3.1 Descriptive analysis
In total, 56,379 HFMD cases were reported from 2008 to 2016 in Xinjiang, with a daily average of 17.2 cases and the annual average incidence was 25.27/10,000. Fig. 2 shows the monthly distributions of HFMD cases and meteorological variables in Xinjiang from 2008 to 2016. The monthly HFMD distribution showed a distinct seasonal pattern over the period and HFMD cases typically occurred between May and July, peaking in June. The annual morbidity among males was about 1.5 times higher than that of females. Children under 5 years old were at the highest risk of HFMD. Most cases (86.2%) were dispersed children who did not go to kindergarten or school. A descriptive summary of the meteorological and socioeconomic variables is shown in Table 1.
3.2.1Factor detector analysis
As shown in Table 2, the determinant power of the average relative humidity was obviously associated with the incidence of HFMD (q = 0.30), indicating that the average relative humidity mainly explains the spatial heterogeneity of the incidence of HFMD. Precipitation, barometric pressure, temperature and sunshine duration were also associated with the incidence of HFMD in Xinjiang, having explanatory powers q of 0.29, 0.29, 0.21 and 0.20, respectively. The study reveals that humidity, precipitation and barometric pressure were three dominant factors influencing the transmission of HFMD in the semi-arid regions.
3.2.2Interaction detector:
The study found that the interaction of any two risk factors has greater explanatory power than any single metrological factor. Compared with their individual impact, they most presented the effect of “nonlinear enhance” or “bivariate enhance”. As shown in Table 3, the q statistics of average relative humidity was 0.3, which increased to 0.5 after accounting for the interactive effect of average barometric pressure on the HFMD incidence. As 0.5 is significantly higher than 0.3 (q statistics of average relative humidity) and 0.29 (q statistics of average barometric pressure), the result indicated that relative humidity and barometric pressure has a significantly bivariate enhanced interactive associations on the incidence rate of HFMD. The explanatory power of average relative humidity increased to 0.39 after considering the interactive effect of precipitation on HFMD incidence. The coupled impact of average relative humidity (q=0.3) and average wind speed (q=0.07) played an important role in HFMD, with an explanatory power of 0.43 (Table 3). High average relative humidity and high average wind speed were associated with a high incidence of HFMD. The interaction of these risk factors could effectively explain the spatial heterogeneity of the HFMD, and the selected risk factors had a tendency of strengthening interaction.
3.2.3Risk detector analysis
Figure 6 showed the relative risk (RR) of HFMD with different meteorological factors from 2008 to 2016 in Xinjiang. Actually, the distribution of RR in space was not the same every year, and there were certain changes. Specifically, the spatial RRs in counties in Northern Xinjiang were higher than the counties in Southern Xinjiang, implying that these counties have relatively higher HFMD risk. Conversely, counties in Southern Xinjiang generally have lower RRs. The Northern Xinjiang had a higher average relative humidity, suitable temperature and precipitation level, resulting in a higher RR of HFMD. The southern regions were affected by the Taklimakan desert, high temperature, low relative humidity, precipitation and air pressure, and the risk of HFMD is relatively low. We found that the lowest RR of HFMD in Khotan, during the study period. According to the following meteorological risk factor charts, Urumqi, Tacheng Prefecture, Changji Prefecture and Ili Kazak Autonomous Prefecture are the high RR areas of HFMD in Northern Xinjiang. It may be ascribed to the higher average relative humidity and sufficient precipitation in the above areas. These areas are suitable for the growth and transmission of the HFMD virus.
We found that when the monthly average precipitation exceeded 0.94mm, the incidence of HFMD decreased. There was an inverted V-shape association between temperature and HFMD. A similar pattern was observed for the association between the monthly average relative humidity and HFMD, the monthly average sunshine duration and HFMD, the monthly average wind speed and HFMD. When the monthly average temperature was 8.81 °C, the HFMD reached a peak. The incidence of HFMD increased along with the monthly average relative humidity and the monthly average sunshine duration, reached a peak when the monthly average relative humidity was at 61.1% and the monthly average sunshine duration was at 7.78 hours and then decreased afterwards, respectively. Risk detector value presented a logarithmic relationship between the monthly average evaporation and HFMD, an exponential relationship between the monthly mean air pressure and HFMD. With regards to the association between the monthly average wind speed and HFMD, the incidence of HFMD was the highest when the monthly average wind speed is less than 2.52m/s.
3.3 Spatial autocorrelation of HFMD incidence
Moran’s I value was calculated by global spatial autocorrelation analysis. Moran scatter diagram (Fig.5) shows the results of the global spatial autocorrelation test, demonstrating a highly statistically significant spatial autocorrelation difference of HFMD at the state level in Xinjiang each year from 2008 to 2016. The Moran’s I values (Table 4) ranged from -0.135 to 0.202 (P < 0.05), indicating the spatial dependency on the occurrence of HFMD in 2008, 2010, 2012, 2014 and 2015. Moran’s I value in 2009 was -0.135, indicating that there was a negative spatial autocorrelation of HFMD in Xinjiang. Moran’s I values in 2011 and 2016 were respectively -0.066 and -0.00018, indicating that the incidence of HFMD in Xinjiang presented a random distribution pattern. Bayingolin Mongol Autonomous Prefecture showed the high-high spatial autocorrelation of HFMD incidence, whereas Kashgar, Hotan, Aksu and Kizilsu Kirghiz Autonomous Prefecture showed the low-low spatial autocorrelation of HFMD incidence in 2008 and 2010. From 2011 to 2016, Urumqi always showed the high-high spatial autocorrelation of HFMD incidence.