A Better Picture of Head-Lice Transmission, Aided by Math and a Fine-Toothed Comb
As evidence mounts that head lice are developing resistance to widely used insecticides, the fight against lice is helped by a better understanding of how they spread—and how to stop them.
Controlled studies of transmission of head lice (Pediculus humanus capitis) in real-world settings aren’t exactly feasible, though, because the most accurate investigation would require somehow tracking the detailed movements of, say, a classroom full of children as well as the individual lice those children might be carrying. So, researchers must find different ways to get at the same kind of information.
That’s what Ariel Toloza, Ph.D., and colleagues in Argentina aimed for in a study published in February in the Journal of Medical Entomology. They used existing mathematical models of how lice colonies grow and spread, based on laboratory studies, and matched them up with real-world data that they could collect: analysis of lice infestations in children in selected classrooms.
What they found suggests that head lice infestations across a classroom likely require the presence of at least one severely infested child, playing the role of “superspreader.”
“This implies that superspreaders are essential in the spread of pediculosis and that the lice transference probably happens within a short period—less than a week—and at a rather large rate,” says Toloza, a research associate at CONICET, the National Scientific and Technical Research Council in Argentina.
Lab studies have shown that the ratio of adults to nymphs in a colony of head lice are somewhat predictable based on the number of female adults initially introduced and the timing of their introduction.
In the sets of classrooms (and one orphanage) Toloza and colleagues studied, they checked children for head lice, performed combing procedures, recorded the numbers of head lice adults and nymphs present on each child (if any), and returned for additional visits to repeat the process. They found that the numbers often fell close to the ratios that predicted by the lab-driven models of lice population growth developed by coauthors María Fabiana Laguna, Ph.D., and Sebastián Risau-Gusman, Ph.D., from the Bariloche Atomic Center and CONICET.
“Our study corroborates that the clustering of infestations in the same school class is probably evidence from head-lice transmission between children in that class,” Toloza says.
In the course of the study, Toloza and colleagues also found that a standard combing procedure was highly effective in removing most lice from a child in light and moderate infestations (less than 20 mobile lice per child)—removing nearly 90 percent of lice, on average, in both boys and girls. In severe infestations, the combing procedure was more effective for boys (80 percent of lice removed, on average) than for girls (53 percent removed, on average).
None the less, these results support the potential of classroom-wide removal procedures to prevent lice incidence from becoming wide-spread.
“It is important to consider the implementation of head lice control strategies that rely on the treatment of the total of individuals within a certain school class,” Toloza says. “In other words, try to apply any of the available control treatments—use combing and/or approved pediculicides—in all the kids within a week. This will remove as many lice as possible and will probably delay the presence of the superspreader state.”
Toloza notes that further research could improve the modeling of head lice transmission if it could account for how the transfer of lice between children specifically affects the growth patterns of individual infestations, “but this would necessitate data about the movement of children at school.”
This may not remain outside the realm of possibility, though. As Toloza and his co-authors note near the end of their report, “recently developed wearable proximity sensors have led to many studies on the temporal patterns of contacts between schoolchildren. Future work will focus in the incorporation of this feature in our model to make it more realistic.”
Journal of Medical Entomology