Head lice are parasitic insects that commonly live close to the scalp. While they don’t spread disease, they’re an itchy, uncomfortable, and spreadable nuisance that can be difficult to get rid of.

If you or a loved one is experiencing a lice infestation, you may also be wondering how long these little pests have been living on your head.

Read on to learn some of the ways you may be able to tell based on the lice’s size and appearance. We’ll focus mostly on lice that live on the scalp, but there are also different lice types that can live on body hair and pubic hair.

There are a few ways you can go about determining how long you may have had lice.

Here are some signs you can use to tell how long you’ve had a lice infestation.

Symptoms

You don’t usually start experiencing scalp itching when lice first arrive in your hair.

You may not experience itching until about 4 to 6 weeks after lice exposure. This is because the lice take time to multiply and cause symptoms of itchiness.

The itching reaction is usually due to your skin becoming sensitized to the saliva that lice release when feeding. If you’ve had lice before, you’ll likely have symptoms faster because your body is already sensitized to lice saliva.

You can usually expect this itching reaction to take place about 2 days after you get the infestation.

It’s also possible that you may never experience symptoms associated with lice. In this case, you’ll have to identify them by sight.

Appearance

It can be hard for someone with an untrained eye to identify lice. You can easily think dandruff, a scab, or residue from hair products are nits.

For your best chance at seeing them, use the following:

  • a very fine-toothed nit comb
  • a magnifying glass
  • piece of paper
  • a location with plenty of light

You may find this task easier on wet hair that has conditioner in it for detangling purposes.

Lice don’t like the light and are very small — two factors that make them hard to identify. But there are ways you may be able to tell how long you’ve had them based on where and how you see them.

Infestation timeline

  • Few nits: less than 2 weeks. An adult louse climbs onto your hair and lays about 6 to 10 nits a day, which take about 9 days to hatch. So if you look on the scalp and see no visible adult lice and several small nits, it’s likely that you’ve caught lice in the earlier stages and had them for less than 2 weeks.
  • Nits and nymphs: 1.5 to 2 weeks. If you see nits and small, moving lice, you’ve likely had lice for 1.5 to 2 weeks. This is because you aren’t seeing a lot of adult lice but are still seeing small, hatched nymphs along with lots more nits than a person who’d only been affected for a few days.
  • Nits, nymphs, and adult lice: 2 weeks or longer. If you’re seeing a mixture of sizes of lice, you may have had an infestation for at least 2 weeks. If you have symptoms like itching along with a variety of lice stages, you’ve likely been living with lice for four to six weeks or possibly longer.
  • Nits more than 1/4 inch from the scalp: old infestation. Only see small nits over a quarter inch away from the scalp? It’s probably an old infestation. You may have treated your lice, and remnants are moving down the scalp. Because lice eggs typically hatch close to the scalp, seeing nits further down your hair can indicate that the infestation is inactive.

Some approaches you can take to treat lice at home include the following:

  • Use a medicated lice treatment shampoo as directed. If you have very long hair, you may need two shampoos to sufficiently treat your head. These often come with small nit combs you can use to manually remove nits.
  • Use a nit comb to remove as many nits as possible. Repeat every 3 to 4 days until you no longer see any nits or lice.
  • Repeat the shampoo about a week after your first application. This can “catch” any remaining lice in their next life cycle where you can ideally eliminate them for good.

Lice have evolved over time to become resistant to some treatments that used to work well in the past.

Some treatments that aren’t as effective as they used to be include pyrethrin, permethrin, malathion, or phenothrin. These medications are still commonly found in over-the-counter lice treatments, so look out for these ingredients when you shop for treatment.

Keep in mind there are a lot of “home remedies” out there that are rumored to kill lice, including mayonnaise, kerosene, oils, and apple cider vinegar.

But these aren’t proven to work and can be very harmful to the scalp (especially kerosene). Stick to approved medicated treatments instead.

