The effects of seasonal climate variability on dengue annual incidence in Hong Kong: A modelling study

Abstract

In recent years, dengue has been rapidly spreading and growing in the tropics and subtropics. Located in southern China, Hong Kong's subtropical monsoon climate may favour dengue vector populations and increase the chance of disease transmissions during the rainy summer season. An increase in local dengue incidence has been observed in Hong Kong ever since the first case in 2002, with an outbreak reaching historically high case numbers in 2018. However, the effects of seasonal climate variability on recent outbreaks are unknown. As the local cases were found to be spatially clustered, we developed a Poisson generalized linear mixed model using pre-summer monthly total rainfall and mean temperature to predict annual dengue incidence (the majority of local cases occur during or after the summer months), over the period 2002-2018 in three pre-defined areas of Hong Kong. Using leave-one-out cross-validation, 5 out of 6 observations of area-specific outbreaks during the major outbreak years 2002 and 2018 were able to be predicted. 42 out of a total of 51 observations (82.4%) were within the 95% confidence interval of the annual incidence predicted by our model. Our study found that the rainfall before and during the East Asian monsoon (pre-summer) rainy season is negatively correlated with the annual incidence in Hong Kong while the temperature is positively correlated. Hence, as mosquito control measures in Hong Kong are intensified mainly when heavy rainfalls occur during or close to summer, our study suggests that a lower-than-average intensity of pre-summer rainfall should also be taken into account as an indicator of increased dengue risk.Dengue is one of the most prevalent vector-borne arboviruses, transmitted between humans by Aedes mosquitoes. The number of dengue cases reported to the World Health Organization (WHO) has increased steadily from an average of less than a thousand cases per year globally in the 1950s to over 3.34 million in 2016 1,2 . Moreover, a recent study estimated that 390 million dengue infections occur each year 3 while another indicated that 3.9 billion people in 128 countries are at risk of infection 4 . The number of cases is still increasing as the disease spreads to new areas and explosive outbreaks occur. This global expansion of dengue virus (DENV) can be due to several causes, including environmental and climate factors. Climate is an important determinant of vector-borne disease epidemics as it directly influences the abundance and distribution of the vector. Together with environmental factors such as the degree of urbanisation, climate variations may result in the geographic expansion of mosquitoes and dengue into new areas 5 .Studies have shown that a positive association exists between temperature and dengue transmission 6-9 . One possible reason is that warmer temperatures lead to a higher abundance of Aedes mosquitoes by increasing their survival and development rates 10,11 . Furthermore, the extrinsic incubation period (EIP; the time required for viruses to become transmis...

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“…However, the effects of climate variables can also be time-dependent. Several studies have demonstrated that the effects of rainfall on dengue incidence can vary throughout the year [23, 24]. The abundant rainfall that occurs during monsoon season is likely to have negative effects on mosquito population size, as the rain can disrupt potential mosquito habitats.…”

“…Latest studies have mentioned the dengue incidence and mosquito abundance can be affected by weather conditions up to 5 months before the season starts [23, 24, 37] using data in or near subtropical areas. However, most of the studies focus on climate factors during dengue season.…”

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