- Pediculosis, or louse infestation, is a common clinical issue that is of limited morbidity but is of significant public health concern, especially in school-age children.
- Humans may be parasitized by three louse types: body lice, head lice, and pubic lice.
- Growing resistance of lice to permethrin, and pyrethrins has led to the development of alternative treatment options that include ivermectin, benzyl alcohol, and spinosad.
Head lice are found worldwide, and are most prevalent amongst elementary school children. Girls may be more susceptible to infection, possibly because they have longer hairstyles. Incidence of pediculosis is seasonal and peaks during the warmer months of the year and during the time corresponding to the start of the school year. Transmission occurs via head-to-head contact. Head lice are obligate parasites that feed on human blood every 6 hours and die in several days without a host. Visible to the naked eye, they appear grayish in color, have six legs, and can reach up to 6 mm in length. The life cycle of the head louse is approximately 21 days in length and has three stages: egg, nymph (or instar), and adult louse. During its reproductive stage, a female louse can produce an average of five eggs (also known as nits) daily. Eggs are attached to the hair shaft close to the scalp with concretions secreted by the louse. Incubation time is 5-10 days. Viable eggs are yellow/brown in color and found close to the scalp, whereas older hatched eggs appear white and are found farther (>1 cm) from the scalp surface.
Body lice infestations are associated with poor living conditions and poverty. Body lice tend to live in clothing and not directly on the body. They are often found along seam lines and in bedding. Body lice may reach 2.5-4 mm in length. They may be associated with the spread of relapsing fever (Borrelia recurrentis), epidemic typhus (Rickettsia prowazekii), and trench fever (Bartonella quintana).
Pubic lice are smaller, about 1 mm in length, and on magnification resemble a crab. In addition to infesting hairs of the pubic area, they may infest other body hairs including eyelashes, axillae, the perianal region, chest, scalp, and beard hairs. Transmission of pubic lice most commonly occurs during sexual contact, and rarely may transfer through the sharing of towels or underwear. Patients with pubic lice should be counseled on prevention of other sexually transmitted infections (STI) and on safe sex practices. Testing for STIs, including HIV, should be offered. Pubic lice diagnosed in a child should raise suspicion of sexual abuse.
Though not considered a serious threat to human health, louse infestation is contagious and can become a public health concern, especially when infestations affect school-age children, their caretakers, and household members. Widespread and localized resistance to treatments has been observed in recent years, and as a result therapeutic strategies may differ based on geographic location.
There is limited morbidity associated with lice. The primary complaint is intense pruritus occurring 4-6 weeks following the onset of infestation, which is primarily due to a hypersensitivity reaction to components of the louse saliva. Severe pruritus may occasionally result in excoriations and can be complicated by secondary skin infections such as impetigo.
Patients with head lice usually complain of severe scalp itching, although asymptomatic cases have been reported. The lice, eggs, or empty egg casings are not always easily visualized. Important clues of infestation include excoriated erythematous papules (shown below) and maculae ceruleae (slate-blue macules indicative of louse bites) on areas of skin that come in contact with the hair, most commonly seen on the posterior neck.
Diagnosis is confirmed when live lice or nymphs are visualized. Evaluation of the hair shaft under magnification may help distinguish between pseudonits (i.e., hair casts), scale (i.e., seborrheic dermatitis) and nits. When active infestation is highly suspected but no lice or nits are seen, the “wet combing” technique, in which wet conditioned hair is combed through with a metal nit comb, can improve detection. Associated complications of head lice include secondary bacterial infections, fever, lymphadenopathy, severe crusting and eczematization.
[Head louse. Reproduced with the permission of the University of Nebraska-Lincoln Extension. Managing head lice safely (018). UNL Extension-Lancaster County. http://lancaster.unl.edu/pest/lice/headlice018.shtml. Accessed November 2, 2012.]
In patients infested with body lice, the lice are usually not found on the body but may be seen on clothing. Clinical presentation consists of excoriations, erythematous papules, crusts and hyperpigmented macules in the affected areas. Secondary infections and lymphadenopathy may occur. To confirm diagnosis, patients can be instructed to look for lice, blood, or insect feces on bed sheets; however, in the absence of the louse itself, these findings are non-specific, as they may be present in the setting of other infestations, such as bed bugs.
