Why don't lice go away after treatment? - briefly
Surviving eggs that the treatment fails to eradicate hatch after application, re‑infesting the host. Improper usage or resistance to the active ingredient can also allow lice to persist.
Why don't lice go away after treatment? - in detail
Lice may persist after an attempt to eradicate them for several reasons that involve the biology of the parasite, the characteristics of the product used, and the manner in which the treatment is applied.
The insect’s life cycle creates a window of vulnerability. Adult females lay eggs (nits) attached to hair shafts within 24 hours of mating. Eggs hatch in 7–10 days, and the emerging nymphs become reproductive adults after another 7–10 days. Most pediculicides act only on live insects; they do not dissolve the protective shell of the egg. If a treatment is applied before all eggs have hatched, newly emerged nymphs will survive and repopulate the host.
Chemical resistance further reduces efficacy. Repeated exposure to the same class of insecticide, such as pyrethroids, selects for resistant strains. Genetic mutations in the lice’s nervous system decrease the binding affinity of the insecticide, rendering standard doses ineffective. Resistance has been documented worldwide and is a major cause of treatment failure.
Incorrect application compromises results. Common errors include:
- Using insufficient quantity of product.
- Failing to cover the entire scalp and hair length.
- Rinsing hair too soon after the prescribed exposure time.
- Skipping the recommended second application, which is needed to target lice that hatch from eggs after the first treatment.
Reinfestation from untreated contacts also contributes to persistence. Lice spread easily through head-to-head contact, shared combs, hats, or bedding. If only one individual is treated while others remain untreated, the parasite quickly returns.
Environmental reservoirs can harbor viable lice or eggs. Furniture, car seats, and pillowcases retain nits that may detach and reattach to hair. Without thorough cleaning of these items—vacuuming, washing at ≥ 60 °C, or sealing in plastic bags for two weeks—recolonization is likely.
Following the initial treatment, a systematic follow-up is essential. Recommended steps are:
- Perform a wet-comb inspection 7 days after treatment; remove any live lice or nits.
- Repeat the treatment after 9–10 days to eliminate newly hatched nymphs.
- Continue weekly inspections for at least four weeks to confirm eradication.
In summary, lice survive post‑treatment because eggs are resistant to most chemicals, resistance to insecticides is widespread, user error reduces product potency, untreated contacts reintroduce parasites, and contaminated environments act as reservoirs. Effective control requires a comprehensive approach that addresses each of these factors.