Using the time series data of HFMD incidence in 31 provinces in China from 2008 to 2017, this study confirmed the correlation between temperature and hand, foot and mouth disease incidence, which is consistent with the previous research results (Xu et al., 2020; Pearson et al., 2020). However, different from other studies, we found that there are threshold differences in the impact of temperature on the incidence of HFMD in different regions and age groups, which can be used as a reference for early warning and guide the public health protection of HFMD.
The study provided evidence for health protection on HFMD attribute to heat wave. HFMD is a high incidence infectious disease in infants. It has the characteristics of high incidence rate, difficulty in diagnosis, rapid progress (Aswathyraj et al., 2016). The first outbreak of HFMD was reported the in Canada in 1957 (Chong et al., 2015). Since then, many studies on HFMD have been widely published all over the world. Asia has the largest outbreak of HFMD in the world, which has been reported in Japan, Singapore, India and other countries (Koh et al., 2016; Gonzalez et al.,2019; Kua et al., 2020; Rao et al., 2017; Huang et al., 2018). A study from China's infectious disease surveillance system shows that has 720092 cases of HFMD from 2008 to 2012 (Xing et al., 2014). The annual incidence rate is 2010-2012 years, the highest incidence is 12-23 months. The incidence rate of HFMD in the northern region is in May, or in September and October respectively. This also suggests that the increase of temperature is an important factor in the incidence of HFMD.
Over the past 50 years, the national average high-temperature days have decreased first and then increased. The number of high-temperature days decreased from the 1950s to the early 1980s and began to increase significantly in the early 1980s. There is a linear increasing trend in the number of high temperature days in Northwest and North China, especially in North China. In midsummer, Tianjin, Jinan, Chongqing, Fuzhou, Changsha, Nanchang, Wuhan, Nanjing and other places have high temperature and hot weather under the control of the Western Pacific subtropical high. High temperature makes the human body unable to adapt to the environment, exceeding the tolerance limit of the human body, resulting in the occurrence or aggravation of diseases and even death (Liu et al., 2015).
MMT varied in different area and population, climate, geographical factors and social economic factors. Our research results show that the optimum temperature in China is mainly concentrated at 17-27°C, there are differences in MMT among different age groups, MMT increased gradually with the increase of age. Under the action of high temperature, the thermoregulation mechanism is temporarily blocked, resulting in heat storage in the body, especially in children under the age of 10, because children have poor adaptability to high temperature and high humidity weather. The adaptability of the body to meteorological conditions is limited, especially for vulnerable children. The organs and regulatory mechanisms are not fully developed. When the temperature is higher or lower than the threshold, the thermal balance regulatory mechanism is easy to be disturbed, and the burden of physiological temperature regulation will increase, resulting in physical discomfort and disease invasion (Tyler et al., 2016).
There are also differences between different provinces. MMT in different regions shows a trend of increasing from north to South and from west to East. When making relevant health policies, it should be considered to break the concept of national unity or geographical area, and to develop the policy according to local conditions. MMT in low latitude and coastal areas is higher than that in other areas. This is mainly due to the long duration of hot summer in low latitude areas, and local residents' adaptation to high temperature is affected by physiological and behavioral adaptation. When a person lives in specific climatic conditions for a long time, he will adapt to the environment (Lee et al., 2019). In addition, there is more precipitation in these areas, which will promote the spread of infectious diseases such as HFMD to a certain extent.
The incidence of children under 5 years old accounts for nearly 90% of the total incidence, resulting in serious disease burden. Therefore, we divided the study into three age groups: under 5 years old, 6-10 years old and over 10 years old. In the three age groups, the northern provinces, such as Heilongjiang and Tianjin, high temperature had a high impact on HFMD, while the southern provinces, such as Guangzhou and Guizhou, high temperature had a low impact on HFMD. There are regional differences in the impact of high temperature on the HFMD, which may be due to the large change of temperature difference in northern regions, especially non-coastal regions, under the influence of continental climate during the change of seasons, especially the alternation of spring and summer, while the temperature difference in southern regions is relatively small under the influence of marine climate. In addition, the population in the South has high adaptability to high temperature due to the population in the north.
There have some limitations in our study. Because we can't measure individual exposure dose, we use meteorological monitoring data to evaluate the impact, which inevitably has an ecological fallacy. We only considered meteorological variables. Other major environmental risk factors of HFMD were not included, such as seasonal life in cities or suburbs, accumulated ultraviolet radiation, socio-economic conditions and air pollution. These factors should be considered or adjusted in future research.