Lice are small, parasitic insects that infest humans. These ectoparasites must live on a host and feed exclusively on human blood to survive. Human infestations are caused by three distinct types: head, body, and pubic lice. Head lice (Pediculus humanus capitis) and body lice (Pediculus humanus humanus) are the two most common types. Understanding their significant biological and behavioral differences is necessary for effective detection and successful eradication.
Distinct Habitats and Identification
Head lice live exclusively on the human scalp and hair, where warmth and ready access to blood meals are optimal. They range in size from about 2 to 3 millimeters. The female head louse cements her eggs, known as nits, firmly to individual hair shafts, typically within a few millimeters of the scalp.
Body lice primarily inhabit clothing and bedding, only moving to the skin surface to feed multiple times a day. They are generally slightly larger than head lice, measuring between 2.3 and 3.6 millimeters. A definitive sign of infestation is the presence of eggs and lice clustered in the seams and folds of clothing, especially around the waist and armpits. Body lice nits are laid directly on the fibers of the fabric rather than on the hair.
Modes of Transmission
Head lice are most commonly transmitted through direct, prolonged head-to-head contact, such as occurs during play among children. Transfer through shared personal items like combs, hats, or hair accessories is possible but less frequent. This is because the insects cannot jump or fly and generally die within one to two days off a host. Head lice infestations are not related to poor hygiene or socioeconomic status.
Transmission of body lice is strongly linked to environmental factors and sanitation. They are typically spread through close physical contact or through sharing infested clothing, towels, or bedding. Body lice infestations are most often found in populations living in crowded conditions where bathing and changing clothes are infrequent, such as in homeless shelters or refugee camps.
Treatment Protocols and Eradication
Treating head lice focuses on the hair and scalp using topical treatments called pediculicides. Over-the-counter options often contain pyrethrins or permethrin, which are applied to the hair according to package directions. After treatment, mechanical removal of nits and dead lice with a fine-toothed nit comb is recommended to ensure complete clearance of the infestation.
A second application of the topical product is often necessary seven to nine days after the initial treatment. This kills any newly hatched lice that survived the first application as nits. Washing bedding and clothing that have been in contact with the head in the two days prior to treatment in hot water and drying on high heat can prevent re-infestation.
Treating a body louse infestation primarily involves environmental sanitation and improved personal hygiene. Since the lice and their eggs reside in clothing, the most effective step is to remove all infested garments and wash them thoroughly. Clothing and bedding must be machine washed in hot, soapy water (at least 130 degrees Fahrenheit), followed by machine drying on high heat for a minimum of 20 minutes. Items that cannot be washed can be dry-cleaned or sealed in a plastic bag for two weeks to starve the lice.
For body lice, treating the human host with topical medications is often secondary to environmental cleaning. Simple bathing with soap and water may be sufficient, but if not, a topical pediculicide can be applied to the affected skin areas.
Health Risks Associated With Each Type
Head lice are generally not considered a public health hazard and are not known to transmit any human diseases. The primary issues are intense itching and irritation caused by an allergic reaction to the louse bites. Excessive scratching can break the skin, leading to secondary bacterial infections such as impetigo.
Body lice carry a much more serious health risk because they are known vectors for certain bacterial diseases. These insects can transmit pathogens responsible for diseases like epidemic typhus, louse-borne relapsing fever, and trench fever. While these louse-borne diseases are rare in developed countries, outbreaks can occur in situations of poor sanitation, such as war or mass displacement. Furthermore, long-term body louse infestation can cause skin changes, including thickening and discoloration, sometimes called “vagabond’s disease.”