Selvarajah Raveendran

High prevalence of Aedes aegypti and Ae. albopictus vectors and dengue incidence as well as the presence of the four dengue virus serotypes underscore the need for effective and simple tools to monitor dengue vectors in Sri Lanka. Field studies were therefore undertaken with inexpensive gravid female trap (GFT) fabricated from local materials in Jaffna city in northern Sri Lanka. The study was initiated in April 2022, and GFT-based mosquito collection was conducted from June 2022 to April 2023. Five blocks of 30 to 40 houses, were selected in areas of high dengue prevalence in Jaffna city. Thirty houses were selected within each block and provided with five traps to be placed indoor and outdoor in each house. Conventional larval indices were determined at the beginning, middle and end of the study. Nine rounds of GFT collections were performed to identify and quantify trapped mosquitoes, their parity, and infection with virus serotypes. A total of 2177 and 3069 Aedes were collected respectively in indoor and outdoor GFTs, during nine rounds of collections and identified at the species level where possible. A total of 679 and 433 Ae. aegypti and Ae. albopictus were collected in indoor traps and 377 and 1288 in outdoor traps respectively. Adult and larval indices decreased respectively by 50% and 100% by the end of the study. Parous mosquitoes comprised 93% (of 120) and 94% (of 288) of the collected Ae. aegypti and Ae. albopictus respectively. In 110 pools of female Ae. aegypti and Ae. albopictus screened for virus serotype by PCR, five pools of Ae. aegypti and six pools of Ae. albopictus from both indoor and outdoor collections were positive for DENV-2. GFTs fabricated with locally available and inexpensive materials described here can be effective for monitoring the prevalence of Aedes vector species, parity and dengue virus infection status. The larval indices suggest that the use of GFTs can contribute to reducing dengue transmission.

Background: Dengue is the major mosquito-borne disease in Sri Lanka. After its first detection in January 2020, COVID-19 has become the major health issue in Sri Lanka. The impact of public health measures, notably restrictions on movement of people to curb COVID-19 transmission, on the incidence of dengue during the period March 2020 to April 2021 was investigated. Methods: The incidence of dengue and COVID-19, rainfall and the public movement restrictions implemented to contain COVID-19 transmission were obtained from Sri Lanka government sources. A Seasonal Autoregressive Integrated Moving Average (SARIMA) model was used to predict the monthly dengue incidence from March 2020 to April 2021 for each of the country’s 25 districts based on five years of pre-pandemic data, and compared with the actual recorded incidence of dengue during this period. Ovitrap collections of Aedes larvae were performed in Jaffna city in the Jaffna district from August 2020 to April 2021 and the findings compared with similar collections made in the pre-pandemic period from March 2019 to December 2019. Results: The recorded numbers of dengue cases for every month from March 2020 to April 2021 in the whole country and for all 25 districts over the same period were lower than the numbers of dengue cases predicted from data for the five years (2015–2019) immediately preceding the COVID-19 pandemic. The number of dengue cases recorded nationwide represented a 74% reduction from the predicted number of dengue cases for the March 2020 to April 2021 period. The numbers of Aedes larvae collected from ovitraps per month were reduced by 88.6% with a lower proportion of Ae. aegypti than Ae. albopictus in Jaffna city from August 2020 until April 2021 compared with March 2019 to December 2019. Conclusion: Public health measures that restricted movement of people, closed schools, universities and offices to contain COVID-19 transmission unexpectedly led to a significant reduction in the reported numbers of dengue cases in Sri Lanka. This contrasts with findings reported from Singapore. The differences between the two tropical islands have significant implications for the epidemiology of dengue. Reduced access to blood meals and lower vector densities, particularly of Ae. aegypti, resulting from the restrictions on movement of people, are suggested to have contributed to the lower dengue incidence in Sri Lanka.

Background: Malaria was eliminated from Sri Lanka in 2013. However, the influx of infected travelers and the presence of potent anopheline vectors can re-initiate transmission in Jaffna city, which is separated by a narrow strait from the malaria-endemic Indian state of Tamil Nadu.Methods: Anopheline larvae were collected from different habitats in Jaffna city and the susceptibility of emergent adults to DDT, malathion and deltamethrin investigated.Results: Anopheline larvae were found in wells, surface-exposed drains, ponds, water puddles and water storage tanks, with many containing polluted, alkaline and brackish water.Anopheles culicifacies,An. subpictus,An. stephensiandAn. varunawere identified in the collections. Adults of the four anopheline species were resistant to DDT.Anopheles subpictusandAn. stephensiwere resistant whileAn. culicifaciesandAn. varunawere possibly resistant to deltamethrin.Anopheles stephensiwas resistant,An. subpictuspossibly resistant whileAn. varunaandAn. culicifacieswere susceptible to malathion. DNA sequencing showed a L1014F (TTA to TTC) mutation in the IIS6 transmembrane segment of the voltage-gated sodium channel protein in deltamethrin-resistantAn. subpictus—a mutation previously observed in India but not Sri Lanka.Conclusion: Anopheles subpictusin Jaffna, likeAn. stephensi, may have recently originated in coastal Tamil Nadu. Besides infected overseas travelers, wind- and boat-borne carriage ofPlasmodium-infected anophelines across the Palk Strait can potentially reintroduce malaria transmission to Jaffna city. Adaptation to diverse larval habitats and resistance to common insecticides in anophelines are identified as potential problems for vector control should this happen.

