Molecular identification of unexposed <i>An</i>. <i>gambiae s</i>.<i>l</i>. mosquitoes collected in six rounds of collection between 2008 and 2010.

<p>Molecular identification of unexposed <i>An</i>. <i>gambiae s</i>.<i>l</i>. mosquitoes collected in six rounds of collection between 2008 and 2010.</p

Sentinel sites for biannual resistance monitoring.

<p>Green dots indicate the geographic locations of each of the four sites. Sites were chosen to represent a range of different selection pressures for insecticide resistance. Embedded charts show the baseline percentage mortality for the year in which monitoring began (2008), where BC = bendiocarb, DDT = DDT, DM = deltamethrin, FT = Fenitrothion, and PT = Permethrin. Green lines represent 98% mortality and red lines indicate 90% mortality. Map source <a href="https://commons.wikimedia.org/wiki/File:Chad_sat.jpg" target="_blank">https://commons.wikimedia.org/wiki/File:Chad_sat.jpg</a>.</p

<i>kdr</i> mutations frequencies among pools of unexposed An. gambiae M and An. gambiae S collected in 2008 and 2010 in three sites.

<p><i>kdr</i> mutations frequencies among pools of unexposed An. gambiae M and An. gambiae S collected in 2008 and 2010 in three sites.</p

Insecticide bioassay results for <i>Anopheles gambiae s</i>.<i>l</i>. in 2008, 2010 and 2014 in four sentinel sites in Chad.

<p>Insecticide bioassay results for <i>Anopheles gambiae s</i>.<i>l</i>. in 2008, 2010 and 2014 in four sentinel sites in Chad.</p

Circumsporozoite analysis from 2008 and 2010.

<p>Circumsporozoite analysis from 2008 and 2010.</p

Results from insecticide resistance bioassays.

<p>A) DDT; B) Deltamethrin; C) Permethrin. No round 4 data from Bongor was available owing to the early end of the rainy season in 2009. Numbers in each column represent the number of mosquitoes exposed to insecticide paper. Data from 2008 have been published previously and are included for comparison.</p

Spatial and Temporal Trends in Insecticide Resistance among Malaria Vectors in Chad Highlight the Importance of Continual Monitoring

<div><p>Background</p><p>A longitudinal <i>Anopheles gambiae</i> s.l. insecticide resistance monitoring programme was established in four sentinel sites in Chad 2008–2010. When this programme ended, only sporadic bioassays were performed in a small number of sites.</p><p>Methods</p><p>WHO diagnostic dose assays were used to measure the prevalence of insecticide resistance to 0.1% bendiocarb, 4% DDT, 0.05% deltamethrin, 1% fenitrothion, and 0.75% permethrin in the main malaria vectors at the beginning and end of the malaria transmission season for three years 2008–2010, with subsequent collections in 2011 and 2014. Species and molecular identification of <i>An</i>. <i>gambiae</i> M and S forms and <i>kdr</i> genotyping was performed using PCR-RLFP; circumsporozoite status was assessed using ELISA.</p><p>Results</p><p>Between 2008 and 2010, significant changes in insecticide resistance profiles to deltamethrin and permethrin were seen in 2 of the sites. No significant changes were seen in resistance to DDT in any site during the study period. Testing performed after the period of routine monitoring had ended showed dramatic increases to DDT and pyrethroid resistance in 3 sites. No resistance to organophosphate or carbamate insecticides was detected. <i>An</i>. <i>arabiensis</i> was the predominate member of the <i>An</i>. <i>gambiae</i> complex in all 4 sites; adult collections showed temporal variation in species composition in only 1 site. <i>Kdr</i> analysis identified both 1014F and 1014S alleles in <i>An</i>. <i>gambiae</i> S only. Circumsporozoite analysis showed the highest vector infection rates were present in Donia, a site with extensive use of agricultural insecticides.</p><p>Conclusions</p><p>During the monitoring gap of four years, significant changes occurred in resistance prevalence in 3 of the 4 sites (p = <0.001), endangering the efficacy of currently implemented malaria control interventions. Significant changes in insecticide resistance profiles and a lack of <i>kdr</i> resistance alleles in adult populations highlight the urgent need for comprehensive entomological monitoring to be implemented and sustained in country.</p></div

Results from adult collections.

<p>A) PSC B) Pit traps. Different coloured series represent different species: Green = Other Anopheles; Red = <i>An</i>. <i>funestus</i>; Blue = <i>An</i>. <i>gambiae</i>.</p