Lice Infestation

Lice infestation is an ectoparasitic infestation caused by three louse species adapted to specific human body sites: the head louse (Pediculus humanus capitis), the body louse (Pediculus humanus corporis) and the pubic louse (Pthirus pubis). Each has a distinct mode of transmission and clinical and public health significance.

Head lice are extremely common and generally benign, though socially disruptive, particularly in school aged children.

Body lice carry greater medical importance. They are an established vector for several serious systemic infections (epidemic typhus, louse borne relapsing fever and trench fever) in populations experiencing overcrowding and inadequate access to washing facilities.

Pathophysiology and transmission

• Head lice are transmitted by direct head to head contact. The louse cannot survive for more than a short period away from the human scalp, so fomite transmission via shared combs, hats or bedding is of relatively minor practical significance.
• Body lice live and lay eggs within clothing seams rather than on the body, moving onto the skin only to feed. Infestation is strongly associated with prolonged clothing wear without washing or changing.
• This explains the association of body lice with homelessness, overcrowding, and population displacement or conflict. Their close, sustained proximity to skin for feeding underlies their capacity to act as a vector for systemic bacterial infections.
• Pubic lice are transmitted predominantly through close physical, typically sexual, contact. They attach preferentially to coarser body hair.
• Pubic lice may occasionally be transmitted to the eyelashes in adults and in children. In children this finding should prompt consideration of possible sexual abuse, given the closer contact typically required.

• Head lice infestation presents with scalp itching. Itching reflects a hypersensitivity response to louse saliva, so it may be minimal or absent, particularly in a new infestation before sensitisation has developed.
• Direct visualisation of live lice, or viable eggs (nits) firmly attached to the hair shaft close to the scalp, confirms the diagnosis. Secondary bacterial infection from scratching occasionally complicates longstanding or heavy infestation.
• Body lice infestation presents with generalised itching. On examination there are excoriations and secondary bacterial infection from scratching.
• Chronic, longstanding body lice infestation causes characteristic hyperpigmented, thickened skin changes ("vagabond's skin"). Lice and eggs are typically found within clothing seams rather than on the skin.
• Pubic lice infestation presents with pubic and perianal itching, with lice and eggs visible attached to pubic hair.
• Characteristic small blue grey macules (maculae caeruleae) are occasionally visible at feeding sites on the skin, reflecting altered blood breakdown products.
• Any patient with body lice or their systemic complications (epidemic typhus, relapsing fever, trench fever) should prompt broader assessment of social circumstances, given the strong association with homelessness, overcrowding and limited access to washing and clothing changing facilities.

• Diagnosis in all three forms is clinical. It rests on direct visualisation of live lice or, for head and pubic lice, viable eggs firmly attached to the hair shaft.
• Hair casts or debris are not firmly attached and slide freely along the hair. This distinction avoids unnecessary treatment after a false positive identification.
• Fine tooth combing of damp, conditioned hair, examining the comb after each pass under good light, detects live lice more sensitively than direct visual scalp inspection alone. It is the recommended diagnostic approach for suspected head lice.
• Body lice diagnosis requires examination of clothing seams, particularly along waistbands and other areas of close, sustained skin contact, because this species lives in clothing rather than on the body.

• Head lice are treated with topical insecticide preparations, including permethrin or malathion, applied according to the product instructions. Standard topical insecticides generally have limited ovicidal activity, so application is repeated after seven to nine days to eliminate lice hatching from surviving eggs.
• Wet combing with a fine tooth comb, performed systematically every few days over a period of two weeks, is a chemical free alternative. Its efficacy is comparable when performed correctly and consistently.
• Only individuals with confirmed active infestation (live lice identified) need treatment. Treating asymptomatic household or close contacts without confirmed infestation is not routinely recommended, given limited evidence of benefit and the risk of unnecessary insecticide exposure and resistance.
• Body lice are treated primarily through improved hygiene. This means regular washing of the body and, critically, laundering of clothing and bedding at high temperature, or discarding it where laundering is not feasible.
• Alternatively, infested clothing can be sealed in a plastic bag for a period sufficient to ensure louse and egg death, since the organism depends on clothing for survival away from skin. Topical insecticide is generally unnecessary once clothing and bedding hygiene is adequate.
• Pubic lice are treated with topical permethrin or malathion applied to all affected areas. These may extend beyond the pubic region to other coarse body hair, in some cases including the eyelashes.
• Eyelash infestation needs a more cautious approach that avoids standard insecticide preparations near the eye.
• Sexual partners need assessment and, where infestation is confirmed, concurrent treatment. Screen for other sexually transmitted infections given the shared risk context.
• Washing recently used bedding, clothing and hats at high temperature, or storing them in a sealed bag for a defined period where high temperature washing is not feasible, supports control of head and pubic lice.
• Extensive environmental decontamination is of much less practical importance for head lice, given the organism's poor survival away from the scalp. Focusing on environmental measures at the expense of correctly performed direct treatment of the affected individual is a common and avoidable management error.
• Current public health guidance generally does not recommend school exclusion for head lice. The condition is extremely common and generally benign, and exclusion does not meaningfully reduce community transmission while causing educational disruption and social stigma.