Comparison of four outdoor mosquito trapping methods as potential replacements for human landing catches in western Kenya

Abstract

Introduction Longitudinal monitoring of outdoor-biting malaria vector populations is becoming increasingly important in understanding the dynamics of residual malaria transmission. However, the human landing catch (HLC), the gold standard for measuring human biting rates indoors and outdoors, is costly and raises ethical concerns related to increased risk of infectious bites among collectors. Consequently, routine data on outdoor-feeding mosquito populations are usually limited because of the lack of a scalable tool with similar sensitivity to outdoor HLC. Methodology The Anopheles trapping sensitivity of four baited proxy outdoor trapping methods—Furvela tent trap (FTT), host decoy trap (HDT), mosquito electrocuting traps (MET) and outdoor CDC light traps (OLT)—was assessed relative to HLC in a 5 × 5 replicated Latin square conducted over 25 nights in two villages of western Kenya. Indoor CDC light trap (ILT) was run in one house in each of the compounds with outdoor traps, while additional non-Latin square indoor and outdoor HLC collections were performed in one of the study villages. Results The MET, FTT, HDT and OLT sampled approximately 4.67, 7.58, 5.69 and 1.98 times more An. arabiensis compared to HLC, respectively, in Kakola Ombaka. Only FTT was more sensitive relative to HLC in sampling An. funestus in Kakola Ombaka (RR = 5.59, 95% CI 2.49–12.55, P < 0.001) and Masogo (RR = 4.38, 95% CI 1.62–11.80, P = 0.004) and in sampling An. arabiensis in Masogo (RR = 5.37, 95% CI 2.17–13.24, P < 0.001). OLT sampled significantly higher numbers of An. coustani in Kakola Ombaka (RR = 3.03, 95% CI 1.65–5.56, P < 0.001) and Masogo (RR = 2.88, 95% CI 1.15–7.22, P = 0.02) compared to HLC. OLT, HLC and MET sampled mostly An. coustani, FTT had similar proportions of An. funestus and An. arabiensis, while HDT sampled predominantly An. arabiensis in both villages. FTT showed close correlation with ILT in vector abundance for all three species at both collection sites. Conclusion FTT and OLT are simple, easily scalable traps and are potential replacements for HLC in outdoor sampling of Anopheles mosquitoes. However, the FTT closely mirrored indoor CDC light trap in mosquito indices and therefore may be more of an indoor mimic than a true outdoor collection tool. HDT and MET show potential for sampling outdoor host-seeking mosquitoes. However, the traps as currently designed may not be feasible for large-scale, longitudinal entomological monitoring. Therefore, the baited outdoor CDC light trap may be the most appropriate tool currently available for assessment of outdoor-biting and malaria transmission risk. Graphic abstract

The Centres for Disease Control light trap (CDC-LT) and the human decoy trap (HDT) compared to the human landing catch (HLC) for measuring Anopheles biting in rural Tanzania

“…funestus catch in this study reflects the predominance of anthropophagic and endophilic behaviours of the local populations as has previously been reported in the same area [ 37 , 38 ]. Other observed sources of variation include location [ 32 ], whether the traps are indoors or outdoors [ 55 , 58 ] and the presence or absence of ITNs [ 27 ]. Overgaard et al [ 55 ] observed that the results of CDC-LT efficacy may also vary based on the different statistical analyses employed.…”

“…The choice of host decoy i.e. whether cow or human [ 35 ], location [ 32 , 33 ] and seasonality [ 31 ] are among the factors observed to influence the HDT performance.…”

“…Host-seeking mosquitoes lured by heat and smell and the dark colour around the target are trapped on its sticky surface. The HDT is a promising entomological surveillance tool based on several studies that demonstrate its capacity to catch a wide range of exophagic mosquito species [ 32 – 36 ].…”

“…funestus An. coustani 1.98 (1.01–3.86) 0.88 (0.37–2.11) 3.03 (1.65–5.56) 1.83 (0.70–4.79) 0.45 (0.13–1.57) 2.88 (1.15–7.22) Not assessed Abong’o et al 2021 [ 32 ] B. HDT 1 Ulanga, Tanzania An. arabiensis An.…”

“…coustani An. pharoensis 5.69 (2.98–10.86) 1.38 (0.60–3.18) 0 18(0.09–0.37) NA 1.32 (0.49–3.59) 0.66 (0.21–2.09) 2.88 (1.15–7.22) NA Not assessed Abong’o et al 2021 [ 32 ] Lakkang area Pucak area 4 Chikwawa, Malawi An. gambiae s.s & An.…”