Pediculosis and Pthiriasis (Lice Infestation) Clinical Presentation

Updated: Feb 15, 2023
  • Author: Lyn C C Guenther, MD, FRCPC, FAAD; Chief Editor: Michael Stuart Bronze, MD  more...
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Patients may come to the attention of a health care provider after discovering lice or nits. Parents and teachers typically make the initial diagnosis of head louse infestation. In the case of head lice, a school nurse usually discovers infestation (routine nit inspections by school nurses are standard in many parts of the United States), or a generic letter is sent home to parents indicating that they should inspect their children for lice, and concerned parents bring their children to their health care provider or the local emergency department.

Pruritus is the most common symptom of infestation. Children often have trouble sleeping because of intense pruritus at night. Areas affected in head louse infestation include the scalp, the back of the neck, and postauricular areas. Scratching can cause secondary infection with bacterial sores. However, lice infestation may be asymptomatic, particularly if it is the first infestation and if the infestation is light. [4]

Patients infested with P corporis experience nocturnal pruritus, particularly in the axillary, truncal, and groin regions, when the lice move from the clothing to the body to feed. The investigating physician should inquire about the patient's socioeconomic status and living conditions, as body louse infestation generally affects people of low socioeconomic status.

Adults infested with P pubis are usually sexually active and have groin and body hair involvement. Involvement with pruritus of the groin, axillae, eyelashes, or eyebrows can help differentiate P pubis infestation from head or body louse infestation. Parents of children infested with P pubis on the eyelids and/or eyebrows should be questioned about also being infested because the parents are usually the source of infestation.

Patients may describe associated features such as papules or wheals, indicating bite reactions.


Physical Examination

A diagnosis of any type of pediculosis rests on the observation of eggs (nits), nymphs, or mature lice. Definitive diagnosis of active infestation requires detection of live lice. Nits are not diagnostic of an active infestation; however, if they are within 6 mm of the scalp, active infestation is probable. Viable nits may be found 8 or more inches from the scalp in warm climates. [6] Nymphs and mature lice, although unable to hop or jump, can move rapidly through dry hair. Wetting the hair and using a fine-tooth "bug-busting" comb is useful to dislodge eggs and to remove live lice/nymphs. The use of a magnifying glass and knowing where to look for lice (based on the biology of each species) assists in the diagnosis.

Head lice can move rapidly, up to 23 cm/min. [6] A helpful technique is to fasten a piece of transparent adhesive tape to the infested areas. Lice stick to the tape, which then becomes a convenient coverslip for a microscopic slide.

Mature lice are 3-4 mm long (approximately the size of a sesame seed), with an elongated body, 3 pairs of legs, and narrow anterior mouthparts. Wide crab-like bodies and claws distinguish pubic lice. Nits are approximately 1 mm in length, transparent, and flask-like in appearance. (See images of lice below.)

The head louse, Pediculus humanus capitis, has an The head louse, Pediculus humanus capitis, has an elongated body and narrow anterior mouthparts. Body lice look similar but lay their eggs (nits) on clothing fibers instead of hair fibers.
The pubic louse, Pthirus pubis, is identified by i The pubic louse, Pthirus pubis, is identified by its wide crablike body.
Nit on a hair. Note the thin, translucent cement s Nit on a hair. Note the thin, translucent cement surrounding the hair shaft. Photo courtesy of David Shum, MD Western University, London Ontario.

P humanus capitis

Manifestations of head louse infestation include scalp pruritus and, rarely, occipital lymphadenopathy and impetigo. Examination of the scalp reveals excoriations, dark specks of louse feces, nits, and adult lice. The heaviest infestation typically is in the retroauricular scalp. [4, 11] For the diagnosis of P capitis infestation, the use of a louse comb is more efficient than direct visual examination of the scalp. [32]

Pruritus commonly leads to excoriations, secondary bacterial infection, and enlargement of the posterior auricular and cervical nodes. [11] A generalized exanthema (pediculid) similar to a viral exanthema or pityriasis rosea rarely accompanies louse infestation. [6]

If excoriations are present, secondary infection (ie, impetigo) should be excluded and treated, if present. Bite reactions manifesting as pruritic red papules and/or wheals may be present, depending on the length of time since the blood meal.

Uncommonly, the hair of patients who are heavily infested and untreated is tangled with exudates, predisposing the area to fungal infection. This results in a malodorous mass known as a plica polonica. Numerous lice and nits are found under the matted hair mass.


Nits can be differentiated from dried hairspray and hair casts by attempting to separate the nit from the hair; hair casts and dried hairspray separate easily, while nits remain securely attached. If the physician remains unsure, a Wood lamp examination can be performed. Live nits are fluorescent white when illuminated with a Wood lamp; empty nits are fluorescent gray.

Eggs depend on body warmth to incubate, so nits are attached to the hair shafts just above the level of the scalp. Since human scalp hair grows at a rate of approximately 10 mm/month (0.37 mm/day), the distance of nits from the scalp can be used to estimate the duration of infestation. Nits found several millimeters from the scalp are nonviable empty egg cases. They indicate chronic infestation.

P humanus corporis

Physical examination findings in body louse infestation include multiple lesions from bites. Uninfected bites present as erythematous papules, 2-4 mm in diameter, with an erythematous base. Bites may be located anywhere on the body but tend to be concentrated in the axillae, groin, and trunk (ie, areas most often covered by clothing). Thus, the face, feet, and arms are not commonly affected. Body lice tend to avoid the scalp, except at the margins.

