Lice Checks

lice cdc

Image: Two lice viewed under an electron microscope. Note the claws used to grasp onto individual hairs. Credit: CDC

What are head lice?

The head louse, or Pediculus humanus capitis, is a parasitic insect that can be found on the head, eyebrows, and eyelashes of people. Head lice feed on human blood several times a day and live close to the human scalp. Head lice are not known to spread disease.

Treatment Frequently Asked Questions (FAQs)

Treatment for head lice may involve:

  • Over-the-counter products. Shampoos containing pyrethrin (Rid, others) or permethrin (Nix) are usually the first option used to combat lice infestations. These work best if you follow the directions very closely.In some geographical locations, lice have grown resistant to the ingredients in over-the-counter lice treatments. If over-the-counter preparations don't work, your doctor can prescribe shampoos or lotions that contain different ingredients.
  • Oral prescription medication. Although it was originally developed for treating worm diseases, oral ivermectin effectively treats lice with two doses, eight days apart. This drug is typically used to treat infestations that haven't responded to other treatments.Children must weigh at least 33 pounds (15 kilograms) to take oral ivermectin. Side effects may include nausea and vomiting.
  • Topical prescription medications. 

Will laundering kill head lice?

Washing, soaking, or drying items at a temperature greater than 130°F can kill both head lice and nits. Dry cleaning also kills head lice and nits. Only items that have been in contact with the head of the infested person in the 48 hours before treatment should be considered for cleaning.

Although freezing temperatures can kill head lice and nits, several days may be necessary depending on temperature and humidity; freezing is rarely (if ever) needed as a means for treating head lice.

Is mayonnaise effective for treating head lice?

CDC does not have clear scientific evidence to determine if suffocation of head lice with mayonnaise, olive oil, margarine, butter, or similar substances is an effective form of treatment.

If the treatment for head lice doesn’t seem to be working, does this mean the lice are resistant and I need a different treatment?

The following are several common reasons why treatment for head lice may fail sometimes:

  1. Misdiagnosis. The symptoms are not caused by an active head lice infestation.
  2. Applying the treatment to hair that has been washed with conditioning shampoo or rinsed with hair conditioner. Conditioners can act as a barrier that keeps the head lice medicine from adhering to the hair shafts; this can reduce the effectiveness of the treatment.
  3. Not following carefully the instructions for the treatment that is used. Some examples of this include not applying a second treatment if instructed to do so, or retreating too soon after the first treatment before all the nits are hatched and the newly hatched head lice can be killed. Another reason is retreating too late after new eggs have already been deposited.
  4. Resistance of the head lice to the treatment used. The head lice may have become resistant to the treatment. If the treatment used does not kill the head lice, your health care provider and pharmacist can help you be sure the treatment was used correctly and may recommend a completely different product if they think the head lice are resistant to the first treatment.
  5. Reinfestation. The person was treated successfully and the lice were eliminated, but then the person becomes infested again by lice spread from another infested person. Sometimes reshampooing the hair too soon (less than 2 days) after correctly applying and removing permethin can reduce or eliminate any residual (continued) killing effect on the lice.

Source for the information and picture: CDC Head Lice website and Mayo Clinic. 

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