- A recent study reports the presence of 14 disease-carrying vector species among more than a hundred mosquito species in Kerala, which has seen a surge in dengue and chikungunya in recent years.
- Mosquito species known to transmit multiple diseases, including multiple vectors of the same disease, had overlapping distributions. This can lead to challenging disease outbreaks and complex transmission dynamics.
- Mosquito diversity was higher in artificial as compared to natural habitats. Some species also showed shifts in their habitat preference to artificial breeding sites such as discarded tyres, household containers, and latex collection cups in plantations, posing an increased risk of disease transmission.
Mosquito species carrying multiple human diseases occur and co-exist in artificial habitats, such as discarded plastic containers or tyres, in Kerala, finds a recent study.
On-ground surveys across five districts of Kerala revealed the presence of 14 mosquito species that are vectors for several diseases, including malaria, dengue, and chikungunya, among the 108 mosquito species the study recorded. The species that act as vectors for multiple diseases, transmitting disease from one organism to the other. Multiple hosts of the same disease often occur in the same area, which may lead to challenging disease outbreaks. This raises alarms for Kerala, which has accounted for three-quarters of the country’s dengue-related deaths (31 of 42 deaths) and is ranked second (8,259 cases) in total dengue cases this year, until August 2025.
The global burden of mosquito-borne diseases
Mosquitoes transmit many human diseases, such as malaria and filariasis (caused by parasites), as well as dengue, chikungunya, Zika, and Japanese encephalitis (caused by viruses). Globally, there are around 249 million cases of malaria with about 608,000 related deaths annually, and 96 million cases of dengue, which causes about 40,000 deaths each year. In 2017, Kerala saw a major outbreak of dengue and has seen a resurgence in dengue and chikungunya in recent years, which is a reflection of a broader global trend. According to the World Health Organisation, the increase in dengue risk may be related to changes in distribution patterns of mosquitoes, climate change, urbanisation, and other socio-economic factors.
“Vector-borne diseases have shown changing trends in India in recent years and manifest as changes in occurrence patterns across geographies, changing burden of diseases and a marked change in vector dynamics,” says Dr. Poornima Prabhakaran, Director, Centre for Health Analytics Research and Trends, Trivedi School of Biosciences, Ashoka University.
“There is high diversity of vector species in India, but we do not have good quality data to understand their habitat preferences. There have been stark shifts in the geographic range of mosquitoes and interactions between different species due to changes in climate and land-use patterns,” explains Farah Ishtiaq, who studies disease ecology at the Tata Institute of Genetics and Society, Bengaluru.
High diversity and overlapping vector distributions
The forests of the Western Ghats in Kerala are a hotspot of biodiversity, and the state also harbours widespread agricultural fields, plantations, and human-dominated areas. The presence of diverse habitats in proximity to forests, along with extensive rainfall, makes the region a hotspot for mosquito breeding sites. To document the diversity and distribution of mosquitoes and their habitat preferences, a team of researchers led by the ICMR-National Institute of Malaria Research-Field unit, Chennai, conducted ecological surveys across five districts of Kerala — Wayanad, Ernakulam, Pathanamthitta, Idukki, and Thiruvananthapuram. The team collected immature (larva and pupa) and adult stages of mosquitoes from water bodies and vegetated areas across natural and man-made habitats and identified species using lab observations.
The study identified 108 mosquito species, of which 14 are known primary and secondary vectors, transmitting diseases such as arboviruses, malaria, and filariasis.
The most abundant mosquito species, Stegomyia albopicta (Aedes albopictus), a vector of diseases such as dengue and chikungunya, was found in all five districts surveyed, with the highest prevalence in Thiruvananthapuram district, and is known to survive in a wide range of environments. However, the second-most prevalent mosquito species (Culex quinquefasciatus, Fredwardsius vittatus and Hulecoeteomyia chrysolineata) varied among districts, highlighting the need for area-specific mosquito control strategies.