Anopheles stephensi, the major vector of urban malaria in India, was recently detected for the first time in Sri Lanka in Mannar Island on the northwestern coast. Since there are different biotypes of An. stephensi with different vector capacities in India, a study was undertaken to further characterise the genotype and biotype of An. stephensi in Mannar Island. Mosquito larvae were collected in Pesalai village in Mannar and maintained in the insectary until adulthood. Adult An. stephensi were identified morphologically using published keys. Identified adult An. stephensi were molecularly characterized using two mitochondrial (cox1 and cytb) and one nuclear (ITS2) markers. Their PCR-amplified target fragments were sequenced and checked against available sequences in GenBank for phylogenetic analysis. The average spiracular and thoracic lengths and the spiracular index were determined to identify biotypes based on corresponding indices for Indian An. stephensi. All DNA sequences for the Mannar samples matched reported sequences for An. stephensi from the Middle East and India. However, a single nucleotide variation in the cox1 sequence suggested an amino acid change from valine to methionine in the cox1 protein in Sri Lankan An. stephensi. Morphological data was consistent with the presence of the Indian urban vector An. stephensi type-form in Sri Lanka. The present study provides a more detailed molecular characterization of An. stephensi and suggests the presence of the type-form of the vector for the first time in Sri Lanka. The single mutation in the cox1 gene may be indicative of a founder effect causing the initial diversification of An. stephensi in Sri Lanka from the Indian form. The distribution of the potent urban vector An. stephensi type-form needs to be established by studies throughout the island as its spread adds to the challenge of maintaining the country’s malaria-free status.

Background: Dengue, chikungunya, malaria, filariasis and Japanese encephalitis are common mosquito-borne diseases endemic to Sri Lanka. Aedes aegypti and Aedes albopictus, the major vectors of dengue, were recently shown to undergo pre-imaginal development in brackish water bodies in the island. A limited survey of selected coastal localities of the Jaffna district in northern Sri Lanka was carried out to identify mosquito species undergoing pre-imaginal development in brackish and saline waters. The effect of salinity on the toxicity of Bacillus thuringiensis israelensis larvicide to Ae. aegypti larvae at salinity levels naturally tolerated by Ae. aegypti was examined. Methods: Larvae collected at the selected sites along the Jaffna coast were identified and salinity of habitat water determined in the laboratory. The LC₅₀ and LC₉₀ of B. thuringiensis toxin, the active ingredient of a commercial formulation of the larvicide BACTIVEC®, were determined with Ae. aegypti larvae. Bioassays were also carried out at salinities varying from 0 to18 ppt to determine the toxicity of Bacillus thuringiensis to fresh and brackish water-derived larvae of Ae. aegypti. Results: Larvae of four Anopheles, two Aedes, one Culex and one Lutzia species were collected from brackish and saline sites with salinity in the range 2 to 68 ppt. The LC₅₀ and LC₉₀ of B. thuringiensis toxin for the second instar larvae of Ae. aegypti in fresh water were 0.006 ppm and 0.013 ppm respectively, with corresponding values for brackish water populations of 0.008 and 0.012 ppm respectively. One hundred percent survival of second instar fresh water and brackish water-derived Ae. aegypti larvae was recorded at salinity up to 10 and 12 ppt and 100% mortality at 16 and 18 ppt, yielding an LC ₅₀ for salinity of 13.9 ppt and 15.4 ppt at 24 h post-treatment respectively for the two populations. Statistical analysis showed significantly reduced toxicity of B. thuringiensis to fresh and brackish water-derived Ae. aegypti larvae at high salinities. Conclusion: A variety of mosquito vectors of human diseases undergo pre-imaginal development in brackish or saline waters in coastal areas of the Jaffna district in northern Sri Lanka. Salinity has a small but significant negative impact on the toxicity of B. thuringiensis toxin to Ae. aegypti larvae at salinity levels where Ae. aegypti larvae are found in the environment. This has implications for the use of B. thuringiensis toxin as a larvicide in brackish waters.