Background
Louse saliva, which contains vasodilators and anticoagulants, is injected into the host while the louse is feeding on the host's blood. The host usually develops a sensitivity to the saliva, resulting in pruritus; however, it may take 4 to 6 weeks for the sensitivity and itching to manifest.
The adult head louse is tan to grayish-white and 2 to 3 mm long (about the size of a sesame seed), with six legs. The female louse can live 3 to 4 weeks, and upon maturation she can lay up to 10 eggs per day. The eggs, which are cemented to the base of the hair shaft approximately 4 mm from the scalp, are pigmented to match the hair color of the host, rendering them difficult to see. Eggs farther than 1 cm from the scalp likely are not viable. Once hatched, the eggs are easier to see, as the empty casings appear white. The term nit denotes the empty casing; however, some experts also use the term to describe the viable egg containing the nymph.
The life cycle of head lice consists of three phases: 1) egg without an eyespot (the eyespot signifies a developed nervous system), 2) egg from time laid to time hatched, and 3) nymph (newly hatched louse) to mature egg-laying louse. From the time of laying, an egg takes an average of 4 days to develop an eyespot and 8.5 days to hatch. It takes a nymph an average of 9.7 days to develop into a mature egg-laying adult. Once mature, the female louse can mate and lay eggs in approximately 1.5 days. Eggs cannot hatch at room temperature; they need body heat to incubate. Even under optimal conditions, 10% to 30% of eggs fail to hatch.