Pubic lice are very small, and when suspected, diagnosis may be aided with the use of a dermatoscope. Lice can typically be found attached to the hair shaft and may sometimes mimic hemorrhagic crusts or scales.
Pubic lice may also affect the eyelashes. [Pubic lice on eyelashes. Wikipedia.org. http://en.wikipedia.org/wiki/Pubic_lice. Accessed November 2, 2012.]
The diagnosis of lice infestation is straightforward. Important diagnostic considerations are hair shaft casts (circumferential keratinous bands that easily slide up and down the hair shaft), seborrheic dermatitis (loose, greasy scales that are very loosely adherent to the hair), and white piedra (superficial fungal infection of the hair shaft).
First-line therapy: The first-line therapy for pediculosis depends on the type of infestation. In general, removal of the lice and nits from the environment and clothing (body lice) or hair (head or pubic lice) is the treatment priority.
Over-the-counter (OTC) Treatments
Common first-line treatments for head lice include the over-the-counter neurotoxic pediculicides such as pyrethrin and pyrethroids; however, the efficacy of these agents has decreased significantly in the last decade due to widespread resistance. Recent studies have reported efficacy rates as low as just 25%.
- Pyrethrin, a natural extract from chrysanthemums, can be applied to wet hair in the form of shampoo, lotion, gel, cream, or mousse for 10 minutes and then rinsed. Since it is not ovicidal (does not kill eggs), the application must be repeated in nine days in order to kill newly hatched lice. Pyrethrins should not be used in patients allergic to chrysanthemum, feverfew, liverwort, lettuce, artichoke, ragweed, endive, echinacea, daisy, dandelion or magnolia.
- Pyrethroids are synthetically derived chemicals that are similar in structure to pyrethrin but more stable. Permethrin, the most commonly used form, is available over-the-counter as a 1% liquid or lotion. Permethrin is applied to shampooed, non-conditioned, towel dried hair for 10 minutes then rinsed. This can be repeated daily for 10 days if active infestation persists, as this medication is not ovicidal.
- Malathion 0.5% is an organophosphate insecticide that works as an irreversible cholinesterase inhibitor. It is both pediculicidal and ovicidal that shows good efficacy with just a single treatment. It can be applied be applied to dry hair for 8-12 hours and then rinsed. Some studies have shown that it can be highly effective after just one or two 20-minute applications. Malathion is approved in children older than 6 years of age; however, patients should be warned that the formulation is malodorous, highly flammable and can cause chemical burns or irritation. Malathion should be applied in a well-ventilated, cool space, and adult supervision is recommended when treating young children. Resistance has been reported in some parts of Europe but not in the USA. If live lice persist, the treatment should be repeated after 7-9 days.
- Benzyl alcohol 5% is approved for use in children over the age of 6 months. The mechanism of action of benzyl alcohol is likely linked to asphyxiation of the louse. This prescribed medication is applied to dry hair for 10 minutes and then rinsed. Benzyl alcohol is pediculicidal but not ovicidal and treatment must be repeated after seven days.
- Spinosad is a product derived from the bacterium Saccharopolyspora spinosa that causes muscle spasms and paralysis in lice. A suspension containing benzyl alcohol is approved for the treatment of head lice in patients older than 6 months of age. It is applied to dry hair and scalp for 10 minutes and rinsed. Only one treatment is typically needed, but if live lice are seen after seven days, treatment may be repeated.
- Lindane 1% is available in a shampoo form that is applied to dry hair for four minutes. Application is typically limited to one-time use because of its toxicity to the central nervous system. Lindane carries a black box warning because of its potent neurotoxicity and reports of death with prolonged use. Use should be limited to patients who have failed other treatments; it should be used with caution in infants, children, and elderly patients weighing less than 50 kg, and in those with a history of seizures. Lindane is banned in the state of California, USA.
- Topical ivermectin was approved for the treatment of head lice in children as young as 6 months of age. One single application of 0.5% lotion is well tolerated and effective for treatment. Oral ivermectin is an oral anthelmintic agent, approved for the treatment of certain parasites. It may be prescribed off-label for the treatment of lice at a single dose of 200–400 μg/kg. Treatment may require 1-2 additional doses repeated after 7 days. It should only be used in children who weigh more than 15 kg and is not recommended for women who are pregnant or breastfeeding.
