Lice (Pediculosis)

What are lice?

Lice are six-legged, wingless, insect parasites of humans, mammals (cattle, pigs etc.), and birds (chicken and other birds). Lice are divided into two groups: lice found on mammals, which need blood for sustenance; and those found on birds, which chew on feathers and dandruff. The three most common external parasites of humans are:

1. Head lice (Pediculus humanus capitus)
2. Body lice (Pediculus humanus humunas)
3. Crab lice (Pthirus pubis)

All lice have flattened bodies to allow them ease of movement in the hair, fur, and feathers of their hosts. Infestation of a person with head and/or body lice is termed "Pediculosis" and with crab lice is called "Pthiriasis."

Pediculousis, or infestation with body and/or head lice, causes little irritation initially. After weeks of continued biting, some individuals may become sensitized and suffer an allergic reaction. Head lice infestation results in intense itching of the scalp behind the ears and base of the hairline or nape of the neck. Sores may form on the head from scratching and may become infected with bacteria. The symptoms of body lice infestation are scratch marks, hives, eczema and a red rash on shoulders and other areas covered with clothes. Pthiriasis, or pubic lice (crab lice) infestation, results in continued itching in the pubic area, a rash, and sometimes discoloration of skin from continued feeding by crab lice.

Head Lice

Head lice are found among hair on the human scalp. Commonly, they can be found on the scalp behind the ears and near the neckline at the base of the scalp. In the United States, head lice are most common among school-aged children. Head lice is more common among girls than boys because they share hats, scarves, hairbrushes, and other hair accessories more frequently.

Head lice are 2mm (male) to 3mm (female) in length. Full-grown adults are approximately the size of a sesame seed. The color of head lice tends to match the hair color of human hosts, ranging from pale tan to dark brown. All lice use the well-developed claws on their legs to grasp hair. Head lice can cling to fine hair on the head. However, they are incapable of clinging to coarser hair on the body and so they are not found on pubic hair. They are slow feeders and cling tightly to head hair while feeding.

Head lice lay oval, yellowish-white, opalescent eggs (nits) on the base of the hair shafts. Eggshells can remain attached to the hair long after the young have hatched. Head hair continuously grows, so any eggs 1 cm above the scalp surface level are mostly hatched or dead. In serious cases, secondary bacterial infections of bite sites may occur.

A common misconception is that lice infestation is a result of poor hygienic practices. In fact, it is actually believed that head lice prefer clean hair to dirty hair. However, their number increases due to a lack of combing and bathing.

Head lice may be spread through shared use of combs and hairbrushes, use of bedding by more than one person, and the sharing of hats and other contaminated accessories. Head lice can survive only for brief periods of time on bedding or upholstered furniture.

What do head lice feed on?

Head lice feed on blood from their human hosts and die in the absence of a human host (ie. they cannot live on pets). They cannot survive more than 48 hours at room temperature (23-24° C) away from their host.

What is the head lice life cycle?

Head lice undergo gradual metamorphosis (change in form) or development and have three distinct stages: eggs (nits), young (nymphs), and adults (male and female). Both young and adult head lice require blood for survival and growth, and females require it to develop and lay their eggs. Females lay 50-150 eggs in their lives.

It takes 7-10 days for nymphs to hatch from their eggs. They must feed within 24 hours of hatching. Nymphs are a miniature image of their parents and they molt three times to become an adult. Adult females are larger than the males. It takes three weeks for lice to develop from eggs to maturity and egg-laying. While living on a human host, adults have a life span of about 30 days.

How can children become infested with head lice?

Head lice infestation usually occurs during a direct head-to-head contact with a lice- infested person. In children, such contact can occur at playgrounds, camps, slumber parties and even during some classroom activities. Children can also get head lice by sharing infested cloths, hats, scarves, combs and hair ribbons. Children in nurseries or day care centers can get head lice from beds, pillows, carpeting or from playing with stuffed animals that previously came in contact with an infested child.

What can I do in my home to help reduce the infestation of head lice?

Consult your physician if you think you or your children have lice. The physician may prescribe a medicated shampoo as a treatment and a schedule of its use. Comb wet hair with a lice comb (extremely fine-toothed comb available in drug stores) to remove nymphs and adult lice. Do not use shampoos that contain Lindane. Lindane use is described as hazardous by the Agency for Toxic Substances and Disease registry of the U.S. Department of Health and Health and Human Services. Your physician or a pharmacist may recommend over-the-counter products (pediculicides). Do not dry your hair with a hair dryer after application of a pediculicide because some treatments contain chemicals that are flammable. It may be necessary to repeat the treatment 7-10 days after the first application.

