Identification of climate factors related to human infection with avian influenza A H7N9 and H5N1 viruses in China

Abstract

Human influenza infections display a strongly seasonal pattern. However, whether H7N9 and H5N1 infections correlate with climate factors has not been examined. Here, we analyzed 350 cases of H7N9 infection and 47 cases of H5N1 infection. The spatial characteristics of these cases revealed that H5N1 infections mainly occurred in the South, Middle, and Northwest of China, while the occurrence of H7N9 was concentrated in coastal areas of East and South of China. Aside from spatial-temporal characteristics, the most adaptive meteorological conditions for the occurrence of human infections by these two viral subtypes were different. We found that H7N9 infections correlate with climate factors, especially temperature (TEM) and relative humidity (RHU), while H5N1 infections correlate with TEM and atmospheric pressure (PRS). Hence, we propose a risky window (TEM 4-14 °C and RHU 65-95%) for H7N9 infection and (TEM 2-22 °C and PRS 980-1025 kPa) for H5N1 infection. Our results represent the first step in determining the effects of climate factors on two different virus infections in China and provide warning guidelines for the future when provinces fall into the risky windows. These findings revealed integrated predictive meteorological factors rooted in statistic data that enable the establishment of preventive actions and precautionary measures against future outbreaks.

Figure 1. Temperature-Humidity distribution of H5N1 and H7N9 influenza infection.

(A) Medial temperature (TEM) of H5N1 and H7N9 influenza infection, (B) medial humidity (RHU) of H5N1 and H7N9 influenza infection, (C) atmosphere pressure (PRS) of H5N1 and H7N9 influenza infection, (D) medial ground surface temperature (GST) of H5N1 and H7N9 infection, and (E) medial wind speed (WIN) of H5N1 and H7N9 infection. The vertical axis represents the number of cases.

Figure 2

PCA and risk windows of H7N9 (A) and H5N1 (C) infection. PCA of the H7N9 infections with the climatic parameters. The first three principle components are used as axes. The green dots indicate cases in North China, while the blue and red dots indicate cases in Central and South China, respectively. Heat map of human infections against temperature and RHU for H7N9 infection (B) and TEM and PRS for H5N1 infection (D). Each cell represents the percentage of human infections. The green box represents a high-risk window, and the blue box represents a moderate-risk window.

Figure 3

Predicted risk windows for possible human H7N9 (A) and H5N1 (B) infections in China. Predicted periods of climate conditions that are conducive for the spread of H7N9 based on the temperature and relative humidity ranges in North (green), Central (blue), and South (red) China. The indicated climate high- and moderate-risk windows indicate periods where vigilance for the control of H7N9 or H5N1 infection should be increased. Adobe Illustrator software was used for map depiction.