Implications of insecticide resistance for malaria vector control with long-lasting insecticidal nets: evidence from health facility data from Benin

Abstract

Background: Insecticide-based interventions have averted more than 500 million malaria cases since 2000, but insecticide resistance in mosquitoes could bring about a rebound in disease and mortality. This study investigated whether insecticide resistance was associated with increased incidence of clinical malaria.

Methods: In an area of southern Benin with insecticide resistance and high use of insecticide-treated nets (ITNs), malaria morbidity and insecticide resistance were measured simultaneously in 30 clusters (villages or collections of villages) multiple times over the course of 2 years. Insecticide resistance frequencies were measured using the standard World Health Organization bioassay test. Malaria morbidity was measured by cases recorded at health facilities both in the whole population using routinely collected data and in a passively followed cohort of children under 5 years old.

Results: There was no evidence that incidence of malaria from routinely collected data was higher in clusters with resistance frequencies above the median, either in children aged under 5 (RR = 1.27 (95% CI 0.81-2.00) p = 0.276) or in individuals aged 5 or over (RR = 1.74 (95% CI 0.91-3.34) p = 0.093). There was also no evidence that incidence was higher in clusters with resistance frequencies above the median in the passively followed cohort (RR = 1.11 (0.52-2.35) p = 0.777).

Conclusions: This study found no association between frequency of resistance and incidence of clinical malaria in an area where ITNs are the principal form of vector control. This may be because, as other studies have shown, ITNs continue to offer some protection from malaria even in the presence of insecticide resistance. Irrespective of resistance, nets provide only partial protection so the development of improved or supplementary vector control tools is required to reduce Africa's unacceptably high malaria burden.

Keywords: Insecticide; Malaria; Pyrethroid; Resistance; Vector.

Fig. 1

Cluster level malaria mosquito mortality on exposure to deltamethrin. The red dotted lines represent 98% mortality (possible resistance) and 90% mortality (confirmed resistance)

Fig. 2

Cluster level malaria mosquito mortality on exposure to deltamethrin versus rate of clinical malaria in children under 5 taken from health facility data from January to July 2013 (left) and August to December 2013 (right). Red lines represent lines of best fit

Fig. 3

Cluster level malaria mosquito mortality on exposure to deltamethrin versus rate of clinical malaria in individuals aged 5 or over taken from health facility data from January to July 2013 (left) and August to December 2013 (right). Red lines represent lines of best fit

Fig. 4

Cluster level malaria mosquito mortality on exposure to deltamethrin versus rate of clinical malaria from a passively followed cohort of children under 5 years of age from January to July 2013 (top), August to December 2013 (middle), and January to December 2014 (bottom). Solid red lines represent lines of best fit; dashed red lines represent lines of best fit omitting outliers (these are the circled points defined as cluster level rate greater than 2 standard deviations above the mean cluster-level rate)