# Heterogeneous impacts for malaria control from larviciding across villages and considerations for monitoring and evaluation

## TL;DR

Larviciding can reduce malaria mosquitoes and clinical cases, but its impact varies across villages due to environmental and operational factors.

## Contribution

The study demonstrates variable village-level impacts of larviciding and provides insights for monitoring and evaluating larval source management effectiveness.

## Key findings

- Larviciding reduced adult mosquito densities by 82-88% in Kenya and 82% in Côte d’Ivoire.
- Malaria prevalence in children dropped by 48-72% in Kenya and 11-78% in Côte d’Ivoire.
- Village-level heterogeneity suggests larviciding impacts cannot be simply extrapolated across regions.

## Abstract

Malaria vector control tools currently focus on insecticide treated nets (ITNs) and indoor residual spraying in malaria-endemic locations, but additional preventative strategies are needed to address protection gaps. Larval source management (LSM) includes larvicide application to aquatic habitat and an array of alternative forms of environmental efforts. An individual-based transmission model for falciparum malaria is used to demonstrate the theoretical benefit of suppressing malaria adult mosquito vector densities through LSM. The model simulates results of epidemiological trials from Western Kenya (a hilly area with papyrus swamps adjacent to human settlements and moderate to high perennial malaria transmission) and Côte d’Ivoire (an area with Sudanese climate, reducing vegetation cover and high transmission) that applied larvicide alongside ITNs, and investigates whether estimated changes in adult density can be used to project changes in human malaria. In the Western Kenya setting generalised linear models estimate 82% (90% credible intervals: 64% – 92%) and 88% (79% – 94%) reductions in the proportion of adult Anopheles funestus and Anopheles gambiae complex mosquitoes respectively as measured by CDC light traps. In Côte d’Ivoire, an 82% (56% – 93%) reduction of the dominant An. gambiae vector was estimated using standard window trap and pyrethrum spray catch. Both studies had variable village-level impacts. The transmission dynamics model predicted that these entomological impacts would result in a reduction in malaria prevalence in children of 6-months to 10-years of age of 48 – 72% in Kenya, and a 11 – 78% reduction in all-age clinical incidence across villages in Côte d’Ivoire, which are broadly consistent with the empirically observed outcomes. High heterogeneity between villages within the same study indicate that the relative or absolute reductions in mosquito adult density observed in these trials cannot be simply extrapolated to other regions. The LSM strategy adopted, unit area covered, and multiple environmental covariates all contribute to differences in indicators that could be used to assess entomological impacts and the corresponding epidemiological outcomes. This important malaria control tool was impactful across all sites examined, though further work is needed to understand how best to use this tool in the fight against malaria.

Additional preventative strategies are needed to address protection gaps in malaria control. One potential opportunity is larval source management (LSM), which reduces the number of mosquito adults emerging by modifying, treating or removing aquatic breeding sites. Measuring the impact from LSM can be challenging because the LSM strategy adopted, the effort deployed (for example, unit area covered), and the environment (how many breeding sites there are) can all contribute to how LSM can change the number of mosquitoes in an area and the clinical malaria cases averted. We estimated the impact from the application of larvicide conducted in two contrasting ecological settings in Kenya and Côte d’Ivoire by fitting difference-in-differences statistical models to adult vector densities before and after the application started. Differences in adult mosquito densities between the control and intervention villages were sufficient to parameterise a Plasmodium falciparum malaria parasite transmission dynamics model and broadly reproduce epidemiological trial outcomes. This work highlighted the variability in potential impact from LSM and provides considerations for surveillance of critical indicators for LSM performance monitoring to facilitate impact interpretation through modelling.