Hyderabad: Something shifted in India’s dengue map between 2021 and 2025.
Pick 20 dengue patients randomly across the country in 2021, and only three would come from South India. Fast forward to 2025, and that ratio flipped dramatically—now three out of every five dengue cases originate from the southern states.
61.8% increase in share of dengue cases in 2025
The latest data from the National Centre for Vector Borne Diseases Control (NCVBDC) reveals this stark geographical transformation. South India accounts for 30,628 cases out of 49,573 national dengue cases recorded till August 2025—a 61.8 per cent share that represents a four-fold surge from the 15.6 per cent the region held just four years ago.
What drove this dramatic shift? The answer lies in three interlocking climate variables that researchers at Bharathidasan University tracked for a decade: rising temperatures, erratic rainfall patterns and relentless humidity.
Together, they created what scientists call ‘optimal breeding conditions’ for Aedes aegypti mosquitoes—the tiny, black-and-white striped carriers of dengue virus.
The climate connection
The Bharathidasan study, published in the International Journal of Environment and Climate Change, examined 8,92,456 dengue cases and 2,847 deaths from 2013 to 2022 across five southern states and Maharashtra.
The numbers tell a story of climate change rewriting disease geography.
“The period from 2013 to 2022 revealed a concerning correlation between climate change and the spread of dengue fever in South India. Rising temperatures and altered precipitation patterns created favourable conditions for the proliferation of Aedes mosquitoes, amplifying the incidence of dengue,” the authors said.
Every 100 mm increase in rainfall is associated with rise in dengue cases
The study quantified what public health officials suspected: every 100 mm increase in monthly rainfall was associated with a 15–25 per cent jump in dengue cases. Every 10 per cent rise in humidity correlates with an 8-12 per cent increase in infections. Temperature operates differently—transmission peaks between 26°C and 30°C, the exact range where South India’s climate hovers.
“Negative binomial regression models demonstrated significant associations between lagged rainfall and dengue case incidence across all study states. The rainfall effect was substantial and consistent, with every 100 mm increase in monthly rainfall associated with a 15-25 per cent increase in dengue cases across states,” the study reads.
But climate operates on a delay. Rainfall today translates into dengue cases four to six weeks later—the time mosquitoes need to breed, mature and start biting. This lag creates a predictable pattern that researchers trace across the region.
Kerala’s paradox
Kerala emerges as the study’s most puzzling case. The state reports 8,259 cases in 2025—moderate numbers compared to Tamil Nadu’s 11,825. Yet Kerala accounts for 31 deaths, representing 77.5 per cent of all dengue fatalities in South India despite holding only 27 per cent of the region’s cases.
Over five years from 2021-2025, Kerala’s death toll reached 368 out of 481 total deaths in South India—76.5 per cent of the region’s dengue mortality. The state’s Case Fatality Rate ranges from 0.38 per cent to 0.88 per cent, significantly higher than other southern states.
What makes Kerala different? Climate, the study suggests.
With average annual rainfall exceeding 2,800 mm and humidity levels around 85 per cent, the state offers Aedes mosquitoes near-perfect conditions.
“Kerala exhibited the strongest climate-dengue associations among the study states. The correlations of rainfall and cases with a 4-week lag were 0.68, along with a humidity-cases correlation of 0.45. These strong associations suggest that Kerala’s notably high rainfall conditions may significantly support dengue transmission,” the study reads.
The warm, humid climate creates ideal habitats for mosquito proliferation throughout the year. While other states experience seasonal breaks in transmission, Kerala’s consistent moisture levels maintain breeding sites continuously.
The 2017 turning point
One year stands out in the decade-long data: 2017.
Dengue activity peaked across all study states, coinciding with abnormal monsoon rainfall and a strong El Niño event. The pattern revealed how global climate phenomena cascade into local disease outbreaks.
“In 2015, there was a gradual increase in rainfall, humidity and dengue cases across all states. However, extreme rainfall events in 2017 led to a sudden surge in dengue cases,” the authors said.
El Niño disrupts normal weather patterns, affecting temperature, humidity and rainfall simultaneously. The 2015-2016 El Niño, one of the strongest on record, coincided with elevated dengue incidence across South India.
“During the El-Niño years, prolonged water scarcity and reduced precipitation contribute to the expansion of breeding grounds for disease-carrying organisms. Most of the severe droughts in India are associated with ENSO events, which have an inverse relation with the Indian Summer Monsoon,” the authors said.
The mechanism operates counterintuitively: drought followed by sudden rainfall creates more breeding sites than steady precipitation. Prolonged dry spells concentrate water in artificial containers—discarded tyres, construction debris, water storage vessels—that become mosquito nurseries when rains finally arrive.
Monsoons and mosquitoes
The southern peninsula’s dual monsoon system—southwest and northeast—shapes dengue’s rhythm.
Transmission peaks during post-monsoon months of October to December, when high humidity and stagnant rainwater support mosquito breeding. Secondary peaks appear between April and June, when early rains follow rising summer temperatures.
