Medical News Today: What causes head lice?

Lice are parasitic insects that live on the human body, and they are highly contagious. While lice do not carry any diseases, having them can be itchy and uncomfortable.

Lice have existed for as long as humans. While some types can live elsewhere on the body, the most common species lives in the hair on the head. Head lice consume blood from the scalp, and this feeding is what makes a person itch.

While anyone can contract lice, they are particularly common in children who attend preschools or elementary schools. It is important to note that having lice is not a sign of poor hygiene.

In this article, learn more about how people get lice.

Causes

close up of head lice on a comb
Head lice will only affect humans.

Lice can spread through physical contact. They cannot fly or jump, but they can crawl from one head to another. This can happen when people’s strands of hair meet during close contact.

Researchers are unsure where lice originated, but they know that lice have affected primates for at least 25 million years, eventually spreading to humans.

Head lice only affect humans, and they will not jump onto pets or other animals.

Lice can also travel on objects that have touched the head. A person may get lice after sharing objects such as hats or towels.

However, lice cannot survive long without feeding. They must move to a new head within around 24 hours, or they will die.

Nymphs, which are young lice, can only survive for a few hours outside of a human scalp.


Risk factors

Elementary school- or preschool-aged girls are the group most likely to get lice. Lice are also common in boys of the same ages.

Some people believe that girls are more vulnerable to lice than boys because they tend to have longer hair and more head-to-head contact.

Each year, around 6–12 million cases of lice occur in children aged 3–11 in the United States.

Sharing items that have come in contact with the head can increase a person’s risk of getting lice. These items can include:

  • helmets
  • hats
  • towels
  • hairbrushes

Poor communication can also be a risk factor. If a person has lice, it is essential to inform everyone in close contact. If a child has lice, a parent or caregiver should notify the school or daycare.

Symptoms

The primary symptom of a lice infestation is the presence of live lice on a person’s head. Other signs and symptoms can include:

  • itchiness
  • scratching the head
  • skin irritation
  • a crawling sensation
  • swollen lymph nodes
  • sleeping problems

These symptoms can point to a range of issues, so it is important to confirm the presence of lice before starting treatment.

The best place to look for live lice is on the scalp behind the ears. If the lice do not seem to be living, a person may not have an active infestation.


Treatment

nit comb
People can use a nit comb to remove dead eggs and lice after pediculicide treatment.

Many effective medications can eradicate lice. A doctor or pharmacist can help a person to choose the best method of treatment.

Standard treatments include over-the-counter (OTC) liquids, shampoos, and lotions called pediculicides. These products contain medication that kills the lice.

The packaging will include instructions about how long to leave the product on the hair, and it is important to follow these guidelines.

After 12 hours, a person can use a very fine comb, known as a nit comb, to remove the dead eggs and lice.

To do this, part the hair into sections. Comb from the roots of the hair to the tips, until all the lice are gone.

In some cases, prescription treatments are necessary. These medications, such as spinosad or malathion, are much stronger than OTC varieties. Doctors only recommend them when OTC methods have not worked.

When to see a doctor

Lice are not harmful, but they are highly contagious and can be uncomfortable. If a person has symptoms of lice or notices these in a child, they should seek treatment. OTC medications are often effective.

Treatment commonly fails if a person does not remove all the lice within a few inches of the scalp. This can lead to reinfestation.

If a person has used medication correctly and the lice remain, they should ask a doctor about prescription-strength treatments. Do not use a second OTC medication if the first has failed.

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