RANDOMISED, CONTROLLED, BLINDED FIELD TRIAL ON THE EFFICACY OF BIOCIDE FORMULATION (BACILLUS SPP.) IN THE CONTROL OF MOSQUITO VECTORS

Larviciding to prevent malaria transmission

BACKGROUND

Larviciding refers to the regular application of chemical or microbial insecticides to water bodies or water containers to kill the aquatic immature forms of the mosquito (the larvae and pupae).

OBJECTIVES

To summarize research evidence evaluating whether larviciding with chemical or microbial insecticides prevents malaria transmission.

SEARCH METHODS

We searched the Cochrane Infectious Diseases Group Specialized Register; the Cochrane Central Register of Controlled Trials (CENTRAL), published in the Cochrane Library; MEDLINE; Embase; CAB Abstracts; LILACS; the World Health Organization International Clinical Trials Registry Platform (WHO ICTRP); ClinicalTrials.gov; and the ISRCTN registry up to 6 June 2019.

SELECTION CRITERIA

We included cluster-randomized controlled trials (cRCTs), interrupted time series (ITS), randomized cross-over studies, non-randomized cross-over studies, and controlled before-and-after studies (CBAs) that compared larviciding with no larviciding.

DATA COLLECTION AND ANALYSIS

We independently assessed trials for eligibility and risk of bias, and extracted data. We assessed the certainty of evidence using the GRADE approach.

MAIN RESULTS

Four studies (one cRCT, two CBAs, and one non-randomized cross-over design) met the inclusion criteria. All used ground application of larvicides (people hand-delivering larvicides); one evaluated chemical and three evaluated microbial agents. Studies were carried out in The Gambia, Tanzania, Kenya, and Sri Lanka. Three studies were conducted in areas where mosquito aquatic habitats were less extensive (< 1 km²), and one where habitats were more extensive (> 1 km²; a cross-over study from The Gambia).For aquatic habitats of less than 1 km², one cRCT randomized eight villages in Sri Lanka to evaluate chemical larviciding using insect growth regulator; and two CBA studies undertaken in Kenya and Tanzania evaluated microbial larvicides. In the cRCT, larviciding across all villages was associated with lower malaria incidence (rate ratio 0.24, 4649 participants, low-certainty evidence) and parasite prevalence (risk ratio (RR) 0.26, 5897 participants, low-certainty evidence) compared to no larviciding. The two CBA studies reported lower malaria prevalence during the intervention period (parasite prevalence RR 0.79, 95% confidence interval (CI) 0.71 to 0.89; 70,902 participants; low-certainty evidence). The Kenyan study also reported a reduction in the incidence of new malaria cases (RR 0.62, 95% CI 0.38 to 1.01; 720 participants; very low-certainty evidence).For aquatic habitats of more than 1 km², the non-randomized cross-over trial using microbial larvicides did not detect an effect for malaria incidence (RR 1.58, 95% CI 0.94 to 2.65; 4226 participants), or parasite prevalence (RR 1.15, 95% CI 0.41 to 3.20; 3547 participants); both were very low-certainty evidence. The Gambia trial also reported the mean haemoglobin level, and there was no difference across the four comparisons (mean difference -0.13, 95% CI -0.40 to 0.13; 3586 participants).We were unable to summarize or pool entomological outcomes due to unreported and missing data.

AUTHORS' CONCLUSIONS

Most controlled studies on larviciding have been performed with microbial agents. Ground larviciding for non-extensive larval habitats may have an effect on malaria transmission, and we do not know if there is an effect in large-scale aquatic habitats. We found no studies using larviciding application techniques that could cover large aquatic habitats, such as aerial spraying using aircraft.

Development of Wuchereria bancrofti in Culex quinquefasciatus that survived the exposure of sub-lethal dose of Bacillus sphaericus as larvae

Development of Wuchereria bancrofti in Culex quinquefasciatus emerged from the larvae that survived the exposure of sub-lethal dose of Bacillus sphaericus was examined in the laboratory. Third instar larvae of Cx. quinquefasciatus were treated with B. sphaericus at a sub-lethal dose of 11.35 microg/250 ml. The female mosquitoes that emerged from the survived larvae were fed on microfilaraemic human blood and parasite development was monitored in the fed mosquitoes. Both treated and untreated mosquitoes could ingest microfilaria (mF) equally as there was no significant difference in mF density between them. But, density of developmental stages of the parasite in treated group was significantly lower. Since, there was no mortality of mosquitoes, the lower density of the developmental stages could be attributed to the loss of parasites in the treated mosquitoes. Consequently, the proportion of mosquitoes with infective larvae (L3) and number of L3 were also significantly lower in treated females. Delay in parasite development was also noticed in treated mosquitoes. The present study indicates that B. sphaericus, when applied at sub-lethal dose kills larvae, and in addition, inhibits development of the filarial parasite and consequently reduces L3 yield in adult mosquitoes that emerged from the survived larvae.