Why is it crucial to test anti-lice repellents?

It is difficult to stop lice propagation just by treating infested heads, since reinfections are possible just a few hours after a successful elimination of all lice from a child's head by application of an active anti-louse product. Therefore, several products have been developed that claim to have a louse repellent activity; however, definite proofs are scarce. The present study involving two louse repellents (Linicin (R) Preventive Spray, Picksan (R) NoLice) and three substances (at 10% dilution) known for their general repellency activity shows that there are much more difficulties to repel lice when compared to other insects or even ticks. Thus, it must be feared that several repellents on the market might have used a problematical test system and thus might not be as effective as they claim.