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A mosquito bite can cause dengue
Dengue cases are seeing a four fold rise from the year 2015 to 2023 in India. Dengue is a mosquito-borne viral disease that spreads through the bite of an infected Aedes mosquito. The virus responsible for causing dengue, is called dengue virus (DENV). There are four strains of DENV virus, which increase the possibility of people being infected four times due to different strains of the virus.
While many DENV infections produce only mild illness, some can cause an acute flu-like illness that can develop into a lethal complication, called severe dengue, which is a leading cause of serious illness and deaths in some Asian and Latin American countries.
The most common symptoms of dengue include fever with headaches, nausea, vomiting, rash, eye pain, muscle and joint pain, bone pain, swollen glands. Dengue stays for about 2 to 7 days. There is no specific medication to treat dengue.
Read more on dengue here
Temperature, humidity, and rainfall proliferate mosquito breeding influencing dengue spread
Dengue spread is significantly influenced by climate related factors such as temperature, precipitation, relative humidity, wind velocity, and climate modes such as the El Niño-Southern Oscillation (ENSO) informs this paper titled 'Dengue dynamics, predictions, and future increase under changing monsoon climate in India' in Nature Scientific Reports.
Temperature and humidity affect mosquito larval development, adult feeding behavior, and survival thus influencing the spread of mosquito vectors and the incidence of disease.
Rainfall provides habitats for the aquatic stages of mosquitoes. While mosquitoes require sufficient rainfall for breeding and larval development, excessive rainfall or rainfall flushes can cause water filled sites to overflow, disrupting mosquito breeding and destroying developing larvae.
Early warning systems for dengue and mosquito breeding
The burden of dengue is feared to increase in response to the continuous increase in global surface temperatures and changes in rainfall patterns. Mosquito control through breeding source reduction is the only efficient method to stop dengue transmission and early warning systems can greatly help in undertaking proactive measures to prevent and manage dengue outbreaks.
This study explores the climate-dengue associations in urban Pune, a dengue hotspot in the state of Maharashtra and devises a dengue model framework for early warnings and future projections of the disease.
The study finds that dengue mortality over Pune is seasonal with low incidence from December to May, and rises after the onset of the summer monsoon rainfall in June and peaks in November.
Temperature
The highest mean temperature recorded in Pune during the study period (2004–2015) was 35 °C, and the optimal temperature range is 27–35 °C, which is specific to Pune’s climate.
The temperature range of 27–35 °C is found to be optimal for increased dengue transmission and influences vector parameters like longevity, capacity and the number of times female lays eggs, timing of egg laying, the time it takes for the virus to develop inside the mosquito and the time from human infection to symptom onset.
The study finds a positive correlation between the annual dengue mortality and the number of days with optimal temperatures (27–35 °C) during the summer monsoon season (June to September) in Pune.
Rainfall flushes and wet weeks
Wet weeks are defined as weeks with a minimum of 0.5 mm and a maximum of 150 mm of rainfall, and when the weekly cumulative rains exceed 150 mm, such events are termed rainfall flushes.
The study reveals that weekly cumulative rains between 0.5 mm and 150 mm lead to an increase in dengue mortality, while rains above that threshold reduced dengue mortality in Pune through the flushing effect.
Monsoon intra seasonal oscillations
The Indian summer monsoon rainfall is characterised by monsoon intraseasonal oscillations that lead to active (wet) and break (dry) phases of the monsoon.
The years with high dengue mortality in Pune (2010, 2014, and 2015) are characterised by a lower number of active and break days while years with low dengue mortality show a higher number of active and break days.
Thus, it is not the cumulative amount of rainfall, but rather the pattern of rainfall, that plays a crucial role in influencing dengue transmission in Pune. Moderate rains spread over time creates favourable conditions for dengue, whereas extreme rainfall events reduce the dengue risk through their flushing effect.
June cumulative rains
Major rain-bearing months in Pune are from June to September, and more than 80 percent of dengue mortalities in the district occur after the onset of monsoon in June.
While mosquito eggs can survive drying out for up to eight months, eggs laid after the monsoon season in the preceding year may rely on the June rains of the current year for survival.
However, heavy rains in June can wash out those eggs and play an important role in reducing dengue-related mortalities for the entire year by controlling the vector population.
The study finds a negative association between annual dengue mortality and the June cumulative rains in the corresponding year in Pune during the study period (2004–2015). However, all monsoon months significantly contribute to mosquito breeding and dengue incidence and wet weeks and flush counts, are calculated based on rainfall throughout the entire year.
Humidity
Humidity influences percentage of hatching, survival rate, and biting frequency of adult dengue mosquitoes and allows the infected female mosquitoes to complete more than one replication cycle of the virus.
A minimum of 60 percent relative humidity is required for Ae. aegypti mosquitoes to survive because low humidity causes water to evaporate from the mosquito’s body and dries out their body fluids.
Below 60 percent humidity decreases the lifespan of mosquitoes who are then unable to transfer the virus from its stomach to salivary glands. However, an increase in relative humidity beyond a specific threshold can result in a decrease in dengue incidence.
The study shows that dengue mortality over Pune is high when relative humidity levels are between 60 and 78 percent during the summer monsoon season (June to September).
Thus moderate rainfall distributed over time contributes to an increase in dengue mortality, while heavy rains have the potential to reduce it through flushing effect. Years with higher dengue mortality rates in Pune are characterised by temperatures ranging from 27 °C to 35 °C and relative humidity levels between 60 and 78 percent during the Indian summer monsoon season.
What do future projections for dengue reveal
The study projects dengue mortality in Pune under three distinct Shared Socioeconomic Pathways (SSPs), representing different levels of socioeconomic development and greenhouse gas emissions. SSP1, SSP2 and SSP5 which correspond to low, intermediate, and high emissions, respectively
Future projections reveal that dengue mortalities over Pune will rise in the future. A 12 to 13 percent increase in dengue mortalities is expected in the near future (2020–40) while the mid-century (2040–60) dengue mortality over Pune is projected to increase by 25 percent under SSP2 and 40 percent under SSP5 relative to the reference period (1995–2014).
In the late century (2081–2100), projections show an increase in mortality by up to a 112 percent under SSP5. In the late century, the percentage increase in dengue mortality under SSP1 and SSP2 will be 30 percent and 58 percent, respectively.
The projected increase in dengue mortality over Pune will be due to increase in temperature and changes in rainfall patterns. This underscores the importance of a comprehensive approach to assess the risks associated with dengue transmission.
The findings of this study have significant implications for policymakers, as they provide insights into the potential impacts of climate change on dengue mortality in Pune and provide a model that can be tried and tested in other parts of the country.