Mosquito repellent effectiveness: A placebo controlled trial comparing 95% DEET, Avon Skin So Soft, and a â€śspecial mixtureâ€ť containing eucalyptus oil The introduction of the West Nile virus in Canada is changing the way we look at mosquitoes. Which repellent is most effective?
ABSTRACT: Background: The introduction of West Nile virus in Canada is changing the perception that mosquitoes are nothing more than nuisance pests. Interest is increasing in ways to protect against mosquitoes. One of the commonest is the use of insect repellents. Methods: Tests were undertaken to determine the effectiveness of three mosquito repellents: 95% DEET, Avon Skin So Soft bath oil, and a â€śspecial mixtureâ€ť of substances thought to have insect repellent qualities. The repellents were compared with each other and against a placebo in eight separate test sessions. During each session, the three repellents and placebo were tested simultaneously. Each session lasted between 90 and 120 minutes. After every session, the repellents were thoroughly washed from the test site and a minimum of 72 hours elapsed between tests. There were four test sites: the authorâ€™s right arm, left arm, right leg, and left leg. All test
sites were of similar surface area. Both mosquito bites and landings were counted as events. Measures were taken to ensure that events were not counted twice. Results: A total of 74 events occurred. The event distribution was as follows: placeboâ€”40 events; special mixtureâ€”28 events; Avon Skin So Softâ€”6 events; DEETâ€”0 events. Conclusions: When tested against a placebo, both DEET ( P .05); ASSS: 4, 2 (P > .05); DEET: 0, 0 (P > .05). Comparing each repellent against the placebo, it was found that both DEET and ASSS (40 vs 0, P < .0001; 40 vs 6, P = .0001) provided significantly better protection than placebo, but that Spec did not (40 vs 28, P = .30) ( Figure ). The use of DEET, ASSS, and Spec resulted in a 100%, 85%, and 30% reduction in event occurrence, respectively, and the number needed to treat to prevent one event was 1.00 (=1) for DEET and 1.17 (~1) for ASSS ( Table 1 ). Comparing the repellents with each other, it was found that both DEET and ASSS protected better than Spec, and that DEET protected better than ASSS ( Table 2 ), which was 85% as effective as DEET. 40 40 28 30 Events 20 10 0 Placebo Spec ASSS DEET 6 0
Figure. Number of events after repellents were applied.
quitoes were eliminated by using a single test subject. Further, the effect of variations in ambient temperature, wind speed, and humidity, as well as variations in mosquito species, hunger, and density were minimized by testing all four body sites, all three insect repellents, and the placebo simultaneously. The systematic rotation of each insect repellent between the upper and lower test sites was designed to mini-
mize any potential bite frequency variation between upper and lower limbs, and to negate the potential effect of the 1% surface area difference between the arm and leg mean surface area. In fact, analyses of the results show that there was no difference in bite frequency between upper and lower limbs (P > .05). This is significant in that future studies of similar design will not need to rotate test limbs.
Table 1. Effectiveness of repellents compared with placebo. Event distribution Placebo vs DEET Placebo vs ASSS Placebo vs Spec 40 vs 0 40 vs 6 40 vs 28
On any given day, numerous confounding variables affect mosquito bite frequency. These include the ambient temperature, wind speed, and humidity; the species of mosquito, the mosquitoesâ€™ level of hunger, and the density of the mosquito population; and the test subjectâ€™s age, sex, activity level, and biochemical attractiveness to the mosquito.6 In this study, measures were taken to minimize or eliminate these variables. For instance, the potential effect of variations in test subject age, sex, activity level, and biochemical attractiveness to the mos-