Near-infrared spectroscopy as a complementary age grading and species identification tool for African malaria vectors

Abstract

Near-infrared spectroscopy (NIRS) was recently applied to age-grade and differentiate laboratory reared Anopheles gambiae sensu strico and Anopheles arabiensis sibling species of Anopheles gambiae sensu lato complex. In this study, we report further on the accuracy of this tool for simultaneously estimating the age class and differentiating the morphologically indistinguishable An. gambiae s.s. and An. arabiensis from semi-field releases and wild populations. Nine different ages (1, 3, 5, 7, 9, 11, 12, 14, 16 d) of An. arabiensis and eight different ages (1, 3, 5, 7, 9, 10, 11, 12 d) of An. gambiae s.s. maintained in 250 x 60 x 40 cm cages within a semi-field large-cage system and 105 wild-caught female An. gambiae s.l., were included in this study. NIRS classified female An. arabiensis and An. gambiae s.s. maintained in semi-field cages as <7 d old or >/=7 d old with 89% (n = 377) and 78% (n = 327) accuracy, respectively, and differentiated them with 89% (n = 704) accuracy. Wild caught An. gambiae s.l. were identified with 90% accuracy (n = 105) whereas their predicted ages were consistent with the expected mean chronological ages of the physiological age categories determined by dissections. These findings have importance for monitoring control programmes where reduction in the proportion of older mosquitoes that have the ability to transmit malaria is an important outcome.

Figure 1

NIRS age and species prediction for semi-field reared An. arabiensis and An. gambiae s.s. as determined from a cross-validation. Panel A (An. arabiensis) and B (An. gambiae s.s.) indicate the actual age against the predicted age. The horizontal dotted line in Panel A and B separates young mosquitoes (<7 d) from old ones (≥7 d). Panel C shows the actual species (1 = An. arabiensis and 2 = An. gambiae s.s.) against the predicted species for 1, 5, 7, 11, 12 days old An. arabiensis and 1, 3, 5, 9, 10 days old An. gambiae s.s. The dotted line in panel C is the classification cut off point for the two sibling species (<1.5 for An. arabiensis and >1.5 for An. gambiae s.s.).

Figure 2

NIRS age and species predictions for wild-caught mosquitoes. Panel A indicates that 92% of all the wild mosquitoes were An. gambiae s.s. as predicted by NIRS. The dotted line in panel A is the classification cut off point for An. gambiae s.s. and An. arabiensis as predicted by NIRS (<1.5 for An. arabiensis and >1.5 for An. gambiae s.s.). Panel B shows the predicted cumulative age structure of wild mosquitoes from Njage collected in two consecutive nights.

Figure 3

NIRS age prediction of wild mosquitoes that were classified by parity dissections as underdeveloped (Christophers' stages ≤IIm) nulliparous, fully developed (Christophers' stages >IIm) nulliparous and parous for the first night (represented by circles) and the second night (represented by squares) [5].