Mapping spread and risk of avian influenza A (H7N9) in China

Abstract

The outbreak of human infections with an emerging avian influenza A (H7N9) virus occurred in China in early 2013. It remains unknown what and how the underlying risk factors were involved in the bird-to-human cross-species transmission. To illustrate the dynamics of viral spread, we created a thematic map displaying the distribution of affected counties and plotted epidemic curves for the three most affected provinces and the whole country. We then collected data of agro-ecological, environmental and meteorological factors at the county level, and used boosted regression tree (BRT) models to examine the relative contribution of each factor and map the probability of occurrence of human H7N9 infection. We found that live poultry markets, human population density, irrigated croplands, built-up land, relative humidity and temperature significantly contributed to the occurrence of human infection with H7N9 virus. The discriminatory ability of the model was up to 97.4%. A map showing the areas with high risk for human H7N9 infection was created based on the model. These findings could be used to inform targeted surveillance and control efforts in both human and animal populations to reduce the risk of future human infections.

Figure 1. The thematic map of the avian influenza A (H7N9) epidemic in China.

Distribution of cumulative numbers of cases with reported onset dates from February 19 to May 30, 2013 in the eight provinces and two municipalities. Colored gradients reflect the number of cases at county level. An asymptomatic infection reported in Beijing and an imported case reported in Taiwan are also shown on the map. The map was created in ArcGIS 9.2 software (ESRI Inc., Redlands, CA, USA).

Figure 2. Epidemic curves of avian influence A (H7N9) for selected provinces or municipality and the whole country.

The red and black bars respect the daily number of urban and rural cases according to their onset date, respectively. The date of closure of live birds markets is marked by the vertical dashed line. (A) Shanghai City; (B) Jiangsu Province; (C) Zhejiang Province; (D) All other provinces; (E) The whole country.

Figure 3. Relationship between risk factors and avian influenza A (H7N9) risk.

The avian influenza A (H7N9) risk based on the BRT model is plotted as a function of (A) number of live poultry markets (LPM), (B) population density (PD_3), (C) percentage coverage of irrigated croplands (IC), (D) percentage coverage of built-up lands (BU), (E) relatively humidity (RH), and (F) temperature (TP). The curves are average predicted lines for above 6 variables by 50 repeats based on the bootstrapping procedure.

Figure 4. The predicted risk map of occurrence of human H7N9 infection in mainland China and receiver operating characteristic (ROC) curves of the predicted risk.

(A) Predicted risk of occurrence of human H7N9 infection is displayed by different color grades, according to BRT modeling. The map was created in ArcGIS 9.2 software (ESRI Inc., Redlands, CA, USA). (B) ROC curves for BRT models: the grey lines are the ROC curve for each repeat, and the solid, dashed and dotted lines indicate the average ROC curves of 50 repeats based on the bootstrapping procedure for the train set, test set and prediction, respectively.