“Incidence tends to increase when precipitation falls below a 1:2 ratio, as this favours the formation of outdoor, rain-filled breeding sites; however, higher rainfall levels may wash these sites out,” the authors noted.
Too little rain concentrates breeding sites. Too much rain flushes larvae away. The sweet spot for mosquitoes—and the danger zone for humans—falls somewhere in between, where intermittent rainfall extends the breeding season without destroying larval habitats.
“Rainfall and humidity showed the strongest positive correlations,” the authors noted, with coefficients ranging from 0.90 to 0.98 across states.
The 2024 surge
The pattern culminated in 2024’s unprecedented surge: 99,211 cases across South India—the highest annual burden in recorded history. Karnataka led with 32,886 cases, followed by Tamil Nadu with 27,378.
Tamil Nadu’s trajectory proved particularly dramatic. Cases jumped from 9,121 in 2023 to 27,378 in 2024—explosive growth that suggested fundamental changes in transmission dynamics.
Karnataka showed steadier expansion, growing from 7,393 cases in 2021 to its 2024 peak. Kerala’s pattern demonstrated volatility, with significant increases in 2023 (17,426 cases) and 2024 (20,674 cases).
The 2025 data, covering only eight months, already shows 30,628 cases region-wide. If seasonal patterns hold through the post-monsoon peak, final numbers could match or exceed 2024’s record.
The mortality mystery
The disease burden distribution reveals a curious pattern: high case counts don’t automatically translate to high death tolls. Karnataka reports 4,281 cases in 2025 with zero deaths. Telangana records 4,296 cases, also with zero fatalities. Tamil Nadu, despite leading with 11,825 cases, maintains a Case Fatality Rate of just 0.06 per cent with seven deaths.
These variations point to differences in healthcare response capabilities and case management effectiveness.
Karnataka and Telangana demonstrate that large patient volumes can be managed without corresponding mortality increases when clinical protocols function effectively.
Tamil Nadu displays a striking correlation between cases and deaths at 0.94 in the decade-long study, suggesting that spikes in infections closely tracked with fatal outcomes during that period. Yet the state’s 2025 mortality remains controlled, indicating improved case management systems.
Urban amplification
Climate change operates as the primary driver, but urbanisation amplifies its effects.
Improper waste disposal, stagnant construction sites and inadequate drainage systems create breeding hotspots throughout South India’s rapidly expanding cities.
“Our study focuses on the local urban environment’s mosquito-carrying capacity. Oviposition sites significantly limit abundance. For instance, a 2003 Kerala study in Thiruvananthapuram emphasised conducive breeding conditions for Aedes mosquitoes during the monsoon season. The warm, humid climate and high annual precipitation create ideal habitats for mosquito proliferation,” the authors said.
Unplanned urbanisation brings human populations into closer contact with vector habitats. Construction sites accumulate rainwater in abandoned materials. Inadequate waste management creates artificial containers where mosquitoes breed. Dense populations ensure abundant blood meals for feeding mosquitoes.
The serotype challenge
Multiple dengue virus serotypes circulating simultaneously add another layer of complexity. Prior immunity to one serotype can trigger more severe disease upon infection with another through antibody-dependent enhancement—a phenomenon where existing antibodies help the virus infect more cells.
This immunological dynamic doesn’t necessarily increase case numbers, but it intensifies disease severity and hospitalisations, thereby increasing detection rates and healthcare system burden. The effect may partially explain South India’s prominent position in national dengue statistics.
Elevated case detection rates in South India may also reflect improved surveillance and diagnostic capabilities compared to other regions.
Better healthcare access and reporting systems increase the likelihood of dengue case identification and documentation. However, this surveillance advantage remains uneven across states, and underreporting continues in some areas.
Looking ahead
The decade-long dataset provides a foundation for anticipating future risks as climate variability intensifies. The findings underscore the urgency of integrating climate intelligence into disease surveillance and prevention.
“Understanding these dynamics allows us to develop effective strategies for dengue outbreak prevention,” the authors said.
The researchers recommend developing climate-based early warning systems that use meteorological data to predict dengue outbreaks weeks in advance. The four-to-six-week lag between rainfall and case surges creates a window for intervention—if health systems can mobilise quickly enough.
Interventions must remain region-specific.
Kerala’s challenge lies in sustained humidity control in an inherently humid climate. Telangana and Andhra Pradesh require better urban water management to eliminate breeding sites. Maharashtra needs localised vector monitoring across its diverse geography.
“As a precaution, especially during periods of reduced rainfall and humidity, it is advisable to take severe dengue precautions to mitigate the risk of outbreaks,” the authors said.
Without proactive adaptation measures, dengue transmission could expand into new ecological zones across South India, straining healthcare infrastructure already under pressure from recurring vector-borne epidemics. The question facing public health officials: Can surveillance and intervention systems adapt as quickly as the climate that drives disease transmission?
