Why infest the loved ones – inherent human behaviour indicates former mutualism with head lice

SUMMARYHead lice transmit to new hosts when people lean their heads together. Humans frequently touch their heads to express friendship or love, while this behaviour is absent in apes. We hypothesize that this behaviour was adaptive because it enabled people to acquire head lice infestations as early as possible to provoke an immune response effective against both head lice and body lice throughout the subsequent periods of their life. This cross-immunity could provide some defence against the body-louse-borne lethal diseases like epidemic typhus, trench fever, relapsing fever and the classical plague. Thus the human ‘touching heads’ behaviour probably acts as an inherent and unconscious ‘vaccination’ against body lice to reduce the threat exposed by the pathogens they may transmit. Recently, the eradication of body-louse-borne diseases rendered the transmission of head lice a maladaptive, though still widespread, behaviour in developed societies.

Susceptibility of the sheep body louse [Bovicola ovis (Schrank), Phthiraptera: Trichodectidae] to temephos and concentration stability during sheep dipping with a commercial temephos product

Laboratory bioassays were used to measure the susceptibility of sheep lice [Bovicola ovis (Schrank)] strains that were resistant to pyrethroid or benzoylphenyl urea insect growth regulator insecticides to the organophosphate temephos, the most commonly used wet dipping insecticide. Compared with the reference susceptible strain, only the pyrethroid-resistant ‘Rowena’ strain displayed low level (2.2×) tolerance of temephos. Lice exposed to the minimum lethal concentration of temephos began to be affected within 6–7 h and all had responded to the insecticide within 24 h. In a dipping trial with sheep, the working concentration of temephos in a 5000-L plunge dip operating according to constant replenishment principles remained close to the nominal application rate of 350 mg/L. The ease with which temephos dip concentration was maintained simplified the dipping process. If typical of similar field populations, the absence, or low level of cross-resistance to temephos reported here for insect growth regulator-resistant and pyrethroid-resistant lice, respectively, should make wool producers confident that thorough dipping with temephos according to product label directions should be effective in controlling lice on their sheep.