Insecticide susceptibility of Phlebotomus argentipes sandflies, vectors of visceral leishmaniasis in India

Knockdown resistance mutations in Phlebotomus argentipes sand flies in Bihar, India

BACKGROUND

Vector control based on indoor residual spraying (IRS) is one of the main components of the visceral leishmaniasis (VL) elimination programme in India. Dichlorodiphenyltrichloroethane (DDT) was used for IRS until 2015 and was later replaced by the synthetic pyrethroid alpha-cypermethrin. Both classes of insecticides share the same target site, the voltage-gated sodium channel (Vgsc). As high levels of resistance to DDT have been documented in the local sand fly vector, Phlebotomus argentipes, it is possible that mutations in the Vgsc gene could provide resistance to alpha-cypermethrin, affecting current IRS pyrethroid-based vector control.

METHODS

This study aimed to compare frequencies of knockdown resistance (kdr) mutations in Vgsc between two sprayed and two unsprayed villages in Bihar state, India, which had the highest VL burden of the four endemic states. Across four villages, 350 female P. argentipes were collected as part of a 2019 molecular xenomonitoring study. DNA was extracted and used for sequence analysis of the IIS6 fragment of the Vgsc gene to assess the presence of kdr mutations.

RESULTS

Mutations were identified at various positions, most frequently at codon 1014, a common site known to be associated with insecticide resistance in mosquitoes and sand flies. Significant inter-village variation was observed, with sand flies from Dharampur, an unsprayed village, showing a significantly higher proportion of wild-type alleles (55.8%) compared with the three other villages (8.5-14.3%). The allele differences observed across the four villages may result from selection pressure caused by previous exposure to DDT.

CONCLUSIONS

While DDT resistance has been reported in Bihar, P. argentipes is still susceptible to pyrethroids. However, the presence of kdr mutations in sand flies could present a threat to IRS used for VL control in endemic villages in India. Continuous surveillance of vector bionomics and insecticide resistance, using bioassays and target genotyping, is required to inform India's vector control strategies and to ensure the VL elimination target is reached and sustained.

Assessing insecticide susceptibility, diagnostic dose and time for the sand fly Phlebotomus argentipes, the vector of visceral leishmaniasis in India, using the CDC bottle bioassay

Visceral leishmaniasis (VL) is a vector-borne protozoan disease, which can be fatal if left untreated. Synthetic chemical insecticides are very effective tools for controlling of insect vectors, including the sand fly Phlebotomus argentipes, the vector of VL in the Indian subcontinent. However, repeated use of the same insecticide with increasing doses potentially can create high selection pressure and lead to tolerance and resistance development. The objective of this study was to determine the lethal concentrations and assess levels of susceptibility, diagnostic doses and times to death of laboratory-reared P. argentipes to five insecticides that are used worldwide to control vectors. Using the Center for Disease Control and Prevention (CDC) bottle bioassay, 20-30 sand flies were exposed in insecticide- coated 500-ml glass bottles. Flies were then observed for 24 hours and mortality was recorded. Dose-response survival curves were generated for each insecticide using QCal software and lethal concentrations causing 50%, 90% and 95% mortality were determined. A bioassay was also conducted to determine diagnostic doses and diagnostic times by exposing 20-30 flies in each bottle containing set concentrations of insecticide. Mortality was recorded at 10-minute intervals for 120 minutes to generate the survival curve. Phlebotomus argentipes are highly susceptible to alpha-cypermethrin, followed by deltamethrin, malathion, chlorpyrifos, and least susceptible to DDT. Also, the lowest diagnostic doses and diagnostic times were established for alpha-cypermethrin (3μg/ml for 40 minutes) to kill 100% of the flies. The susceptibility data, diagnostic doses and diagnostic times presented here will be useful as baseline reference points for future studies to assess insecticide susceptibility and resistance monitoring of field caught sand flies and to assist in surveillance as VL elimination is achieved in the region.

