High vector indices signal dengue alarm in Thiruvananthapuram district

Vector indices are showing a rising trend in many parts of the district, giving rise to apprehensions that there could be an uptick in dengue cases in the district, which has the dubious distinction of being the State’s dengue capital.

Senior Health officials said that while there is no unusual surge in cases as of now, the rise in mosquito density is a clear indication that dengue cases can spike any time. Dengue is being reported with a fair regularity from urban slums and coastal areas in the district, while sporadic cases have been reported from many urban wards within the Corporation limits.

The State has already reported 10,499 confirmed cases of dengue and 28,380 probable cases of dengue so far, with 51 confirmed and 23 probable dengue deaths too. Yet, when compared to the previous years, 2025 has been a “better” year, when there was no seasonal surge in cases and the case numbers have remained at a “low””, according to Health officials.

In 2024 the State had reported 20,674 confirmed and 53,536 probable dengue cases. Confirmed dengue deaths numbered 128, with 13 probable deaths too.

The worst recorded dengue epidemic to date was in 2017, with 21,993 confirmed cases and 66,329 probable cases; 165 confirmed deaths and 108 probable deaths. Cases had surged mid-year, overwhelming hospitals, especially in southern districts like Thiruvananthapuram, marking it as Kerala’s most severe outbreak since the virus’s introduction in 1997.

State hyperendemic

Dengue fever has been a persistent public health challenge in Kerala, with cyclical outbreaks, serotype shifts, and seasonal spikes tied to monsoon patterns. The State is hyperendemic for all four dengue virus serotypes (DENV1–4), meaning they circulate year-round, but explosive epidemics occur roughly every 4 years, driven by factors like urbanisation, climate change, population mobility, and rise in vector indices, when stagnant water pools become breeding sites for Aedes mosquitoes.

Though a majority of the dengue infections are either mild or asymptomatic, the rising burden of cases and high dengue sero prevalence (proportion of individuals in a community who have had prior exposure to dengue virus, as seen in blood samples) in the community presents unique challenges.

A joint study conducted by the WHO and the Health department between 2017-2020 among school children aged 9-12 years in three districts – Thiruvananthapuram, Kollam and Kozhikode – revealed a State-wide sero prevalence of 30.9%, the highest being in Thiruvananthapuram (46.9%). Majority of the children were unaware that they had had prior dengue infection, indicating widespread silent exposure to the virus.

Four sero types

Dengue virus has four distinct sero types and while infection with one sero type renders one immune to that serotype, it leaves him vulnerable to severe secondary dengue infection, with heightened risk of complications like dengue shock syndrome, when infected by a different sero type.

Though all four sero types are in circulation year round, DENV 1 and DENV 2 were the dominant sero types in the State till 2017. But a study conducted in 2022 indicated a shift from previous years, with DENV 3 and DENV 4 emerging as the dominant sero types.

Public health experts point out that it is important that primary care physicians are made aware of the sero type-specific clinical features of dengue so that early identification of the infection can avert possible complications.