Efficacy testing of several Ixodes ricinus tick repellents: different results with different assays

In the European Union (EU), tick repellents for humans need to be registered and approved by the authorities in order to be marketed. As there are currently no specific technical guidelines for product evaluation, we compared 3 different test methods: the mechanical moving object bioassay (MOB), and 2 assays involving human volunteers. For the latter, procedures according to the U.S. Environmental Protection Agency (EPA) and the Stiftung Warentest (StiWa), a German consumer care organization, were used. Two repellents, Autan(®) (AU), based on 20% Picaridin [2-(2-hydroxyethyl)-1-piperidinecarboxylic acid-1-methylpropyl ester], and ZeckWeck (ZW), based on 12.5g/100g Citriodiol™ (main compound: p-menthane-3,8-diol) were tested with all 3 assays. Three repellents, Anti Brumm(®) naturelle, based on 20% Citriodiol™ (main compound: p-menthane-3,8-diol), G090141, based on 20% EBAAP (ethyl buthyl acetyl aminopropionate), and G090152, based on 10% decanoic acid (capric acid), which is contained in Zanzarin(®), were tested according to the EPA and the StiWa procedures. The EPA assay indicated a significantly higher repellency of the products AU and G090141 than the StiWa test, but no difference between assays could be detected for the remaining 3 products. Also the corresponding protection times were significantly longer (approximately 4h) when determined according to EPA versus to StiWa for 3 of the products, whilst the difference was insignificant for ZW and G090152. Additionally, significantly lower numbers of ticks initially walked onto the repellent-treated skin when tested according to EPA versus to StiWa in all products except ZW and G090152. Thus, the StiWa protocol appears to pose higher demands on a repellent than the EPA method. Contrary to expectation, the MOB showed the same or even lower product efficacy when compared to the EPA and StiWa tests. Particularly, the percentage of ticks clinging to repellent-treated filter paper was significantly higher than the proportion of ticks walking onto treated skin in the other assays. This could mean that in nature more ticks may probably cling to a human protected by a given repellent than the EPA or the StiWa assay might suggest. Nevertheless, the MOB produced results that are quite similar to the tests involving human volunteers.