Dengue and Climate
Climate (temperature, rainfall, and humidity) plays a significant role in the geographical range of vector mosquitoes, extend the disease transmission season, shorten the gonotrophic cycle, and reduce the time taken for ingested viruses to progress to infective stages, thereby increasing the rates of arboviral diseases transmitted by A. aegypti and A. albopictus (Sirisena and Noordeen, 2014; Gotto et al.,2013; Johansson et al.,2009; Sun et al., 2017).
Hence, climate primarily influences the vector and humans rather than the parasite in the transmission of dengue. Human activities such as agriculture and traditional water storage practices are common in south Asian countries and these elements are climate-sensitive in an indirect sense. However, it is the vector that has the higher climatic sensitivity and south Asian region provide the ideal condition for the vector especially during monsoon period with heavy rainfall and high humidity levels (Raheel et al.,2010; Sun et al., 2017).
Preliminary work in Sri Lanka on the analysis of dengue incidence data and climate data (Kusumawathie, Zubair et al., 2009) indicated an association between rain, temperature and dengue incidence with 1-2 months lag (Figure.). In Western Sri Lanka there are two seasons of active dengue transmission, May to August with a peak in June/ July and October to January with a peak in November/ December (figure ….) with slight variations in other parts of the country. These transmission peaks are associated with the seasonality of rains.
Dengue shows a rapid increase with intermittent epidemics at 2-3 irregular year intervals. Epidemics are understood to be periods of unusual rapid rise in transmission. Transmission intensity depends on rainfall, temperature and relative humidity as these impacts the biology, ecology and bionomics of the vector mosquito, replication of the virus. It is already established that rainfall is a driver of transmission – for example, the two seasonal peaks in cases is driven by the seasonal rainfall peaks as it leads to an abundance of surface water needed for mosquito breeding. This impact can be confounded by factors such as new serotypes and mosquito subspecies
Also, temperature affects both the life span of the mosquito larvae and the virus development period within the vector mosquito (Figure). Increase of minimum temperature decreases the time for larval development and the gonotrophic cycle (time between a blood meal and egg-laying) of the mosquito resulting in more mosquito generations as compared to that of normal conditions. This affects the human-biting activity of the vector mosquito causing frequent human-biting by infected mosquitoes resulting in the high transmission of dengue (Kusumawathie et al., 2009; Kusumawathie et al., 2013; Hopp and Foley, 2002; Delatte et al.,2009, Weeraratne et al.,2013). Hence, with high water temperature, larva takes a shorter time to mature and female mosquitoes tend to digest blood faster during warmer climates (Madanayake et al.; Massad et al., 2011). Likewise, Delatte et al (2009), reported the adults of Ae. albopictus was inversely correlated with temperature, with the highest survival rates found at 15C and the lowest at 35C and longevity of females were longer than males at all observed temperatures.