Key Takeaways

  • A typical head lice infestation involves a small number of lice, usually fewer than 10, although some individuals may have more.
  • It’s possible to have a lice infestation for a prolonged period, even a month or more, without knowing it, as symptoms may not appear immediately.
  • While lice themselves do not transmit diseases, prolonged infestations can lead to scalp irritation, potential bacterial infections, and, rarely, iron deficiency anemia.

A lice infestation (pediculosis) can make you feel like your head is crawling with a million lice. But the number of lice usually found on a person’s scalp is actually very small.

It can be hard to see head lice, let alone count them. But estimating the size and duration of an infestation can help you determine the best treatment for you or your child.

In this article, we provide information about the number of lice and nits (lice eggs) you might see during an outbreak. We also review the life cycle of lice so you can figure out how long they may have been on your head.

Adult lice range in size from 2 to 4 millimeters, about the size of a sesame seed. In addition to being tiny, they move quickly and shy away from light. These characteristics make it hard to find lice crawling on the scalp.

You can generally expect to find fewer than 10 lice on your or your child’s head during an infestation. But some people with an infestation can host 20 lice or more on their scalps at any given time.

Lice have short lives but multiply quickly:

  • Adult lice can live for around 30 days on the scalp.
  • After mating, a mature female louse starts to lay eggs roughly 2 days later. She can lay up to 10 eggs each day for most of her life.
  • Nits hatch and become nymphs (young lice) in 7 to 12 days.
  • Female nymphs grow by shedding their skin (molting) several times until they mature. This takes up to 2 weeks.
  • Once they mature into adults, the females are ready to mate and lay new nits.

Is it possible to have only one louse in your hair?

It only takes one mature female to start a lice infestation.

It’s possible to have lice without nits or nits without living lice. However, do not assume you only have one louse on your head because that’s all you found.

If you see one louse, act quickly. Taking this step can help ensure that an infestation doesn’t take hold.

It’s possible to have lice for a month or longer without knowing it.

Lice feed on human blood. While they’re feeding, they simultaneously inject their saliva into the scalp. The itching you feel results from sensitization to compounds in the saliva.

Sensitization is an allergic reaction that builds up over time. Many people become sensitized to lice saliva within 4 to 6 weeks. You can have lice during this early period without experiencing symptoms. If you don’t notice lice or nits, you may not know you have a growing infestation on your scalp.

Some people never become sensitized to lice saliva. If you do not become sensitized, you will not experience itching. If the ongoing infestation remains unchecked, a bacterial infection may be your first clue that you have lice.

You can roughly calculate how long you’ve had lice through observation.

If you see no live lice and very few nits, you may have an early infestation of 2 weeks or less.

The location of the nits is key. If they’re flush to the scalp, they’re newly laid. If they’re one-quarter inch or more away from the scalp, they have already hatched, and your infestation is of a longer duration.

If you see nits and nymphs but no adult lice, you may have had lice for roughly 2 weeks. Adult lice are the size of sesame seeds. Newly formed nymphs are the size of poppy seeds and very hard to see.

If you see adult lice in addition to nits and nymphs, you may have an ongoing infestation of 2 weeks or more.

Lice do not carry disease. However, they can irritate or damage the scalp over time. If you have untreated head lice, a bacterial infection may develop, requiring antibiotics.

In rare severe cases, head lice and nits can lead to iron deficiency anemia due to blood loss.

An unchecked infestation can also spread to your eyebrows and eyelashes, although this is rare. Head lice will not spread to body hair.

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How many eggs does one head louse lay?

Mature female lice can lay up to 10 eggs per day and typically live for 3 to 4 weeks.

In addition to laying eggs, the female louse secretes a cement-like glue that adheres each nit to a hair follicle. This glue hardens like a shell over the egg, protecting it until it is ready to hatch.

How many lice are in one egg?

If lice eggs hatch, each has the potential to become one louse. Some nits die before they hatch.

How long do head lice live?

According to the Centers for Disease Control and Prevention (CDC), lice spend about 7 days as nymphs before maturing into adults. Adult lice can then live for up to 30 days on your scalp. They die after 1 to 2 days without a food source.

Can lice enter your ear or brain?

Lice like to live and lay their eggs on the scalp. They often gather along the hairline, in the back of the head, around the ears, and at the nape of the neck. They do not crawl into the ears and will not enter your brain.

Adult lice do not lay their eggs in the ears, so you don’t have to worry about that.

Lice can live on your scalp for a month or longer before you realize it. Most infestations comprise 10 or fewer adult lice and multiple nits. To avoid a more severe infestation, treat your or your child’s scalp as soon as you see or suspect lice.