak but there is something new and worrying about the pattern of human infections. Although most of this year's eight human cases of H5N1 have a history of poultry contact, six of them occurred in provinces that have not reported outbreaks in poultry. “The information in the public domain that I have seen suggests that virtually all of the cases have some known or probable link to poultry, in some cases sick poultry, in others poultry in markets, or in handling carcasses”, said Les Sims, of the Asia Pacific Veterinary Information service. There have been past human cases not obviously linked to poultry outbreaks and the rise in cases maybe a result of better diagnosis and investigation than an actual rise in infection rates, says Nan-Shan Zhong, director of the Guangzhou Institute of Respiratory Diseases. However, the lack of noise in the poultry flocks has got both veterinary and human influenza specialists wondering whether the situation in China is signalling a substantial change. Whether that change is in the virus, how it is spread, how poultry are responding to it, or in human behaviour to bypass public-health measures and avoid a loss of income when flocks are culled, or is simply, as Zhong suggests, better surveillance and testing, is not yet clear. “We are all monitoring the situation and trying to understand the epidemiological pattern”, said Vincent Martin, senior technical adviser for the Food and Agriculture Organization in Beijing. “We all agree there is a disconnect between the human and animal cases. We are seeing very few outbreaks. This is a problem—the animals should serve as a sentinel and raise the alert. But we also know that the virus has been circulating this year for several months and in several provinces.” Sims has warned about the risks of relying on mass culls, particularly where backyard farmers stand to lose their entire livelihood if they report poultry infection. “This is best demonstrated by the occurrence of human (zoonotic) cases of influenza caused by avian H5N1 viruses without detection of poultry cases, indicating a failure of veterinary reporting and surveillance systems”, he said at the World Poultry Conference in 2008. “This may be due to failure by owners of sick and dying birds to report disease, failure to recognise the disease by veterinary authorities, or in some cases deliberate suppression of reports due to the severe consequences associated with reporting.” Hong Kong, devastated economically and socially by the spread of the Severe Acute Respiratory Syndrome in 2002, is taking no chances. In early December, infected birds were detected on a farm in Yuen Long, near the territory's border with mainland China. The flocks were culled, the farm was closed, and the territory's health authorities raised the H5N1 threat level to “serious response”. They also infuriated live poultry retailers and consumers by refusing to lift a tightly enforced cross-border quota of 7000 live mainland chickens a day for the Chinese New Year festival. China began reporting cases of H5N1 in 2003. As of Feb 3, 2009, it has identified 38 people with H5N1, 25 of whom have died. Of the eight cases that have emerged this year (one is classified as a 2008 case because her symptoms date from December, 2008), five have died. So far there is no evidence that any of those with H5N1 have infected any of their contacts. Mass monitoring of contacts—a big job in crowded China—has yielded no further cases. Although the wide geographic spread—human cases have emerged from all parts of China including Beijing and Hunan in the central-north, Xinjiang in the north-west, and Guizhou province in the southwest—supports the official line that they are sporadic and there is no outbreak, the influenza season is only just beginning. In southern China human influenza rates peak between February and June, giving the H5N1 virus plenty of time to mix in human beings.