Course info

Treatment of Head Lice

Louse saliva, which contains vasodilators and anticoagulants, is injected into the host while the louse is feeding on the host’s blood.

The host usually develops sensitivity to the saliva, resulting in pruritus; however, it may take 4 to 6 weeks for the sensitivity and itching to manifest.

Head lice breath through holes in their bodies called spiracles. Ordinary shampoo causes them to close these spiracles and wait until the washing stops.

The female louse can live 3 to 4 weeks, and upon maturation she can lay up to 10 eggs per day. The eggs, which are cemented to the base of the hair shaft approximately 4mm from the scalp, are pigmented to match the hair color of the host, rendering them difficult to see. Once hatched, the eggs are easier to see, as the empty casings appear white.

Head lice simply crawl from one person’s hair into another’s when the two people’s heads are in close contact. Head lice can’t jump or fly.

Life cycle of head lice

1. Egg without an eyespot (eyespot signifies a developed nervous system).

2. Egg from time laid to time hatched.

3. Nymph to mature egg-laying louse.

From the time of laying, an egg takes an average of 4 days to develop an eyespot and 8.5 days to hatch. It takes a nymph an average of 9.7 days to develop into a mature egg-laying adult. Once mature, the female louse can mate and lay eggs in approximately 1.5 days. Eggs cannot hatch at room temperature; they need body heat to incubate. Even under optimal conditions, 10% to 30% of eggs fail to hatch.

Treatment

 

Pediculicidal/Ovicidal agents

Those chemicals destroy lice by affecting their nervous system.

Pediculicidal agents only kill lice that are alive, not lice eggs. For this reason they should be applied twice. First application kills all hatched lice. Nine days later (rather than 7 days, as some manufacturers recommend), a second application kills all newly hatched eggs not killed with the first application. If the second application is done on day 7, some eggs laid just before the first application have not yet hatched, based on the average hatching time of 8.5 days.

Ovicidal agents are able to kill both lice and lice eggs.

 

Permethrin

Permethrin is first-line treatment for head lice. This chemical causes hyperstimulation of lice’s nervous system, resulting in paralysis and death. It leaves a residue on the hair that can kill any nymphs remaining after the first application.

Avoid conditioners and hair products with silicone-base additives because they may hinder permethrin’s adherence to the hair and impair the residual effect.

Adverse effects of permethrin include pruritus, erythema and edema. It should be avoided in very young children (under 2 years old).

Application instructions:

1. Shampoo hair with non-conditioning shampoo and then towel it dry.

2. Apply to damp hair and leave on for 10 minutes.

3. Rinse off with warm water.

4. Repeat if live lice are seen after 7-10 days.

Occlusive agents

Those chemicals destroy lice by asphyxiation. Effectiveness of those agents can be affected by lice ability of closing their respiratory spiracles (usually they do so once they get submerged in water or occlusive agents).

Benzyl Alcohol + Oils

Chemicals containing benzyl alcohol combined with large size molecules like oils are good occlusive agents since benzyl alcohol is able to prevent lice from closing the spiracles allowing the vehicle (containing mineral oils) to penetrate and obstruct them, resulting in asphyxiation. 

This agent is approved for use in children older than 6 months and is Pregnancy category B.

Most common side effects include eye and local irritation and application-site anesthesia.

Application instructions:

1. Apply to dry hair, using enough to saturate hair and scalp.

2. Rinse off with water after 10 minutes.

3. Repeat after 9 days.

Treatment Counseling Points

1. Shake bottle before using and wash hands after applying.

2. Use warm (not hot) water to rinse off products over a sink (rather than in a bathtub or shower). This limits systemic exposure.

3. Be sure to treat all of the hair and scalp. Pay close attention to the back of the neck and behind the ears.

4. Family members who share a bed with the patient should be treated.

5. Pediculicidal agents ideally involve 2 treatments administered 9 days apart.

6. Using a nit comb after treatment may increase the likelihood of complete eradication.

7. Itching/burning sensations do not necessarily mean that treatment has failed. Topical corticosteroids and/or antihistamines may alleviate these adverse effects.