A Second Visit to the Clinic
1Bram Oduya's younger sister came home from summer camp with head lice. Their father bought a lice shampoo at the pharmacy and followed the directions exactly, but a week later the lice were back. At the follow-up visit, the children's doctor, Dr. Esperanza Lindholm, told the family this was common. "Lice have nothing to do with being dirty," she said. "They spread when heads touch. And in our county, this shampoo often does not work anymore." Bram, who was taking eighth-grade science, asked why.
2Head lice are tiny wingless insects that live only on the human scalp and feed on small amounts of blood. A female glues several eggs a day to hairs near the scalp. The eggs hatch in about a week, and the young lice can lay eggs of their own about two weeks after that, so a new generation appears roughly every three weeks. Young lice inherit their gene versions from their parents.
3The shampoo contains an insecticide that acts on a protein in the nerves of lice and kills them. In many lice, though, the gene for that nerve protein is a changed version. Lice with the changed version build a slightly different protein, and the insecticide affects it much less.
4Dr. Lindholm showed Bram data from the county hospital lab, which has tested lice collected at the clinic for years, with permission from each family. In one test, lab workers place live lice on filter paper soaked with the insecticide, then count how many are still alive one hour later. Table 1 shows the results by gene version. Each test began with live, active lice, and every louse was handled by trained lab staff, not by families. The lab also tested lice from a family that lives on a remote farm and had never used any lice treatment. Some of those lice already carried the changed version.
5Every few years the lab also checks what share of the lice brought to the clinic carry the changed version, and the clinic records how often the shampoo cleared a child's lice in one treatment (Table 2). The same shampoo was the most common treatment in the county during all of those years.