(Review Jurnal) Evaluation of standard field and laboratory methods to compare protection times of the topical repellents PMD and DEET
- Biting mosquitoes (Diptera, Culicidae) are important vectors of several diseases such as malaria, filariasis and arboviral infections, including dengue, West-Nile, chikungunya and Zika, mainly in the tropical and subtropical regions but also increasingly in Europe as recent autochthonous cases of dengue and chikungunya have shown. Fly control has been found to be protective against the transmission of enteric infections, but there is insufficient evidence that the reduction in fly numbers limits disease transmission
- The most widely used active ingredient in commercially available mosquito repellents is the synthetic compound N,N-diethyl-3-methylbenzamide (DEET) which is generally regarded as the “gold standard” due to its high efficacy against a broad range of insects.
- While the guidelines recommend conducting efficacy studies both under laboratory and field conditions, mosquito repellents are hardly being compared side-by-side in both settings.
The objective of the present study was to shed more light on the relationship between the protection provided by a topical repellent against mosquito bites in laboratory and field settings, and to evaluate the efficacy of the active ingredient PMD against DEET under equal conditions, we conducted an experimental study that compares the protection times of 15% ethanolic solutions of PMD and DEET by exposing the same 18 study participants using similar parametres both in the field and in the laboratory
- Materials & Methods
- In the field, the repellents were evaluated at two locations in Switzerland that differ in their ecology; the Langholz forest (E 7.87170, N 47.28607), a restored forest area in the Canton of Aargau and in the meadows of the Thurauen Nature Reserve (E 8.59496, N 47.59600) between the rivers Thur and Rhine in Ellikon am Rhein, Canton of Zürich
Field experiments using the human landing catch method.
- Using the human landing catch (HLC) method, observations were made hourly for 30 minutes over 6 hours, starting 1 hour post application of the formulations at 17:00 hours and 14:15 hours in Langholz and Thurauen, respectively
- In preparation of the field experiments, the test surface (i.e. the bare lower leg) of the study participants were washed with neutral soap, rinsed, dried and swabbed with Arixtra wipes containing 70% isopropanol (Sanofi-Synthelabo, Meyrin, Switzerland). One of the lower legs was then treated with either one of the two repellent formulations or the negative control at a rate of 1 ml per 600 cm2.
- Sixty minutes after application of the treatment, the study participants were assigned to one of 6 positions that were set at least 20 m apart, as recommended in the WHO guidelines20, in order to avoid bias due to competition in attractiveness to the mosquitoes.
Laboratory experiments using the arm-in-cage test.
- The laboratory experiments were conducted at Swiss TPH in Basel between 5 August and 21 November 2015, following the WHO guidelines for the arm-in-cage (AIC) test20. In the AIC test, the protection time of a repellent is assessed by exposing a treated forearm to hungry mosquitoes at regular intervals. The cages measured 40 cm × 40 cm × 40 cm and were made of clear acrylic glass sides with an opening on the front side. At the bottom of the cage was a mirror positioned allowing for observation of mosquitoes landing on the lower side of the arm. The back side of the cage was made of a fine metal grid to ensure air supply during the experiments
- The test cages contained 200 host seeking 5 to 10-day-old females of one of the 3 mosquito species; Ae. aegypti, An. stephensi or Cx. quinquefasciatus. The three mosquito species are WHO recommended model organisms20. Adult mosquitoes were fed with 10% sucrose solution and water ad libitum. Testing and rearing conditions for all mosquito colonies were 26.8 ± 1.2 °C (mean ± SD) and 64.8% ± 7.2% relative humidity (mean ± SD) and a 12:12 (light:dark) photoperiod. Male and female mosquitoes were kept in the same rearing cages to allow mating to occur.
- Before exposure to the mosquitoes the forearm was washed with odourless soap, dried with a towel, swabbed with an Arixtra isopropanol wipe and then dried again. Then, to assess the readiness of the mosquitoes to land, the forearm of a study participant was exposed in the experimental cage for 60 seconds or until 10 landings were counted. A landing was defined as a mosquito alighting on the skin and remaining for at least 2 seconds. After measuring the landing activity with the untreated forearm the forearm was treated from wrist to elbow with either 15% PMD or 15% DEET at an application rate of 1 ml per 600 cm2. In order to estimate the application volume the surface area of the forearm was calculated as the average circumference of the elbow, wrist and middle of the forearm multiplied by the distance between the wrist and the cubital joint. Each volunteer tested only one repellent per day. Thirty minutes after application of the repellent the participant exposed the treated forearm in the test cage for 3 minutes or until 10 mosquitoes landed. The procedure was then repeated every 30 minutes over 6 hours. The duration until the first, second and tenth landing of a mosquito on the treated forearm was noted. During the exposure time volunteers were allowed to shake off the mosquitoes before they started biting, preventing an excessive number of bites. At the end of the experiment the arm was again washed and dried as before, and a second control measurement of the mosquitoes’ landing activity was taken.