Characterization of the glutathione S-transferase genes in the sand flies Phlebotomus papatasi and Lutzomyia longipalpis shows expansion of the novel glutathione S-transferase xi (X) class

Leishmaniasis control often relies upon insecticidal control of phlebotomine sandfly vector populations. Such methods are vulnerable to the evolution of insecticide resistance via a range of molecular mechanisms. There is evidence that two major resistance mechanisms, target site insensitivity and metabolic resistance, have evolved in some sandfly populations and further genetic characterization of resistance would be useful to understand and combat it. To facilitate the study of the mechanisms of metabolic resistance, here we improved the annotation and characterized a major detoxification gene family, the glutathione-s-transferases (GST), in the genomes of two sand fly species: Phlebotomus papatasi and Lutzomyia longipalpis. The compositions of the GST gene family differ markedly from those of Aedes and Anopheles mosquitoes. Most strikingly, the xi (X) class of GSTs appears to have expanded in both sand fly genomes. Our results provide a basis for further studies of metabolic resistance mechanisms in these important disease vector species.

Susceptibility status of the wild-caught Phlebotomus argentipes (Diptera: Psychodidae: Phlebotominae), the sand fly vector of visceral leishmaniasis, to different insecticides in Nepal

Background

Visceral leishmaniasis (VL) is targeted for elimination as a public health problem in Nepal by 2023. For nearly three decades, the core vector control intervention in Nepal has been indoor residual spraying (IRS) with pyrethroids. Considering the long-term use of pyrethroids and the possible development of resistance in the vector Phlebotomus argentipes sand flies, we monitored the susceptibility status of their field populations to the insecticides of different classes, in villages with and without IRS activities in recent years.

Methodology/Principal findings

Sand flies were collected from villages with and without IRS in five VL endemic districts from August 2019 to November 2020. The WHO susceptibility test procedure was adopted using filter papers impregnated at the discriminating concentrations of insecticides of the following classes: pyrethroids (alpha-cypermethrin 0.05%, deltamethrin 0.05%, and lambda-cyhalothrin 0.05%), carbamates (bendiocarb 0.1%) and organophosphates (malathion 5%). Pyrethroid resistance intensity bioassays with papers impregnated with 5× of the discriminating concentrations, piperonyl butoxide (PBO) synergist-pyrethroid bioassays, and DDT cross-resistance bioassays were also performed. In the IRS villages, the vector sand flies were resistant (mortality rate <90%) to alpha-cypermethrin and possibly resistant (mortality rate 90–97%) to deltamethrin and lambda-cyhalothrin, while susceptibility to these insecticides was variable in the non-IRS villages. The vector was fully susceptible to bendiocarb and malathion in all villages. A delayed knockdown time (KDT₅₀) with pyrethroids was observed in all villages. The pyrethroid resistance intensity was low, and the susceptibility improved at 5× of the discriminating concentrations. Enhanced pyrethroid susceptibility after pre-exposure to PBO and the DDT-pyrethroid cross-resistance were evident.

Conclusions/Significance

Our investigation showed that P. argentipes sand flies have emerged with pyrethroid resistance, suggesting the need to switch to alternative classes of insecticides such as organophosphates for IRS. We strongly recommend the regular and systematic monitoring of insecticide resistance in sand flies to optimize the efficiency of vector control interventions to sustain VL elimination efforts in Nepal.

Visceral leishmaniasis (VL), transmitted by P. argentipes sand flies, is endemic in South-East Asian countries such as Bangladesh, India, and Nepal, and is on the verge of elimination as a public health problem in Nepal by 2023. As part of the WHO Global Vector Control Response, entomological surveillance including insecticide resistance monitoring is one of the four main pillars of this strategy. In the early 1990s, the historical use of DDT for sand fly vector control was replaced with deltamethrin or alpha-cypermethrin, which have now been in use for almost three decades in Nepal. Suspecting that this long-term use of pyrethroids might have selected resistance in sand fly populations which would jeopardize control efforts, we conducted the first comprehensive survey to generate contemporary evidence of insecticide resistance in Nepal. For this, we performed WHO susceptibility tests in five VL endemic districts and found strong evidence of pyrethroid resistance in vector populations from the areas receiving IRS. Resistance mechanisms involved would probably be kdr mutations and monooxygenase. This study also endorses regular insecticide resistance monitoring to inform evidence-based decisions on insecticide use for vector control and to maintain the effectiveness of vector control measures as a core intervention in the fight against VL.