Spatial and kinetic factors for the transfer of head lice (Pediculus capitis) between hairs.

Although the global increase in pediculosis is well known, little is known about the exact nature of head lice (Pediculus capitis) transmission. Several mechanisms have been proposed such as head-to-head and fomite transmission, but some contention remains concerning the primary transmission route. This study investigated spatial and kinetic factors influencing the dynamics of hair-to-hair transfer to clarify further how head lice transmit from head to head. Forty-eight factorial experimental trials, with 10 replicates each, were conducted using 480 freshly caught P. capitis from primary school children. In the trials, each louse was placed on a stationary suspended hair or a mobile hair and was presented with mobile or stationary hairs for transmission. All hair passes involved contact between the uninhabited hair and the lice. Hairs without a louse were presented dorsally, laterally, and ventrally to the louse. They were also passed from head to tail or from tail to head and were moved at speeds of 8 m and 4 m per min. The proportion of P. capitis transmission was highly dependent on the specific setting. The tail-to-head direction, slow movement, and a parallel direction all proved favorable for transmission. The highest transfer proportion of P. capitis (85%) was observed in the setting where the presented hair was laterally slow moving in a parallel way from tail to head. No transmission at all was observed under an angle of 90 degrees. Hair-to-hair P. capitis transmission occurred more frequently when hairs were in particular physical and kinetic relationships. This suggests that head lice are less likely to take advantage of many proposed fomite transmission scenarios and are most likely to rely on head-to-head contact for transmission.