Bourquin, Martine

2013, Kroeber, Thomas, Bourquin, Martine, Guerin, Patrick

The threat of transmission of Lyme borelliosis and tick-borne encephalitis by ixodid ticks has resulted in an increasing number of tick repellents coming onto the market. To allow proper evaluation of the efficacy of different types of compounds and their formulations, there is a need for standardised methods for testing ticks repellents. Ticks show a marked negative geotactic response following contact with a potential host, i.e., they climb up in order to locate attachment and feeding sites, whereas exposing ticks to repellents induces positive geotaxis, i.e., ticks walk downwards or drop off the treated host or substrate. We describe here complementary tests that employ these geotactic responses to evaluate repellents: one in vitro on a warm glass plate and the other on the lower human leg (shin). The compounds tested were DEET, EBAAP, icaridin, capric acid, lauric acid, geraniol, citriodiol, citronella essential oil and lavender essential oil, all non-proprietary ingredients of widely distributed tick repellent formulations. In controls on both the warm glass plate and the human leg, the majority of Nodes ricinus nymphs walk upwards. By contrast, in both the in vitro and in vivo tests, effective doses of repellents cause ticks to either walk downwards or fall off the substrates, termed here "affected ticks". The ED75 values for affected ticks on the human leg indicate that the test products can be divided into three groups: (1) icaridin, EBAAP, DEET and capric acid with values between 0.013 and 0.020 mg/cm(2), (2) citriodiol and lauric acid with values between 0.035 and 0.058 mg/cm(2), and (3) geraniol, citronella oil and lavender essential oil with values between 0.131 and 1.58 mg/cm2. The latter three products can be considered as less effective repellents. The tests on the warm glass plate resulted in very similar efficacy rankings for the products tested in vivo, and the ticks' behavioural responses also corresponded closely to those observed on the treated human leg. The ED75 values on the glass plate ranged from half to one sixth needed on the leg. The warm glass plate test thus provides a reliable alternative to human subjects for an initial evaluation of new repellents, and is particularly appropriate for testing products with still to be determined human toxicity and dermatological effects. (C) 2013 Elsevier Inc. All rights reserved.

2010, Kroeber, Thomas, Kessler, Sébastien, Frei, Jérôme, Bourquin, Martine, Guerin, Patrick

We describe here an in vitro behavioral assay for testing mosquito repellents applied in a dose-based manner to a warm body (34 C) in test cages. The system was used to assess the sensitivity of 4-6-day-old Anopheles gambiae to the insect repellent diethyl methyl benzamide (deet). These tests were made in the absence and presence of additional carbon dioxide (CO(2)) applied as a pulse to activate mosquitoes in the cages. In the absence of the CO(2) pulse the mosquitoes hardly responded to the warm body. Increasing the CO(2) level in the cage by 1,000 parts per million caused a 25-fold increase in the number of landings by mosquitoes on the warm body in 2-min tests. This mosquito activation allowed the measurement of a significant reduction in the number of landings to bite on the warm body with increasing doses of deet (0.4 to 3.8 mu g/cm(2)). An asymptotic nonlinear model fitted to the repellency data in the presence of CO(2) allowed estimation of the effective dose of deet that reduced landings to bite by 50% (ED(50)) at 0.95 mu g/cm(2) (5 nmol/cm(2)) and the corresponding ED(95) at 4.12 mu g/cm(2) (21.5 nmol/cm(2)). This in vitro bioassay has the advantage of permitting a fast throughput of test products under standardized conditions and is suitable for screenings designed for the purpose of discovering lead products with as yet unknown human toxicological and dermatological profiles.