Reduced bioefficacy of used LLINs against natural populations of Anopheles gambiae sensu lato in the centre and east regions of Cameroon

TL;DR

Used insecticide-treated mosquito nets in Cameroon show reduced effectiveness against local mosquitoes, highlighting the need for better maintenance and new net technologies.

Contribution

The study provides empirical evidence on the reduced bioefficacy of used LLINs in real-world settings with insecticide-resistant mosquitoes.

Findings

LLINs in Bertoua showed higher usage but more damage compared to Eyang.

Olyset Plus nets were more effective against local mosquitoes than other brands.

Nets with pyrethroid + PBO showed better performance against resistant mosquitoes.

Abstract

Long-lasting insecticidal nets (LLINs) are crucial for malaria prevention in Cameroon, yet their operational performance may be compromised because of deterioration of the physical integrity and bioefficacy of nets. This study evaluated LLINs, physical integrity, and bioefficacy following mass distribution campaigns in two regions in Cameroon: East (Bertoua) and Centre (Eyang).

Household surveys were conducted to assess ITN ownership, usage patterns, and maintenance practices. Net condition was measured using the proportionate hole index (pHI), and bio-efficacy was assessed using WHO cone bioassays against Anopheles gambiae (s.s.) (Kisumu strain) and local field mosquitoes.

A total of 55 LLINs from Bertoua and 30 from Eyang were sampled. LLIN ownership was comparable between sites (66.7% in Bertoua vs. 67.9% in Eyang), with a higher usage rate in Bertoua (73.3%) compared to 58.2% in Eyang. In Bertoua, a large percentage of LLINs (59.6%) were too torn, with Olyset Plus being the most common brand. In contrast, Eyang had a lower proportion of torn nets (44%), and households used a combination of different brands, including the Olyset net, Permanet 2.0, and Royal Sentry. Against the susceptible Kisumu strain, Bertoua’s Olyset Plus nets showed optimal efficacy with a 94.6% mortality rate, exceeding the World Health Organization's (WHO) threshold of ≥ 80%. In contrast, nets from Eyang had a 79.3% mortality rate, falling short of the optimal threshold. However, when tested against local field mosquitoes, Olyset Plus (pyrethroid + PBO) showed higher efficacy (mortality rate of 31.8%) than the other brands (7.1% from Olyset net; 18.6% for Permanet 2.0; and 8.6% for Royal Sentry) (p > 0.0001).

These findings underscore the crucial importance of proper LLINs maintenance, particularly regarding washing practices. The results also indicate the need to deploy newer generation LLINs to address emerging insecticide resistance and strengthen malaria control efforts.

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