What Are Head Lice?

Overview

Head lice are small insects that are specific to humans and live in the hair on the head. They can be visible to the naked eye and can cause itching of the scalp. They are transferred between people by head-to-head contact.

Head lice are benign and are not associated with adverse medical events. Despite this, head lice are associated with much stigma. Therefore, people find it very important to treat them quickly. Wet combing and medications are recommended methods of treatment. Some of these medications are available OTC whilst others are on prescription. It is not usually necessary to involve a doctor for head lice treatment.

It is most common in young children and their families. It is very important to note that you should not keep your child off school because they have hair lice.

Causes of head lice

Head-to-head contact with an infected individual is generally required for the transmission of head lice between people. This is because head lice can only crawl – not hop or jump.

It is much less likely that head lice will spread indirectly (e.g. via a hairbrush or clothes).¹ For example, lice on hairbrushes are much more likely to be injured or dead and only 4% of infected people had lice found on their pillowcases.^2,3

There are many misconceptions regarding the causes of head lice which may account for the stigma attached to them. Some of these misconceptions are laid out as follows:^4,5

• You cannot get head lice from pets.
• There is no evidence that school attendance affects the risks of having head lice.
• There is no evidence that head lice prefer clean or dirty hair.

Signs and symptoms of head lice

It is possible to see head lice in the hair. 

• Head lice go through different stages throughout their lifecycle in which they will look different
• Head lice eggs (or nits) appear oval-shaped (0.8 mm long) and are found close to the scalp
• Eggs may be seen hatched or unhatched, but these are not necessarily indicative of a current infestation. Nymphs hatch from the eggs and are the size of a pinhead. Adult head lice are usually 3 mm long and are used to diagnose someone with head lice

Itching of the scalp

• This occurs due to an allergic reaction to the saliva of the head lice when they bite the scalp. The allergic reaction only occurs once the scalp has been sensitized to the saliva of the head lice
• In the case of initial infestations, itching may only occur after 4-6 weeks because this is how long it takes for the scalp to become sensitized to the head lice’s saliva
• In cases of reinfestation, the scalp will become itchy within 2 days
• Itching only affects 14-36% of infected people

Anaemia⁶

• Only occurs in people with very severe head lice infestations

Head lice rash⁷

• This can be seen on the back of the neck
• The cervical nodes swell and hairs may stick together
• The head lice rash is a consequence of secondary infection by Staphylococcus aureus or streptococci

Management and treatment for head lice

It is important to treat head lice as soon as you notice them. It is also important to check those who live in the same household for head lice and start treatment on everyone with the infestation as soon as possible.

It is important to treat head lice even though they are benign and are very rarely associated with adverse medical events; partly this is due to the attached stigma.

Although there is a stigma around head lice and school children, no healthy child should be excluded from school or kept at home because they have head lice as there is no evidence that school attendance is associated with the increased likelihood of getting head lice.⁸

There are many methods described online that claim to treat head lice, despite not being proven to do so (botanical or occlusive agents and desiccants). However, wet combing and medications described below are recommended methods.

Wet combing

Wet combing aims to remove mobile head lice from the scalp to prevent more eggs from being laid.

You can buy a detection comb (fine-toothed comb) from a pharmacy which comes with instructions for wet combing. Generally, wet combing takes between 10-30 minutes depending on the length and the texture of the hair. You should do wet combing on days 1, 5, 9, and 13 and then check everyone for head lice on day 17 to check there are none left.⁹

Medications

There are a variety of medications with FDA approval for the treatment of head lice. Some of these are available over the counter (OTC) and some are only available upon prescription. It is important to discuss these with a healthcare professional to ensure you are using medication suitable for the patient.

