London School of Hygiene & Tropical Medicine Malaria Centre

Implications of insecticide resistance for malaria vector control with long-lasting insecticidal nets

10 April 2018
A WHO-coordinated, prospective, international, observational cohort study[1]

Authors: Kleinschmidt I, Bradley J, Knox TB, et al

Lancet Infect Dis 2018.

 

Long-lasting insecticidal nets (LLINs) have provided protection against malaria for millions of people in malaria endemic countries for nearly two decades. Standard LLINs shield the person s under the net from the blood-seeking female mosquito which is likely to be killed through the insecticidal action of the chemical (known as a pyrethroid) on the net. In recent years there have been widespread reports of insecticide resistance in malaria transmitting Anopheles mosquitoes (vectors). There have been justifiable concerns that the emergence of insecticide resistance could herald a catastrophic rebound in malaria disease burden.

 

Insecticide resistance and LLINs

Our study, published in Lancet Infectious Diseases, investigated whether LLINs still provide protection against malaria in the presence of insecticide resistance. The study, which was coordinated by WHO, was conducted in five countries (Benin, Cameroon, India, Kenya, and Sudan). In 279 study locations in these countries we simultaneously assessed insecticide resistance in the local mosquito vector population on the one hand, and the effectiveness of LLINs in protecting against malaria infection and disease on the other.

Our approach

Insecticide resistance was measured in a standardised and controlled way which entails exposing samples of local mosquitoes to the insecticide that is used on LLINs. In all study locations LLINs were made available to local communities in sufficient numbers to provide at least one net for every two persons.

Effectiveness of LLINs was assessed by following cohorts of children in each of these locations and comparing malaria infection and disease between children sleeping under nets, with those not using nets.

LLINs should still be used

Our main finding was that those sleeping under LLINs were exposed to considerably lower rates of malaria parasite infection and disease than those not using LLINs, even in places with insecticide resistance, i.e. where the insecticide that is used on the net no longer kills all mosquitoes.

We did not find a measurable difference in the level of protection provided by nets between places where relatively few mosquitoes survived exposure to insecticide (i.e. few showed resistance to insecticide) and places where large proportions of mosquitoes survived (were resistant).

 Nevertheless, users of nets, although significantly better protected than non-users, continued to be subjected to malaria infection risk. We therefore conclude that regardless of resistance, people living in malaria endemic areas should continue to use long-lasting insecticidal nets to reduce their risk of infection. However, as nets provide incomplete protection, the development of additional vector control tools should be prioritised to reduce the unacceptably high malaria burden.

Once new generation LLINs with proven superiority to standard LLINs become available, their use should be prioritised in areas of pyrethroid resistance, in accordance with recent WHO guidelines2. In the meantime, the use of standard pyrethroid LLINs should continue to be promoted to save lives until more effective malaria prevention methods are widely available.


1. 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. Lancet Infect Dis 2018; published online April 9. http://www.thelancet.com/journals/laninf/article/PIIS1473-3099(18)30172-5/fulltext

2.  Conditions for deployment of mosquito nets treated with a pyrethroid and piperonyl butoxide. WHO 2017. http://www.who.int/malaria/publications/atoz/use-of-pbo-treated-llins/en/