Background: Near-infrared spectroscopy (NIRS) has the potential to be a useful tool for assessing key entomological parameters including age, infectious status, and species of wild malaria mosquitoes. Before NIRS can be reliably used in the field at scale, methods for killing mosquitoes and conserving samples prior to NIRS scanning need to be further optimized. Studies have historically killed mosquitoes using chloroform, though this is not without health hazards and should not be used in human dwellings. It is also unclear which mosquito preservation method to use.
Methods: Here we investigate the use of pyrethrum sprays, a commercially available insecticide spray in Burkina Faso, for killing mosquitoes. Laboratory-reared Anopheles gambiae and Anopheles coluzzii were killed using either Kaltox Paalga®, a pyrethrum insecticide spray routinely used as part of indoor mosquito collections, or chloroform (the “gold standard”). Preservative methods were also investigated to determine their impact on NIRS accuracy in predicting laboratory-reared Anopheles and wild-caught mosquito species. After analysis of fresh samples, mosquitoes were stored in 80% ethanol or in silica gel for two weeks and reanalyzed by NIRS. In addition, experimentally-infected An. coluzzii and wild-caught An. gambiae sensus lato were scanned fresh to determine whether they contained sporozoites then stored in preservatives mentioned above for two weeks before being reanalyzed.
Results: There is not a substantial difference in NIRS accuracy in differentiating between laboratory-reared An. gambiae and An. coluzzii killed with either insecticide (90%) or chloroform (92%). After preservation, NIRS accuracy was 90% when mosquitoes were killed using chloroform and preserved in silica gel or ethanol. The same accuracy was obtained using Kaltox to kill mosquitoes and silica gel for preservation, but was diminish when ethanol was used as a preservative (80%). For infection status, NIRS is able to differentiate infectious and uninfectious mosquitoes with slightly lower accuracy for both laboratory and wild-caught mosquitoes preserved in silica gel or ethanol.
Conclusions: NIRS can classify An. gambiae s.l species killed by pyrethrum spray with no loss of accuracy. This insecticide may have practical advantages over chloroform for mosquitoes killing in NIRS analysis.