- Dimethicone (silicone oil) works by causing osmotic stress and death in lice. Dimethicone 4% liquid gel or oil is available in the UK. It is applied twice within a week for 15 minutes or overnight. This formulation is not currently available in the USA.
- Hand-picking head lice and nits may be performed for those who do not want to apply chemicals, although this method is time consuming.
- A heated air device has been cleared by the US FDA for treating lice with hot air.
- Shaving the head will cure head lice though this is rarely necessary. It can be considered, but only if acceptable to the child and parent.
- Hats and possibly infested clothes and bed linens should be machine-washed and dried. Items that cannot be washed should be stored in plastic bags for 2 weeks.
- All combs and brushes should be soaked in hot water for 5 minutes.
- Rooms and furniture should be vacuumed to prevent lice spread.
- Topical anti-lice treatment is usually not required for body lice, as these mites do not live on the skin or hair. The presence of nits or lice on the patient’s hair represents a second lice infestation (head or pubic lice) and should be treated accordingly.
- Topical steroids and oral antihistamines may be required for days to weeks to control pruritus. If extensive excoriation is noted, the patient should be carefully examined or tested for bacterial superinfection and treated with systemic antibiotics, if necessary.
- The patient must bathe well with soap and water. All infested clothing and bedding should be disposed of in sealed biohazard bags. Any remaining bedding and clothes should be washed in hot water and dried with heat, and the seams should be hot ironed. An alternative strategy is to seal infested clothing and bedding in an airtight plastic bag for 2 weeks.
- Body lice typically occur in persons with poor hygiene. These persons may also be at risk for nutritional deficiencies and tuberculosis. Their social situation should be investigated.
- First-line treatment is permethrin 5% cream applied overnight to all hairy areas with re-treatment in one week.
- Other insecticides (usually permethrin 1%, pyrethrin, malathion or ivermectin) may be use as alternative treatments.
- For eyelash pediculosis, petrolatum is applied 3–5 times a day for 10 days. The patient should mechanically remove as many nits and lice as possible.
- Examine and treat all infected contacts.
- Examine the patient for other sexually transmitted infections.
- Children with pubic lice should be referred to a child protective service/agency.
- Pyrethrins (RID, Triple X, A-200) should be applied to the affected genital area and washed off after 10 minutes.
- In cases where topical treatment has failed or is not feasible, weight-dosed oral sulfamethoxazole/trimethoprim given daily for 14 days is effective.
- It is advisable to schedule a follow-up evaluation by a physician or other health care provider at the end of the treatment period to assess for evidence of ongoing louse infestation whether additional or alternative treatments are needed.
- Head lice can be resistant to lindane, pyrethrin and pyrethroid-based agents. In Europe, resistance to malathion has also been reported. If 2 treatments with any of these agents are ineffective, lice resistance is confirmed and an alternative agent should be used. Recurrence may also stem from ongoing exposure to a contagious individual or fomite.
- The extent of pubic lice infection depends upon how far hair extends from the pubic area. In hairy men, infestation may extend down the legs, up onto the chest, and into the axillae. The whole affected area must be treated. Systemic treatment with ivermectin may be considered.
- The presence of pubic lice in children should raise suspicion of child sexual abuse and should be investigated further.
- 15-year-old healthy female high school student
- No significant past medical history
- Review of systems is noncontributory
- Presents for management of a 6-week history of an intensely pruritic eruption on the scalp and neck
- Recent history of a louse infestation outbreak on school athletic team
- Denies recent travel, new cosmetics for hair or skin
- Healthy appearing female teenager
- Scattered erythematous macules and excoriated papules on the neck; several lice and egg casings are noted in the scalp hair
- No evidence of superinfection is present
- Complete skin and hair examination does not reveal additional sites of infestation
- Diagnosis: head lice
- Recommend permethrin 1% lotion applied on days 0, 7, and 15 for 10 minutes to clean, non-conditioned, towel-dried hair
- Topical triamcinolone 0.1% lotion applied twice daily to the scalp and posterior neck to alleviate pruritus
- Follow-up in 9 days
- Several lice, eggs, and nits noted, raising concern for permethrin-resistance
- Malathion 0.5% is prescribed (day 0 and repeat application on day 9)
- Follow-up in 2 weeks (no evidence of lice infestation is found)
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