Permethrin 1% lotion¹⁰

• This is available OTC
• This medication is available for people 2 months and older and is the preferred medication for pregnant people
• You should wash your hair without using silicone-based products or conditioners then towel dry, before applying the lotion to damp hair from the scalp working outwards
• It is important not to shampoo the hair for 48 hours after treatment because the lotion has residual activity over this period
• Retreatment on day 9 has been shown to be most effective
• But treatments on day 0, 7, 13, and 15 has also been shown to be effective based on the lifespan of the head louse

Pyrethin + Piperonyl butoxide (Shampoo)^6,10

• This is OTC
• For people 24 months and older
• Apply this to dry hair and leave on for 10 mins. Apply to the scalp then work outwards
• 20-30% of nits remain viable after treatment so a second treatment on day 9-10 is necessary
• If head lice persist, similar retreatment patterns to permethrin 1% lotion is recommended

Resistance to Permethrin 1% lotion and Pyrethin + Piperonyl Butoxide (Shampoo) has developed over the past 40 years. Resistance to these medications may require you to seek professional medical advice.¹⁰

Other FDA-approved medications.¹⁰

• Ivermectin 0.5% lotion (OTC)
• Malathion 0.5% lotion (Prescription)
• Spinosad 0.9% suspension (Prescription)
• Ivermectin 3mg tablet (Prescription)

Diagnosis

An itchy scalp is not sufficient for diagnosis. Identification of live head lice on the scalp is necessary for head lice diagnosis. Adult head lice are 3 mm long so a magnifying glass can help see them. Sometimes it is tricky to see the head lice because they crawl away from light, sit close to the scalp, and because an infected scalp usually has fewer than 10 live lice.¹⁰

Using a detection comb (fine-toothed comb available to buy at a pharmacy) can be used for easier identification of head lice compared to using a standard comb. This is because the detection comb helps to pull off the head lice as they thread through the hair.  The nape of the neck and behind the ears are also places that are generally easier to observe head lice.⁹

It is important not to confuse head lice with other head debris such as dandruff, hair casts, fungal infections, cosmetic products, or other insects.³ Also do not confuse live head lice with dead head lice because live head lice are indicative of treatment failure whilst dead head lice are not. The head lice eggs retain a viable appearance weeks after the death of all the head lice so these are also not sufficient for diagnosis.⁴

FAQs

How can I prevent head lice?

It is not possible to prevent head lice. Washing your clothes in hot water will not help, neither will keeping your child off school. You should not use medications to prevent head lice as these may irritate the scalp. Combing the hair regularly will help in early detection and therefore treatment of head lice.

How common are head lice?

A UK study of primary school children found a 2% prevalence and 37% annual incidence of head lice.¹¹  Similarly, a study in the US found 1.6% of children had head lice and 3.6% of children had nits. There is geographical variability in prevalence.¹²

Who is at risk of having head lice?

Head lice are very common in school-age children and their families. There has been evidence that the risk is higher for children with longer hair, with more siblings, and from a lower socioeconomic background.⁴

How long does head lice last?

The head lice infestation depends on how long it takes for diagnosis and treatment to begin.

Louse life cycle

• Female lice lay its eggs 6 mm away from the scalp and glue them to the hairs (nits = head lice eggs) 
• Nits take a week to hatch
• Nymphs, the size of a pinhead, emerge from the nits
• The nymphs take one week to mature into adults
• An adult female head louse can start to lay eggs 1.5 days after becoming an adult

When should I see a doctor?

Generally, seeing a doctor is unnecessary because head lice infestations are benign and not associated with adverse medical events.³

The involvement of a medical professional is more likely to occur if people use alternative methods of treatment with unproven efficacy/safety or if people improperly use medications resulting in resistance.

Summary

Head lice can be treated by wet combing or medications which are generally topical treatments. It is important to appreciate that identifying live head lice is required to diagnose someone with head lice or to observe the effectiveness of the treatment. This is because the eggs can retain their appearance for weeks even if they do not hatch. If medication does not seem to be working after reapplication, it may be necessary to consult a healthcare professional as these head lice may have developed resistance to the medication you are using, and therefore, a prescription of a different medication may be required. However, it is important to note that head lice are benign and not associated with adverse medical events.