Evaluation of the Repellency Effectiveness of Linalool and Metofluthrin against Anopheles Gambiae S.L. in Kisian Village, Western Kenya

Abstract

Malaria continues to pose a major public health threat. Despite the efforts that have been made to prevent humans from mosquito bites, changes in the vector biting behavior, and mosquitoes tending to bite earlier before bed time could lead to successful malaria transmission. Vector control for the prevention of malaria has relied mainly on the use of chemical insecticides with extended residual life on walls or nets. These Insecticides act to some extent through repellent properties which has spurred increased interest in spatial repellents. In Kisian village, insecticide-treated nets (ITNs) have been scaled up. The main malaria vectors in this area are Anopheles gambiae s.1. The objective of this study was to test the effectiveness of two outdoor repellents; Linalool and Metofluthrin and four indoor formulations of Linalool repellent namely: natural Linalool, synthetic Linalool, 70-80% dLinalool and 55% d-Linalool in selected houses in a village located west of Kisumu town near KEMRI's Center for Global Health Research. A cross-over design study was carried out in 80 pairs of houses within this site. Half of the randomly selected houses were provided with Linalool in a gel emanator placed on top of the partitioning wall inside the houses while the other half were not given any treatment. Natural Linalool and synthetic Linalool formulations were evaluated during the short rains while the 70-80% d-Linalool and 55% dLinalool formulations were evaluated during the long rains. All houses were grouped in two sets. During the first evaluation, houses in set 1 received Linalool repellent and during the second evaluation, houses in set 2 were treated with Linalool repellent while the rest of the houses served as controls. Houses were sampled for mosquitoes each morning for two days in a week during the rainy season by pyrethrum spray catches (PSC) and the data collected from each house was entered into PDAs. To further assess the effectiveness in these spatial repellents in deterring outdoor host-seeking females from the host, three tents were used each night outside the houses. The tents were located at least 100 m apart. One repellent formulation (Linalool or Metofluthrin), was placed on poles approximately 1 meter high next to one of two tents and the third tent was not treated. Volunteers remained inside the tents from 7pm until 7am. At 7am, they collected mosquitoes from the tent traps using mouth aspirators and transferred them to paper cups labeled with date of collection and the tent from -which the mosquitoes were collected. All the data entered in the PDA were downloaded into a database on a secure server and data from the outdoor collections were entered using visual CE and appended to the same database. Poisson regression was used for data: analysis. Analysis was done using univariate procedure in SAS version 9.1. The major findings of this study is that lTN and 70-80% d-Linalool combined reduced Anopheles mosquito densities by 49% in treated houses relative to untreated controls (RR=0.49; CI, 95% 0.25-0.97; P=0.04). ITN and 70-80% d-Linalool combination reduced the density of fed Anopheles mosquitoes compared to ITN-55% d-Linalool treatment combinations (mean difference=-0.19; P< 0.0001) compared with ITN only, 70-80% and ITN (RR=0.73; CI, 95% 0.53-0.99; P=0.04) and Natural linalool and ITNs (RR=0.61; Cl, 95% 0.41-0.92; P=0.02) reduced densities of fed and half-gravid mosquitoes but all treatments had no effect on reducing the number of blood-fed Anopheles mosquitoes during the long rains. Tents treated with linalool had higher numbers of mosquitoes compared to tents with no treatment (RR=1.61; CI, 95% 1.27-2.05; P=O.OOOl). In conclusion, this study found Linalool and Metofluthrin repellents not effective in reducing indoor and outdoor densities of Anopheles mosquitoes while ITNs remain to be effective in the control of malaria vectors. The results from this study provide important information for malaria programs that aim at finding effective repellents for the control of malaria.