Influence of climate and heatwaves on dengue transmission in Sao Paulo and Natal, Brazil

PLOS One | https://doi.org/10.1371/journal.pone.0334838 October 24, 2025 9 / 13 behaviors start to diminish [41], thereby limiting dengue transmission. This could account for the higher relative risk linked to minimum and average temperatures compared to maximum temperatures [8]. In Natal, peaks of infections are more regular and recurrent than in São Paulo. However, the annual patterns of den- gue cases do not necessarily indicate that each year exhibits the same level of incidence. For example, in 2017, dengue incidence was notably lower compared to other years. This could be attributed to various factors, such as the presence of a population susceptible to the predominant circulating serotype [42], given that dengue is caused by four distinct virus serotypes. Additionally, the intricate interplay between environmental factors and the four dengue serotypes might have played a role [43]. Furthermore, the reported number of dengue cases in 2017 was below expectations for both Brazil and Colombia [44], which could be linked to the prior Zika virus infections in these populations [45]. We observed a negative correlation between rainfall and dengue incidence. Although water is crucial for Ae. aegypti development in outdoor containers, heavy rainfall can wash away these breeding sites, thereby reducing mosquito populations [46]. Between 2014 and 2015, São Paulo experienced its most severe drought since 1930 [47]. Interestingly, confirmed dengue cases drastically increased during this period (Fig 2), despite precipitation levels being below average.

Typically, the seasonal increase in dengue cases coincides with the rainy season, which occurs in São Paulo during the first half of the year (Fig 4). Similar patterns have been documented in the Magdalena River watershed in Colombia [48–50], where higher dengue incidence is also observed during the warmer and wetter months. These authors suggest temperature as the primary driver, as human-filled water containers decouple mosquito reproduction from precipitation, implying an association without causation. The connection between temperature and dengue incidence has been observed in various other studies [51–53]. In São Paulo, the slightly higher-than-normal temperatures observed during the 2014 and 2015 dengue outbreaks are particularly relevant given the concurrent El Niño conditions. These conditions persisted from late 2014 through all of 2015 [54,55], potentially exacerbating the temperature’s influence on dengue transmission. Our model indicates that the risk of dengue infections decreases dramatically as the number of days with heatwaves increases. The extremely low RR values from three days onwards suggest that the impact of heatwaves stabilizes after a certain point, with hotter weeks consistently presenting a much-reduced risk of dengue infections. Similar patterns have been reported in Singapore [24], though research on the effects of heatwaves on vectors, including mosquitoes, remains scarce. A recent study from Hanoi, Vietnam, suggested that heatwaves could lead to larger dengue outbreaks in the

following weeks [3]. Interestingly, the inhibitory impact of heatwaves on dengue infections observed in our study con- trasts with these earlier findings. We attribute this discrepancy to potential differences in the climates of the cities being compared, and, importantly, to variations in the definitions used to identify heatwave events. Jia et al. [15] demonstrated that heatwaves generally suppress mosquito populations. Laboratory research has established that temperature plays a critical role in shaping important mosquito characteristics related to dengue transmission, such as fecundity, survival, development speed, and adult lifespan [56–58]. Although moderate temperature increases can promote mosquito growth, exposure to extreme heat significantly limits this development [15]. Furthermore, earlier studies have shown that exposure to extreme temperatures leads to the downregulation of multi- ple genes associated with heat shock proteins, which play a crucial role in protecting Aedes mosquitoes from heat stress [33,59]. The increased mortality risk during prolonged periods of extreme heat offers a biologically plausible explanation for the observed reduction in dengue transmission. This effect, coupled with potential changes in human behavior during heatwaves, may contribute to the negative association we found between dengue and extreme heat [3]. During periods of extreme heat, people tend to spend more time in air-conditioned spaces—which are widespread in São Paulo—thereby

reducing contact between humans and mosquitoes and lowering the risk of dengue transmission [24]. Several studies in Brazil have examined the relationship between temperature and dengue incidence using a variety of modeling approaches. Distributed lag non-linear models (DLNM) have been applied, such as in São Luís, Maranhão, where temperature and rainfall were evaluated using negative binomial models, revealing significant associations between rainfall (with a 3-month lag) and dengue cases, although temperature effects were less pronounced [60]. Similarly, a