Here’s some lousy news for parents of itchy-headed kids: Lice eggs can take 2 weeks to hatch in human hair, making standard 7-day delousing treatments ineffective in some cases. New research shows that if conditions are right, the eggs, called nits, can sit dormant during treatment, only to pop later and reinfest the scalp. A third application may be necessary after 14 days to eliminate any slow-hatching nits, they say.

Lice don’t lay their eggs directly on skin—instead, they deposit nits at the base of hair shafts. The timing of louse hatching on a human head is difficult to track because adult lice lay eggs continuously, obscuring earlier hatches, and the effectiveness of traditional insecticides on eggs is variable. Previous estimates of how long nits remain viable did range up to 14 days, but much of that work dated to the 1920s and 1930s, when researchers reared body lice inside boxes strapped to a person’s arm or ankle. More recent work relies on head lice raised in lab incubators, which are more stable than the wide range of temperatures and cleanliness found on a human scalp.

For a more reliable estimate, medical entomologist Ian Burgess of Insect Research & Development Ltd. in Cambridgeshire, U.K., analyzed data from 20 previous studies of treatments that kill lice through physical means, such as lotions that suffocate the insects, but do not kill eggs. They didn’t include insecticide treatments because lice across the United Kingdom have developed resistance to standard drugs, Burgess says, leading more doctors to try a brute-force approach that does not rely on insecticides.

The data from 1895 patients revealed cases in which technicians found newly hatched louse nymphs on the 14th day after treatment began, even though the second scalp application had occurred 7 days before. “Some [nymphs] had emerged only an hour or two before checking,” Burgess says. To rule out cases where re-infestation from another child had occurred, or where a few adult lice had escaped treatment, he excluded cases with lice that appeared older than the number of days since the last treatment. Nearly two dozen cases remained—enough to verify that a handful of nits can outlast standard treatment protocols, Burgess reports in an upcoming issue of Medical and Veterinary Entomology.

Although the treatments themselves may play a role, a person's scalp temperature is likely to be the most important factor in how long it takes eggs to hatch, Burgess says. Location and hairstyle matters, too: Lice develop faster at warmer temperatures, so they will hatch more quickly when laid on the warm, thick hair at the nape of the neck than on the thinner hair on top and in front of the scalp.

The analysis is the most rigorous yet to quantify louse hatching times, says Rich Pollack, a public health entomologist at Harvard University. “It should be considered by those who are trying to make a management treatment decision,” Pollack says, observing that just a small number of patients are likely to need a third dose.

New oral insecticides may render the question of hatching times moot, Pollack notes. Those drugs, now available by prescription in the United States, are up to 85% effective at killing lice and eggs with one dose, sparing parents from dousing a squirming child's scalp multiple times.