Head lice are tiny wingless insects that live and breed in human hair. They feed on blood from the scalp and cause itching and discomfort, but do not transmit disease. Head lice are most common in children ages 3-11 years old. It is estimated that 6-12 million infestations occur each year in the United States, most commonly spread by close head-to-head contact. Fortunately, there are several effective treatment options available to get rid of head lice. However, after treatment it is common to wonder – how long can head lice live after treatment? Here we will discuss the life cycle of head lice, treatment options, and how long lice can survive after treatment.
Life Cycle of Head Lice
The life cycle of head lice has 3 stages:
Eggs – Female lice lay eggs called nits, cementing them to the base of hair shafts near the scalp. Nits are small (about the size of a knot in thread) and oval-shaped. They are often yellow to white in color.
Nymphs – After 7-10 days, nits hatch into baby lice called nymphs. Nymphs look like small adult lice, but only reach full size after 7-10 more days. Nymphs feed on blood and mature into adults.
Adults – Mature adult lice mate and reproduce, with females laying 3-6 nits per day. Adults live close to the scalp and have a lifespan of about 30 days.
So the full life cycle from nit to adult takes about 3 weeks. Understanding this life cycle helps target treatment and follow up.
Treatment Options
There are a variety of treatment options available to kill lice and eliminate infestations:
Over-the-Counter Medicated Shampoos
These products contain insecticides that kill lice, usually pyrethrins or permethrin. Follow instructions carefully, using on dry hair and leaving on for 10+ minutes before rinsing out. Usually a second treatment is recommended 7-10 days later to kill any newly hatched nits.
Prescription Medicated Shampoos
Prescription shampoos have higher doses of insecticides like spinosad or ivermectin. They may be more effective than OTC products and require only 1 application in some cases.
Non-Insecticide Treatments
These may involve shampoos/rinses with ingredients like benzyl alcohol, mineral oil, or silicone that work by suffocating lice. Supportive treatments like nit combing, heat, or oral medication are also sometimes used.
Combination Approaches
Using medicated shampoo together with combing and nit removal can increase effectiveness. Rotating treatments from different chemical classes helps prevent resistance.
How Long Can Lice Live After Treatment?
The goal of lice treatment is to kill live (crawling) lice. However, no product kills 100% of nits. So how long can lice live after treatment?
Eggs/Nits
– Nits are very resistant to treatment and can survive chemical shampoos. up to 25% may remain viable after medicated shampoo.
– Nits can survive off the body for up to 10 days. On the hair shaft they can survive for 7-10 days until hatching.
– It takes at least 7 days for nits to start hatching after treatment. Removal by combing or picking nits out can help prevent surviving nits from hatching.
Nymphs
– Nymphs that emerge 7-10 days after treatment are vulnerable because they have to bite to survive.
– Nymphs that emerge later than 10 days after treatment are unlikely to have survived most chemical treatments.
Adult Lice
– No adult lice should survive correctly applied medicated shampoo treatment. They should die within minutes.
– Any lice found alive more than 1 day after treatment likely hatched from surviving nits. Retreatment is recommended.
– Adult lice cannot live longer than 1-2 days off the human scalp due to lack of feeding.
Here is a summary:
| Type of Lice | How Long Can Live After Treatment |
|---|---|
| Eggs/Nits | 7-10 days before hatching; can remain on hair up to 10 days after treatment |
| Nymphs | Unlikely to survive more than 10 days after treatment |
| Adults | Should die within minutes; unlikely to survive more than 1-2 days |
Follow up After Treatment
Following treatment, follow up is important:
– Comb hair thoroughly with a nit comb 2-3 times per week for 2-3 weeks after treatment to catch any surviving nits and emerging nymphs before they reach adulthood.
– Check the scalp carefully for signs of reinfestation, which would indicate treatment failure. Presence of adult lice or new nits beginning 1 week after treatment means repeat treatment is needed.
– Most guidelines recommend routine retreatment 7-10 days after the initial treatment. This targets newly hatched nymphs before they mature.
– Avoid head-to-head contact until cleared. Household disinfection of shared items can also help prevent spread.
– Continue checking for lice/nits 1-2 times per week for 1 month after end of treatment, as reinfestation can occur if lice are still circulating among contacts.
Conclusion
While no treatment completely eliminates all lice, medicated shampoos are very effective at killing live lice. Any lice found alive 1 or more days after treatment have likely hatched from nits that survived. Eggs are more resistant than adults, but proper follow up with repeat treatment and nit combing can prevent surviving nits from reaching maturity. With diligent treatment and combing, most infestations can be cleared within 1-2 weeks. Monitoring closely for 1 month after treatment ensures lice are fully eradicated.