In addition to treating the lice on your hair

In addition to treating the hair, you should also take the following steps for personal items that may have come in contact with lice:

  • Wash any clothing, sheets, towels, or other similar items worn by an affected person in hot water of at least 128.3°F (53.5°C).
  • Place any items that can’t be laundered in sealed bags and leave them in the bag for at least 2 weeks, or take them for dry-cleaning.
  • Vacuum all living areas thoroughly to remove hairs that may have retained nits.
  • Place combs and brushes in hot water that’s reached at least 130°F (54.4°C) for 5 to 10 minutes to kill off any remaining lice or nits.

You may also want to check with the school or daycare if you or your child has had lice. These organizations may have policies regarding head lice and when a young person can return to school after recognition and treatment.

In the absence of such policies, most people don’t have to isolate themselves, provided they’re treating the lice and engaging in preventive transmission methods.

If lice persist even after home treatments, it’s time to see a doctor. You may need prescription treatments or professional removal to take the nits away.

If your skin gets very irritated from the lice, the itching can lead to a potential infection.

Swelling, redness, and pain in the areas where you have lice may indicate that you’re experiencing or are at risk for an infection and may need antibiotic treatments.

Some examples of prescription-strength medical treatments for lice include:

  • benzyl alcohol lotion
  • malathion lotion
  • spinosad topical suspension
  • lindane shampoo (a second-line prescription treatment)

Prescription lice treatments tend to have stronger medications that can irritate the skin. You should talk with your doctor about any potential side effects and how to minimize them before using prescription treatments.

Your doctor may also be able to recommend additional methods to treat and remove lice depending on your symptoms.

Here are some preventive steps you can take to ensure you don’t get or transmit lice:

  • Refrain from hair-to-hair contact at school, on the playground, while playing sports, or at sleepovers.
  • Refrain from sharing personal items that come in contact with hair, such as combs, hats, scarves, helmets, ribbons, or barrettes.
  • Don’t use any personal items that haven’t been washed if a person who had known lice used them, such as pillows or towels.

Because they’re parasites, head lice require a host to live. They usually won’t survive more than 2 days after falling off a person. Washing and isolating items can help ensure lice won’t live beyond your scalp.

Lice belong to the insect category Pediculus humanus capitis. As a parasite, they live on a human host and feed on their blood.

People transmit lice by direct contact because the lice can’t fly or hop. Children most commonly transmit them by head-to-head contact when playing.

While less likely, it’s also possible for a person to spread lice by contact with personal items (like a comb or brush) or clothing.

Lice appear on the hair in one of three forms:

  • Eggs/nits. Nits are small, oval-shaped lice eggs that a female louse lays usually near the scalp. They’re often white, yellow, or clear in appearance, and they’re easily mistaken for dandruff or flakes of hair products, if visible. According to the Centers for Disease Control and Prevention (CDC), nits take between 8 and 9 days to hatch once a female lays these eggs.
  • Nymphs. Nymph are nits that has just hatched. They’re smaller than an adult louse and spend about 9 to 12 days feeding on blood and growing into an adult after hatching.
  • Adults. An adult louse has matured beyond about 12 days. At its full growth, an adult louse is usually no larger than a sesame seed. They’re usually tan, gray, or white in appearance. Adult females are typically larger than adult males. Most adult lice don’t live for more than 30 days on the scalp.

Head lice have small, hook-like claws on the ends of their legs that make them very hard to remove from the hair shaft.

Because lice are so small (and typically on the back of your head), they can be very difficult to detect. Some symptoms that may indicate lice include:

  • frequent tickling feeling in the hair
  • problems sleeping, as lice move mostly at night
  • rash on the back of the head
  • sores that develop over time due to scratching
  • unexplained itchy scalp, especially close to the nape of the neck

Lice don’t carry diseases, but that doesn’t make them any less bothersome. They aren’t typically the result of poor hygiene or health, but rather because you or a loved one came into contact with someone who had them.

If you can see nits or lice, the amount and symptoms may indicate to you how long you’ve been infested. This can help you track where you might have gotten the lice and indicate potential treatment challenges.

If your lice seem to be persistent or you’re concerned about how to treat them, talk with your doctor.