Despite Pyrethroid Resistance, Insecticidal Nets Protect Against Malaria

The use of long-lasting insecticidal net may provide protection against malaria, even in areas with pyrethroid resistance, according to a study recently published in the Lancet Infectious Diseases.

Almost 70% of the more than 660 million cases of malaria that have been prevented since 2000 were avoided as a result of the scale-up of insecticide-based interventions. Therefore, the likely effects of increasing insecticide resistance could herald a rebound in disease and mortality and should be quantified. This observational study coordinated by the World Health Organization aimed to investigate whether insecticide resistance was associated with the loss of effectiveness of long-lasting insecticidal nets and increased malaria disease burden. Specifically, this study investigated whether long-lasting insecticidal nets are protective against malaria in the presence of vector resistance to pyrethroids, and whether higher frequencies of vector resistance to pyrethroids were associated with greater infection prevalence and incidence of clinical malaria at the community level.

Between 2012 and 2016, cohorts of children (n=40,251) from each of the 279 included clusters (villages or groups of villages in which phenotypic resistance was measurable) in Benin, Cameroon, India, Kenya, and Sudan were randomly selected and followed up with to measure incidence of clinical malaria and prevalence of infection. Pyrethroid long-lasting insecticidal nets were the principal form of malaria vector control in all study areas, whereas this approach was supplemented with indoor residual spraying in Sudan. Mosquitoes (n=80,000) were collected seasonally in each cluster and were assessed for the susceptibility to pyrethroids, using the standard World Health Organization bioassay test.

Results showed that despite vector resistance, long-lasting insecticidal nets provided protection against malaria in all study areas except Sudan. Users of long-lasting insecticidal nets had lower infection prevalence (adjusted odds ratio, 0.63) and disease incidence (adjusted rate ratio, 0.62) than did nonusers across a range of resistance levels.

In addition, no association between infection prevalence (odds ratio, 0.86) or clinical incidence (rate ratio, 0.89) with higher pyrethroid resistance was found. Although users of nets were significantly better protected than nonusers, they were still at high risk for malaria infection. Among net users, the average incidence per person-year in India was 0.023, and in Kenya was it 0.80, whereas the average infection prevalence in India was 0.8% and in Benin was 50.8%.

The study authors concluded that “as nets provide only partial protection, the development of additional vector control tools should be prioritised to reduce the unacceptably high malaria burden.”

Reference

Kleinschmidt I, Bradley J, Knox TB, et al. Implications of insecticide resistance for malaria vector control with long-lasting insecticidal nets: a WHO-coordinated, prospective, international, observational cohort study [published online April 9, 2018]. Lancet Infect Dis. doi: 10.1016/S1473-3099(18)30172-5