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

OBJECTIVES: Indoor residual spraying (IRS) with insecticides is the main vector control intervention for the elimination of visceral leishmaniasis in India. After a change in IRS policy in 2015 due to widespread resistance of Phlebotomus argentipes to DDT, IRS with DDT was replaced with alpha-cypermethrin IRS in 2016. The objective of the present study was to evaluate the susceptibility of P. argentipes to DDT and its alternatives, namely malathion and pirimiphos-methyl (organophosphates); alpha-cypermethrin, deltamethrin, lambda-cyhalothrin and permethrin (pyrethroids), and bendiocarb and propoxur (carbamates), in support of visceral leishmaniasis elimination in India. METHODS: Phlebotomus argentipes sandflies were collected from the visceral-leishmaniasis endemic states of Bihar, Jharkhand and West Bengal. In the WHO tube tests, the phenotypic susceptibility of F1, 2-day old, non-blood fed females were determined against filter papers impregnated with DDT 4%, malathion 5%, pirimiphos-methyl 0.25%, alpha-cypermethrin 0.05%, deltamethrin 0.05%, lambda-cyhalothrin 0.05%, permethrin 0.75%, bendiocarb 0.1% and propoxur 0.1%, which were sourced from Universiti Sains Malaysia. The knockdown of sandflies after 1-h exposure and mortality at 24 h after the 1-h exposure period were scored. RESULTS: Mean mortality of P. argentipes 24 h after exposure in tube tests was 22.6% for DDT and ≥ 98% for other insecticide-impregnated papers tested. CONCLUSION: Phlebotomus argentipes continues to be highly resistant to DDT with no reversal of resistance after DDT's withdrawal from IRS. P. argentipes was fully susceptible to pyrethroid, organophosphate and carbamate insecticides tested. Regular monitoring is warranted for insecticide resistance management in sandfly vectors.

Measurement of recent exposure to Phlebotomus argentipes, the vector of Indian visceral Leishmaniasis, by using human antibody responses to sand fly saliva.

Antibody (IgG) responses to the saliva of Phlebotomus argentipes were investigated using serum samples from regions of India endemic and non-endemic for visceral leishmaniasis (VL). By pre-adsorbing the sera against the saliva of the competing human-biting but non-VL vector P. papatasi, we significantly improved the specificity of a P. argentipes saliva enzyme-linked immunosorbent assay. Using this method, we observed a statistically significant correlation between antibodies to P. argenitpes saliva and the average indoor density of female sand flies. Additionally, the method was able to detect recent changes in vector exposure when sera from VL patients were assayed before, during, and after hospitalization and protected from sand fly bites under untreated bed nets. Collectively, these results highlight the utility of antibodies to P. argentipes saliva as an important tool to evaluate VL vector control programs.

Phlebotomus argentipes seasonal patterns in India and Nepal.

The current control of Phebotomus argentipes (Annandale and Brunetti), the vector of Leishmania donovani (Laveran and Mesnil), on the Indian subcontinent is base on indoor residual spraying. The efficacy of this method depends, among other factors, on the timing and number of spraying rounds, which depend on the P. argentipes seasonality. To describe P. argentipes' seasonal patterns, six visceral leishmaniasis (VL) endemic villages, three in Muzaffarpur and three in Sunsari districts in India and Nepal, respectively, were selected based on accessibility and VL incidence. Ten houses per cluster with the highest P. argentipes density were monitored monthly for 15-16 mo using Center for Disease Control and Prevention light traps. Minimum and maximum temperature and rainfall data for the months January 2006 through December 2007 were collected from the nearest available weather stations. Backwards stepwise regression was used to generate the minimal adequate model for explaining the monthly variation in P. argentipes populations. The seasonality of P. argentipes is similar in India and Nepal, with two annual density peaks around May and October. Monthly P. argentipes density is positively associated with temperature and negatively associated with rainfall in both study sites. The multivariate climate model explained 57% of the monthly vectorial abundance. Vector control programs against P. argentipes (i.e., indoor residual spraying) should take into account the seasonal described here when implementing and monitoring interventions. Monitoring simple meteorological variables (i.e., temperature, rainfall) may allow prediction of VL epidemics on the Indian subcontinent.

Effect of village-wide use of long-lasting insecticidal nets on visceral Leishmaniasis vectors in India and Nepal: a cluster randomized trial.

BACKGROUND: Visceral leishmaniasis (VL) control in the Indian subcontinent is currently based on case detection and treatment, and on vector control using indoor residual spraying (IRS). The use of long-lasting insecticidal nets (LN) has been postulated as an alternative or complement to IRS. Here we tested the impact of comprehensive distribution of LN on the density of Phlebotomus argentipes in VL-endemic villages. METHODS: A cluster-randomized controlled trial with household P. argentipes density as outcome was designed. Twelve clusters from an ongoing LN clinical trial--three intervention and three control clusters in both India and Nepal--were selected on the basis of accessibility and VL incidence. Ten houses per cluster selected on the basis of high pre-intervention P. argentipes density were monitored monthly for 12 months after distribution of LN using CDC light traps (LT) and mouth aspiration methods. Ten cattle sheds per cluster were also monitored by aspiration. FINDINGS: A random effect linear regression model showed that the cluster-wide distribution of LNs significantly reduced the P. argentipes density/house by 24.9% (95% CI 1.80%-42.5%) as measured by means of LTs. INTERPRETATION: The ongoing clinical trial, designed to measure the impact of LNs on VL incidence, will confirm whether LNs should be adopted as a control strategy in the regional VL elimination programs. The entomological evidence described here provides some evidence that LNs could be usefully deployed as part of the VL control program. TRIAL REGISTRATION: ClinicalTrials.gov CT-2005-015374.

Long-lasting insecticidal nets fail at household level to reduce abundance of sandfly vector Phlebotomus argentipes in treated houses in Bihar (India).

OBJECTIVE: To determine whether the use of long-lasting insecticidal nets (LLINS) at household level are effective in reducing the abundance of Phlebotomus argentipes, vector of anthroponotic visceral leishmaniasis in India, Nepal and Bangladesh. METHODS: The impact of two long-lasting nets (Olyset and PermaNet) on indoor sandfly abundance was evaluated in selected houses of three endemic hamlets in Bihar (India). It was assumed that most sandflies breed inside the houses and that LLINs would progressively reduce the indoor density during the reproduction season. A campaign of indoor spraying with dichloro-diphenyl-trichloroethane (DDT) interfered with the trial but did not affect the sandfly population. Results Only the density of males of P. argentipes was significantly reduced by both the LLINs but not the females. CONCLUSIONS: These findings suggest that most female sandflies are coming from outside and that LLINs do not reduce their entry rate.