Large-scale intermittent larviciding intervention and associations with key malaria epidemiological parameters in...

Malaria remains one of the major public health problems both globally and in Tanzania. In addition to existing malaria control interventions, the country plans to introduce larviciding in its efforts towards malaria control and elimination. To guide a possible national upscaling of the intervention, a pilot larviciding study using biolarvicides produced in-country was implemented and evaluated to generate a body of evidence on operations and the impact of the intervention. The current manuscript reports on the association between larviciding and three major malaria epidemiological parameters.

Larviciding was conducted in three councils in Tanga Region in the North-East of the country and covered a population of over 1.1 million. The councils represented different malaria risk strata: high, moderate, and low. Larviciding was conducted between June 2022 and April 2024 through a community-based approach using existing local government structures. Larviciding was applied intermittently in the intervention councils to all identified breeding habitats for three rounds per year, based on the local rainfall seasons. Both Bacillus thuringiensis var. israelensis and Bacillus sphaericus were used and each round comprised of 8 consecutive weeks of biolarvicide application. Epidemiological data, including malaria incidence, laboratory malaria test positivity rate and antenatal malaria test positivity rate, were collected from routine health facility data. Each intervention council was paired with multiple control councils based on malaria risk and rainfall patterns, allowing a controlled before-after study design. The association between larviciding and malaria incidence was assessed using negative binomial regression while logistic regression was used for the other epidemiological outcomes.

The intervention did not have a significant association with the incidence of reported clinical malaria among all age-groups. The estimated incidence rate ratio (IRR) was 1.16 (95% CI 0.98; 1.37) in the high, 0.86 (95% CI 0.72; 1.02) in the moderate, and 0.96 (95% CI 0.76; 1.22) in the low stratum. Similar results were observed across other epidemiological parameters. There was a slight tendency for estimates in the moderate and low strata to be in the direction of a reduction.

Under the current design of intermittent larviciding application, there was no strong evidence of an association between larviciding and malaria epidemiological indicators. More studies considering continuous year-round implementation and randomizing a large number of clusters could generate further evidence on the contribution of larviciding to malaria control in similar transmission settings.