On the other hand, primary vectors of diseases had limited prevalence across districts. For example, the mosquito species Stegomyia aegypti (Aedes aegypti), the primary vector of dengue, comprised only 1.43% of the mosquitoes collected during the survey. The vectors of Japanese encephalitis (Culex tritaeniorhynchus), malaria (Anopheles stephensi and A. culicifacies), and lymphatic filariasis (Mansonia uniformis) had even lower prevalence. This suggests that despite their relatively low abundance, many vectors may act as pathogen reservoirs, causing disease outbreaks when vector populations rise during favourable environmental conditions.
Understanding the distribution of disease vectors is crucial, as a recent study reported dengue antibodies in about a third of children in Kerala. It also found that infections showed geographic variation, involved multiple pathogen strains, and frequently went undetected, highlighting the importance of regional surveys and mitigation measures.
The Kerala mosquito biodiversity study also reports the co-existence of the primary vectors of malaria with its secondary vector (Anopheles varuna), which can potentially prolong or intensify transmission and lead to complex disease dynamics. Apart from the ecology of known disease vectors, the research team flagged other mosquito species for potential disease transmission. Among these are Stegomyia krombeini, and species from the albopictus subgroup closely related to Stegomyia albopicta (S. subalbopicta, S. novalbopicta, S. pseudalbopicta), that were reported in the surveyed areas for the first time.
The study also observed anthropophilic behaviour in rare mosquito species (Heizmannia and Verrallina) usually found in forests and plantations. Its association with humans needs to be studied for possible disease transmission.
Shifts in habitat preferences and climate
The field survey revealed a greater diversity of mosquitoes around human habitation, in water collected in artificial habitats such as discarded plastic containers, tyres, and household items, or water stored for agriculture, as compared to natural habitats such as ponds, lakes, rockpools, tree holes, and leaf axils. A preference towards artificial habitats has also been observed in other studies, including a survey of mosquito diversity in the Lakshadweep islands off the coast of Kerala.
Stegomyia albopicta, the most dominant mosquito species from the survey, was found to breed across 77 habitat types, primarily in artificial habitats such as discarded tyres, household containers, and latex collection cups used in plantations, apart from natural breeding sites such as tree holes. S. aegypti was mostly found in containers for storing water and discarded items in urban areas. Interestingly, some mosquito species such as Heizmannia chandi and S. subalbopicta, known to specifically breed in tree holes, were also observed in discarded tyres, suggesting an adaptation to man-made habitats.
“Some species like Stegomyia albopicta and Stegomyia aegypti are very versatile in their preferences and can invade other habitats,” says Ishtiaq. “It has largely happened because the boundaries between forests and urban areas have blurred. The habitats of these species are changing rapidly, and these kinds of studies are very important to understand how different species are adapting to such habitat modification.”
A recent modelling study found large geographic overlaps in the environmental suitability of dengue, chikungunya, and Zika, and another study predicted changes in vector distribution and abundance under climate change. Earlier this year, a study from the dengue hotspot of Pune found that dengue-related mortality is strongly influenced by temperature, rainfall, and humidity. The study predicts up to 40% rise in mortality due to a rise in temperature and changes in rainfall patterns under climate change. These connections between climatic variables and disease vector dynamics highlight the importance of studying the close association of vector biology with the environment.
“Much of vector dynamics has been attributed to a changing climate with a greater propensity for favourable conditions for vector transmission, replication and spread. This has manifested in a growing burden of mosquito-borne illnesses in parts of India that were not vulnerable earlier,” says Prabhakaran. “Higher temperatures result in shorter incubation periods, allowing rapid replication, while changing precipitation patterns and increased rainfall and humidity result in more favourable conditions for mosquito breeding and spread.”
“Vector ecology is kind of undermined in the study of diseases, where there is a lot more focus on the pathogens. But we need to pay more attention to vectors and need more such data from other parts of India,” emphasises Ishtiaq. “We need good evidence-based studies to influence policy.”
Read more: Warming temperature and humidity expand dengue’s reach
Banner image: The study found that the most abundant mosquito species was Stegomyia albopicta (Aedes albopictus), a vector of diseases such as dengue and chikungunya, and was found in all five districts. Image by Marcello Consolo via Flickr (CC BY-NC-SA 2.0).
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