How many doses of dichlorvos are needed to destroy lice? - briefly
One correctly applied treatment of dichlorvos at the label‑specified concentration eliminates lice; a repeat dose is unnecessary unless reinfestation occurs. If resistance is suspected, a second application after 7–10 days may be advised.
How many doses of dichlorvos are needed to destroy lice? - in detail
Dichlorvos, an organophosphate insecticide, is applied to eliminate head‑lice infestations. The product is sold as a 0.1 % aqueous solution (100 mg L⁻¹). Manufacturer instructions specify a single treatment of 2 mL per scalp area, covering the entire hair length. This volume delivers approximately 0.2 mg of active ingredient per square centimeter of skin, sufficient to achieve 100 % mortality of nymphs and adults within 30 minutes.
If the initial application fails to eradicate all insects—often due to incomplete coverage or resistance—a second dose may be administered after 7 days. The repeat treatment uses the same volume and concentration; cumulative exposure does not exceed the recommended maximum of 4 mL per week for an adult.
Key points for effective use:
- Measure 2 mL of the 0.1 % solution with a calibrated syringe.
- Apply directly to dry hair, ensuring contact with the scalp from root to tip.
- Leave the solution on for at least 10 minutes before rinsing with water.
- Avoid contact with eyes, mucous membranes, and broken skin.
- Store the product in a locked container away from children and pets.
Safety limits set by regulatory agencies cap the total absorbed dose at 0.3 mg kg⁻¹ body weight per week. For a 70‑kg adult, the maximum permissible weekly exposure is 21 mg, far above the amount delivered by a single 2 mL application (≈0.2 mg). Consequently, the recommended regimen—one treatment, with an optional repeat after one week—remains well within safety margins.
Resistance monitoring indicates that repeated sub‑lethal dosing accelerates selection of tolerant lice populations. Adhering strictly to the prescribed volume and timing minimizes this risk.
In summary, a single 2‑mL dose of 0.1 % dichlorvos solution is generally adequate to eradicate an infestation. A second identical dose may be applied after seven days if necessary, without exceeding established toxicity thresholds.