Nits are louse eggs. They are small (about 1/32 inch), white to cream color, and oval in shape with a distinct cap. Nits are often the first sign of a head lice infestation.
The part encircling the hair shaft is called the nit cylinder. The egg thus is mechanically attached to the hair and protected from outside influences. The eggs generally hatch in about eight to ten days at body temperature, producing nymphs that develop through 3 stages prior to becoming adult lice (Ibarra 1993). The adult form lives for 30 days, during which the female lays approximately 10 eggs per day. The female louse attaches her eggs, to the hair of the host with a glue-like, water-proof substance produced by the louse’s accessory glands (Burgess, 1995). After the nymph emerges from the egg, the nit as a hatched and empty eggshell, remains attached to the hair unless physically removed by fine combing or manual pulling. These empty egg sheaths are highly reflective and visually apparent as white oval specks along the hair shaft.
As a response to the failure of topical insecticides to be 100% ovicidal, nit removal has become an important part of head lice therapy. Such a “no-nit” policy imposes potentially prolonged absenteeism, as no present commercial product significantly facilitates the task of nit removal (Burkhart and Burkhart, 1998; Burkhart et al., 1998). Indeed, it can take up to 9 hours on average for a parent to search thoroughly and remove all nits from a child’s scalp (Burgess, 1995).
Treatments for head lice usually kill and remove lice, but dead or empty nits often remain firmly attached to the hair for several months. In addition to the cosmetic problem, nits are also responsible for false positives when children are screened for head lice (William et al. 2001). In 25-30% of children attending kindergarten or primary school, nits are found in the absence of head lice (Mumcuoglu et al. 1990, 2001). Nits are firmly attached to the human hair shaft, usually close to the scalp. In one study, approximately 14% of the nits found on children contained a dead egg. The distance of the nit from the scalp end of the hair roughly reflects the age of the nit. Nits located further away are usually older than nits close to the scalp (Mumcuoglu 1990). The solid cylinder that attaches nits to the hair shaft does not easily break or tear when forces are exerted on it. The position of the nit along the hair is important to consider. In one study, less force was needed to remove nits located further away from the scalp. This could be related to the fact that more distal nits might be older or had been moved previously as a consequence of combing/brushing the hair.
Little is known about the exact composition of the glue; however, electrophoresis of secretions from the female louse glands has demonstrated the presence of four proteins (Carter 1990). Various commercial chemical formulations have been marketed based on the belief that the sheath was made of chitin (Barat and Scaria, 1962) (DeFelice et al., 1989; Parish et al., 1989); however, there is no scientific evidence that has shown a chitinous character of the louse nit. One study did not reveal the presence of chitin-derived components in the pyrolyzate of the nit sheath, but rather that it is proteinaceous with a relatively high contribution of myristic and palmitic saturated fatty acids and other aliphatic components, e.g., alkanols. This means that the nit sheath is very similar in structure to human hair. This explains why no effective nit removal products have been developed, as any product that is able to dissolve the nit sheath or glue will also dissolve hair and/or skin.
Gratz, N. J. 1997. Human lice: their prevalence, control and resistance to insecticides: a review 1985Ð1997. WHO/ CTD/WHOPES/97.8. World Health Organization, Geneva, Switzerland
Downs, A.M.R., K. A. Stafford, G. H. Stewart, and G. C. Coles. 2000. Factors that may be inßuencing the prevalence of head lice in British school children. Ped. Dermatol. 17: 72Ð74.
Mumcuoglu, K. Y., M. Friger, I. Ioffe-Uspensky, F. Ben-Ishai, and J. Miller. 2001. Louse comb versus direct visual examination for the diagnosis of head louse infestations. Ped. Dermatol. 18: 9Ð12.
Ibarra, J. 1993. Lice (Anoplura), pp. 517Ð28. In R. P. Lane and R.W.Crosskey [eds.], Medical insects and arachnids. Chapman & Hall, London, United Kingdom.
Carter, D. 1990. An investigation of the nature and secretion of insect egg glues, with special reference to the human louse, Pediculus humanus and the cabbage white butterfly, Pieris brassicae. Ph.D. dissertation. University of Cambridge, United Kingdom.