The Threat of avian influenza A H7N9

Abstract

A novel low-pathogenic avian influenza (LPAI) virus A (H7N9) causing zoonotic disease was detected in Shanghai in 2013. Since then, the virus has gradually spread to poultry in 31 municipalities, provinces and autonomous regions in mainland China with travel-associated cases reported from Hong Kong SAR, Macau SAR, Taiwan, Malaysia and Canada. The LPAI virus caused no symptoms of illness in poultry making it impossible to detect unless virological testing was carried out; thus human cases have often being the first clue to the presence of the virus. So far, 1622 human cases have been reported, leading to 619 deaths. The fifth wave of the outbreak in the winter of 2016 was the largest so far. The virus has still not adapted to efficient transmission between humans but risk assessment of current zoonotic viruses has identified H7N9 to be the most concerning pandemic threat. In 2017, one lineage of the virus has undergone mutation in the virus haemagglutinin leading to the emergence of a variant that is highly pathogenic (HPAI) in chickens. So far, 25 human cases with the HPAI variant have been detected; it is unclear if this is associated with increased pathogenicity for humans. However, emergence of resistance to oseltamivir has been reported in a number of these patients, posing a dilemma of how best to treat these patients. The Ministry of Agriculture has recently commenced vaccinating poultry with a H5/H7 bivalent vaccine and it remains to be seen what impact this intervention may have on the zoonotic wave this winter. In Hong Kong, interventions in live poultry markets such as “rest days,” where the market is empty of poultry, and banning of holding live poultry overnight within the live poultry market, have been effective at minimizing virus amplification and persistence, thus reducing zoonotic risk.