Insecticide resistance status and bar-coding of dengue vectors in three districts of Tamil Nadu, India

Occurrence and distribution of vector population are crucial for entomological study in context of prevention, control and elimination of vector-borne diseases. To update some entomological aspects the study was undertaken in three districts of Tamil Nadu state namely Kumbakonam, Nagapattinam and Thriuvarur. The objective of the study was to understand the prevalence of mosquitoes; to assess insecticide resistance and phylogenetic analysis of dengue vectors ( Aedes aegypti and Ae. albopictus ).

The immature stages of mosquitoes were collected from different localities by standard WHO methods marking with GPS and mapping was done using ArcGIS 10.4 software for all three districts. Insecticide resistance test was conducted using WHO susceptibility test kits. The F1 generation of female adult mosquitoes of Ae. aegypti and Ae. albopictus were exposed to DDT 4% and malathion 5% with the control paper of Risella oil and olive oil, respectively. Further, genomic DNA of individual mosquito was isolated, and sequencing was done through Eurofins, Bangalore, India. The FASTA sequence was analyzed and phylogenic tree was constructed using the Maximum likelihood method in Molecular Evolutionary Genetics Analysis (MEGA) software (version 10.0).

A total 5307 specimens were collected through expanded survey in all three study areas. The collection yielded 16 species from six genera of mosquitoes. In total collection, Ae. albopictus was the dominant species in Kumbakonam and Thiruvarur districts and Ae. aegypti was dominant in Nagapattinam. The predominant breeding sources were discarded tyres with rainwater, plastic cups, coconut shells, aluminum vessels, sliver containers, bottles, grinding stones and earthen pots. The study revealed high pupal indices in all three study areas. Insecticide resistance monitoring revealed possible resistance in Ae. aegypti against DDT in all three districts whereas against malathion, possible resistance was recorded in Kumbakonam and Nagapattinam and Thiruvarur district, the species was found to be susceptible. Ae. albopictus showed resistance against DDT in all three districts but susceptible to malathion. The sequences obtained for dengue vectors showed 99% similar with GenBank. The phylogenetic tree was constructed using COI region sequences. Certainly, we observed the different genetic relationship among Ae. aegypti and Ae. albopictus between the study areas.

The study confirmed the presence of Ae. aegypti and Ae. albopictus in all three districts. The study further revealed that these vectors are susceptible to malathion but resistant to DDT. The continued surveillance of dengue vector and monitoring of insecticide resistance will strengthen the control programme for appropriate vector control measurements.