The finding of maculae cerulea is believed to be pathognomonic for infestation with lice. Maculae cerulea are blue-gray macules, which are actually a discoloration of the skin due to the insect's bite. Enzymes in the louse saliva are believed to induce the breakdown of human bilirubin to biliverdin, causing the change in skin color associated with maculae cerulea. An allergic reaction to louse bites may result in intense itching. Secondary infections due to excoriations may occur.

The diagnosis of body lice depends on the close examination of the patient's clothing for crawling lice and nits. The inner seams of clothing worn on the axillae and groin regions are common sites of residence. Occasionally, a body louse may be seen feeding or crawling on the skin. The number of body lice per host is usually approximately 10, although as many as 1000 lice can be present in clothing. [11]

Body louse infestation is also known as vagabond disease, and individuals who have an infestation for many years can develop a condition termed vagabond skin. The skin becomes thickened and darkened after years of bites and subsequent rubbing and excoriations. [10]

It is also important to examine for systemic illness that may be related to one of the vector-borne diseases associated with P corporis (see Complications).

P pubis

The primary symptom in patients with pubic lice is pruritus in the affected areas. Another clinical feature of pubic louse infestation is the presence of pathognomonic maculae cerulea (bluish-gray macules) secondary to bites. [6] Crusts and pinpoint blood staining may also be noted on underwear. [6]

Pubic hair is the most common site. Pubic lice and nits generally are plainly visible throughout the pubic hair. Because of the less-mobile nature of pubic lice, they are more likely to be found on affected areas clasping onto the hairs near the skin's surface.

The infestation may spread to hair around the anus, abdomen, axillae, chest, upper arms, eyebrows, and eyelashes. [33] Rarely, facial hair (ie, beard, moustache, eyebrows, eyelashes) is a site of infestation. [33] Scalp involvement is rare and is usually confined to the marginal areas. In adults, eyelash involvement in the absence of genital involvement is rare. In prepubertal children, the eyebrows and eyelashes are the typical sites of infestation. Eyelash nits are a manifestation of pubic louse infestation, not head louse infestation.

In children, P pubis infestation may be acquired from an infested parent or during sexual exposure or abuse; the child should be examined for signs of abuse. [13]

Excoriations are common. Inguinal lymphadenopathy and axillary lymphadenopathy have also been reported with pubic louse infestation.



There is no evidence indicating that any species of louse has the ability to transmit HIV. However, lice may carry S aureus and group A Streptococcus pyogenes on their surface and transmit these coagulase-positive pathogens to humans.

Pubic louse infestation is usually spread as a sexually transmitted disease (STD). Thirty percent of infested individuals may have other concurrent STDs (eg, HIV infection, syphilis, gonorrhea, chlamydia, herpes, genital warts). [6]

Louse-borne disease is a potential problem whenever body lice spread through a population. The body louse, P humanus corporis, is a known vector of 3 major bacterial diseases, all of which have caused epidemics: louse-borne typhus, trench fever, and louse-borne relapsing fever. Evidence shows that some infectious organisms are altered by their arthropod vector and that disease manifestations may be vector-specific. For example, bartonellosis spread by a louse has different manifestations from bartonellosis spread by a flea or biting fly. [34]

In a 2018 study of pathogens infecting 524 body lice collected from homeless persons in Algeria, several emerging pathogenic bacteria were found. B quintana, which causes trench fever, was found in 13.35% of lice specimens. In addition, Coxiella burnetii was found in 10.52% of specimens, Anaplasma phagocytophilum in 0.76%, and Acinetobacter species (Acinetobacter baumannii, Acinetobacter johnsonii, Acinetobacter bereziniae, Acinetobacter nosocomialis, Acinetobacter variabilis) in 46.94%. Neither Rickettsia prowazekii nor B recurrentis was detected. [35]

Louse-borne typhus

The intracellular pathogen R prowazekii causes typhus. Typhus fever epidemics have consistently been related to times when overcrowded conditions and body louse infestations were prevalent. For example, mass migration, refugee camps, and times of war have been linked to body louse infestations and secondary epidemics of typhus. Human reservoirs of typhus also exist. Following natural disasters, body lice have the potential to spread rapidly throughout the population, causing great epidemics similar to those seen during World War I. Typhus is now considered to be a rare disease, with most epidemic reports in the US now being reported in association with exposure to the nests of flying squirrels. (see 

The illness begins with a high fever and progresses over hours to days to malaise, backache, headache, and myalgia. A petechial rash appears approximately on day 4, beginning in the flank and axillary regions and quickly spreading to the trunk and extremities. By the second week, the fever begins to wane, profuse sweating occurs, and convalescence ensues. CNS involvement during this period places the patient at high risk of mortality. In the event of epidemic typhus, the treatment of choice is doxycycline. 

Trench fever

The extracellular pathogen B quintana causes trench fever and infective endocarditis. Although rarely fatal, this disease has been the cause of many epidemics, including trench fever during World War I. [36] Infection in humans results from autoinoculation of louse feces into abraded or scratched skin. The infection has a 10- to 30-day latency period and results in a fever similar to that of typhus, with headache, myalgia, and pain in the back and the legs.

Louse-borne relapsing fever

The spirochete B recurrentis causes relapsing fever. This disease is highly fatal in malnourished persons. Although not common in North America, epidemics have been described during the last few decades in Asia, South America, Africa, and Europe.

Human infection with this spirochete occurs only when a crushed louse comes into contact with an abrasion or if it is ingested. The bacteria replicate in the louse hemolymph, not in the gut; therefore, no transmission occurs through the salivary glands or via the feces. The bacterial infection causes a high fever, headache, dizziness, and myalgia. Rash and sweating also appear and wane approximately on day 5. As the name indicates, this fever often returns several times.