In addition to the treatment of an infested individual, all bedding, towels, and clothing from the infested individual should be cleaned with soap and hot water and placed in a dryer for at least 20 minutes to help kill any remaining lice. Dry-clean all clothes that need to be dry-cleaned. Seal the infested individual's stuffed toys in a plastic bag and leave them for 10 days to allow all lice to die of starvation. Dispose of or soak combs and hairbrushes in rubbing alcohol or the medicated shampoo used to kill lice. Throw out any hair accessories, such as hair elastics and ribbons. Thoroughly vacuum carpets and upholstered furniture. Pets cannot become infested with head and body lice, so no precaution is required.

Also see the UFT's head lice page for further information. 

Body Lice

Body lice (Pediculus humanus humans) are larger than head lice, with longer bodies and antennae. They are mostly found in areas of clothing that continuously come in contact with the body, such as underwear, fork of trousers, armpits, waistline, and near neck and shoulders. If the temperature becomes high near the skin, layering in the clothing allows lice to move to the upper layers of clothing. As a result, body lice can survive starvation longer (3.5 days) at room temperature (23 to 24°C) than head lice. Dirty clothes and unclean bodies provide a stable environment for body lice. Body lice cannot withstand high humidity. In hot weather, they move to the upper layers of clothing, resulting in increased transmission.

What do body lice feed on?

Body lice feed on human blood. Most feeding takes place in areas where the clothing comes in contact with the body.

What is the body lice life cycle?

Two days after blood feeding, body lice deposit eggs in the seams of clothing that are in contact with the host's body. Female body lice lay 50-150 eggs in their lifetimes, but some females lay 275-300 eggs. The time required for incubation of eggs and emergence of nymphs is directly dependent on incubation temperature, which is dependent on the proximity of the eggs to the body. Eggs take 5-7 days to hatch. Eggs only hatch at temperature 23°C-38°C. In the body lice's life cycle, eggs are the most resistant stage to changing environmental temperatures.

Nymphs are miniature replicas of their parents. They start feeding frequently (day or night) just after hatching. The nymph stage lasts 16-18 days, including three molts. Females start laying eggs 1-2 days after maturity. Body lice live from 30-40 days and survive best at the body temperature of humans. A four to five degree rise in temperature is fatal for lice. They prefer cold environments where clothing layers provide a humid to dry gradient. They also flourish best in hot dry climates where a temperature gradient exists in clothing.

How do you become infested with body lice?

Body lice are transferred between people by close interpersonal contact. For example, in close, crowded conditions, such as in subways and buses, high temperature and humidity close to the body forces body lice to crawl out of the inner layers of clothing to outer layers and may facilitate transfer from one person to the next. An increase in body temperature from a high fever or the death of a host also results in emigration of the body lice from sick people to healthy people.

Can body lice transmit diseases?

Body lice initially cause little irritation. Continuous infestation with body lice results in hardened and deeply pigmented skin (Vagabond's disease or Morbus errorum). Body lice serve as hosts of Rickettsia and Spirochetes and transmit diseases that are caused by these pathogens (organisms that cause diseases). The three diseases transmitted by body lice are epidemic typhus, trench fever, and epidemic relapsing fever. If you would like further information about these diseases please visit the web sites listed at the end of this document.

Pubic lice

Where do you find pubic lice?

Pubic lice are most commonly found in the pubic and peri-anal areas, and sometimes can infest eyebrows, eyelashes, mustaches and beards. Pubic lice are 1.5 - 2mm long and are as wide as they are long. Their middle and hind legs are very stout and give them the appearance of a crab and thus are also called "crab lice". The third pair of legs is equipped with a claw that helps lice hang on to coarse pubic hair. These lice can sometimes infest dogs. Pubic lice remain attached to the same place for a long time during blood feeding, resulting in accumulation of their feces and matting of the pubic hair. Skin discoloration occurs in the place of their continued attachment. Pubic lice survive for a shorter time away from their hosts compared to head and body lice. Pubic lice are not disease carriers.

What do pubic lice feed on?

Pubic lice feed on human blood and remains attached at the sites of their feeding for a long period of time and can be considered as almost stationary.

What is the pubic lice life cycle?

Pubic lice lay less than 30 eggs per female. The eggs hatch after seven to eight days, when the nymphs emerge and molt over a 13-17 day period. After the third molt, they reach maturity. The total life span of pubic lice is over one month.

How do you become infested with pubic lice?

Sexual contact is the primary mode of transmission of pubic lice. Infestation can also occur by sleeping close to an infested person. Infestation does NOT occur as a result of using the same toilet as an infested individual. Condoms do not protect from pubic lice.

How do you control pubic lice infestation?

The easiest method to control lice infestation is to remove all hair from the pubic region. Physicians may also offer alternative treatment options. Most products used for louse control can be used to control pubic lice infestation (follow manufacturer's instructions). However, it is recommended to consult a physician before choosing a treatment product.