The Spread of H7N9 in China

Hong Kong’s Centre for Health Protection weekly Avian Influenza Report published on August 1 is the first weekly report in 2017 with no reported human H7N9 cases from the Republic of China. The last weekly Avian Influenza Report with no reported H7N9 cases was previously published on December 1, 2016. In the intervening 8 months more than 750 human H7N9 cases were reported from China. This is an extremely large number of human infections in a short time period.

Human cases of H7N9 were first identified and reported from China late in the 2012-2013 influenza season. Since then, there have been several waves of human H7N9 infections generally corresponding with seasonal influenza outbreaks. Although human cases of H7N9 have been sporadically reported over the past four years, the 750+ cases reported in the last eight months represent about 49% of all reported H7N9 cases. To date, all human infections have occurred in China although some individuals infected in China were identified in other countries. 

   Chart: Number of human H7N9 cases by influenza season.

As depicted in the maps below, between 13 and 16 provinces, autonomous areas, or municipalities were reporting H7N9 cases during these previous waves on a seasonal basis. This recent wave with over 750 cases, the 2016-2017 influenza season, struck China particularly hard. All provinces, autonomous areas, municipalities, or special administrative regions in China with the exception of Hainan, Heilongjiang, Qinghai, and Ningxia Hui reported at least one human H7N9 case during the past influenza season.

Map: Distribution of human H7N9 cases in China in early 2013.

Map: Distribution of human H7N9 cases in China in the 2013-2014 influenza season.

Map: Distribution of human H7N9 cases in China in the 2014-2015 influenza season.

 Map: Distribution of human H7N9 cases in China in the 2015-2016 influenza season.

Map: Distribution of human H7N9 cases in China in the 2016-2017 influenza season.

Almost all of these recent H7N9 infections are attributed to contact with infected poultry; human to human transmission is apparently very rare. There is very little public information about the distribution of low pathogenicity avian influenza (LPAI) H7N9 among domestic poultry in China. However, we can deduce from the extensive geographic distribution of human H7N9 in China this past season, that H7N9 is wide spread and perhaps endemic in poultry flocks in China.

Domestic poultry may soon be infected in adjoining countries of Myanmar, Laos, and Vietnam.  Human cases from these countries in the future would not be surprising. With the next influenza season officially starting in a few weeks, we can anticipate that additional human H7N9 cases will be reported from China in the upcoming influenza season.