Description and life history continued…

All lice develop through three life stages: egg, nymph, and adult. Nymphs and adults actively move away from light sources. Given the constant body temperature of a human host, these three species of lice can reproduce year-round. About 10-12 generations are produced each year.

Head louse

The head louse is the most common lice species the United States, and infestations tend to be most prevalent in schoolchildren. Adults are tan or gray and measure 2-3 mm (~1/10 inch) long. Although adult head lice are visible to the naked eye, using a magnifying hand lens may aid in spotting an infestation. The nymphs resemble adults but are smaller, about the size of a pinhead. Eggs are white or yellow, oval, and measure about 0.8 mm (< 1/20 inch) long. Eggs deposited beyond this region on the hair shaft are either nonviable or have already hatched. The eggs are yellow to brown or white and are more easily seen than adults or nymphs when scouting on dark hair. Given their small size, eggs are often misidentified as dandruff, droplets of hair spray, or dirt particles.

Eggs are most frequently laid at night on the base of a hair shaft, about 6.5 mm (1/4 inch) from the scalp. After hatching about one week later, the egg case remains attached to the hair shaft, and the newly emerged nymphs begin feeding immediately. The nymphs undergo three molts before developing into adults, which can take up to 10 days. A mature female head louse can lay up to eight eggs per day and lives up to 30 days on a human host. Adults feed several times a day and will die within 48 hours in the absence of a host. The most common way to transmit head lice is through head-to-head contact with an infested individual. Transmission is also possible through sharing infested clothing or other belongings such as a bed, couch, or pillow.

Body louse

The body louse is morphologically indistinguishable from the head louse and is a vector of three life-threatening human diseases: louse-borne typhus (Rickettsia prowazekii), trench fever (Rochalimaea quintana), and louse-borne relapsing fever (Borellia recurrentis). Currently, these diseases are a major public health concern in areas with poor hygiene due to war, social disruption, poverty, or a lack of public health outreach. In the United States, incidences of trench fever primarily occur in homeless individuals. Laboratory research suggests that the body louse can also vector other emerging or re-emerging human pathogens, such as Acinetobacter baumannii and Yersinia pestis, which is the causative agent of bubonic plague.

Unlike the head louse and P. pubis, the body louse can lay eggs on hair and clothing. A single female lays about eight eggs per day and up to 300 eggs throughout her life. Eggs hatch in about a week under favorable environmental conditions. Nymphs actively feed on blood and complete their development in a little under two weeks. Body lice can also live up to seven days without a host, which allows them to persist in untreated clothing. The total lifespan of an individual body louse ranges from one to three months. Body lice are transmitted through close person-to-person contact, especially among individuals living in crowded conditions.

Pthirus pubis

Adults of P. pubis are about 1-2 mm (~1/20 inch) long and have a more rounded body shape in comparison with the head louse and body louse. The rear two pairs of legs have robust claws for clinging to coarse body hair. The eggs of P. pubis are roughly 0.7 mm long and yellow in color. Nymphs resemble adults but are smaller.

This species tends to reside at the base of densely packed hair and requires a human host to survive. After removal from a host, adults will typically die within 48 hours. A single female can lay approximately 30 eggs, singly, on hair shafts in the pubic, armpit, or facial region over a three- or four-week lifespan. About one week later, nymphs emerge from eggs and develop through three molts before reaching adulthood. Adults are mobile and tend to move several millimeters or more each day, though they are generally less active than head and body lice. This species is most frequently transmitted from person to person via sexual contact, although the presence of P. pubis in bedding and clothing may also play a minor role in transmission.

Symptoms

Head louse

The major condition caused by infestations of head lice is scalp pruritus (i.e., itchy scalp). This may be accompanied by a tickling sensation in the hair caused by actively moving lice and difficulty sleeping. Scratching the affected area can break the skin and increase susceptibility to secondary bacterial infections.

Body louse

Infestations of body lice can cause intense itching and an allergic reaction that results in a rash. Heavily bitten areas of the skin may be thickened and discolored when infestations have been present for extended periods of time. This may be especially prevalent around the waist, groin, and upper thighs. Signs of typhus, relapsing fever, and trench fever include a combination of fever and chills, malaise, meningitis, joint pain, headache, rapid breathing, aching muscles, rash, cough, vomiting, and confusion.

Pthirus pubis

Infestations of P. pubis cause intense itching and purple lesions on the affected areas of skin. Itching sensations may not occur until two to six weeks after exposure, and scratching can break the skin and increase the likelihood of secondary bacterial infections. In the pubic region, bites inflicted by P. pubis can cause blue macules to occur on the thighs and lower abdomen. Infestations in the pubic region can result in P. pubis excrement on underwear, appearing as dark red or brown spots. Some symptoms of P. pubis infestation in the eyelashes include lice and eggs appearing as transparent debris in the field of vision, burning sensations, lesions from louse bites, loss of eyelashes and eyebrows, and conjunctivitis.

Monitoring

Head louse

Using a magnifying lens and a fine-toothed comb will greatly improve the chances of diagnosing an infestation since nymphs and adults are very small and tend to actively avoid light. It is worth noting that eggs remain attached to hair even after they have hatched. When eggs are found on hairs more than 6.5 mm (1/4 inch) from the scalp and adults are absent, this indicates that the infestation is no longer active, and treatment is unnecessary.

Body louse

Infestations of body lice are diagnosed by scouting clothing for the presence of eggs and actively moving lice. Body lice can also be observed crawling on skin. A magnifying lens may be helpful in spotting eggs and lice on clothing.

Pthirus pubis

A magnifying lens can be used to aid in scouting pubic or facial hair for P. pubis eggs and lice. The presence of eggs is generally enough evidence that an infestation is present since adults and nymphs of P. pubis are less active than head and body lice. Individuals with P. pubis infestations should consider being tested for other sexually transmitted diseases.

Management

Head lice

All persons sharing a household with a host should be checked and undergo treatment. If non-insecticidal management is desired for head lice, a blow dryer can be used on wet or dry hair to kill all life stages. Using a skin care product and blow-drying hair on high heat for 15-20 minutes every seven days for three weeks will kill eggs and nymphs.

Insecticides designed for lice management (called pediculicides) are often effective against adults and nymphs but rarely kill eggs. Those that have weak or no ovicidal activity should be applied routinely, while those displaying high toxicity toward eggs only need to be reapplied if live lice are observed several days after the initial treatment. Such reapplications are most effective after all eggs have hatched and before new eggs are produced. Over the counter medicated shampoos are the most common treatment for persons over two years of age. If medicated shampoos are ineffective, doctors may prescribe additional treatment products. In addition, hats, scarves, pillowcases, bedding, clothing, and towels used or worn by the infested individual can be machine washed and dried under at temperatures greater than 53.5°C (128 °F). Sharing of any items worn by the infested person should be avoided. Furthermore, complete control requires the use of a lice comb every other day for up to two weeks. Between each pass through the hair, the comb should be dipped in hot, soapy water to kill any lice or eggs.

Body lice

Body lice thrive in crowded conditions with poor hygienic practices. Changing into clean clothes and washing infested clothing and bedding is recommended at least once a week. Hot water with a minimum temperature of 130 °F and a hot dryer cycle is recommended. Items that cannot be washed can be dry-cleaned or sealed in a plastic bag for two weeks. Sharing clothing, beds, bedding, or towels with individuals who have body lice, and such items can be treated with an insecticide registered for controlling infestations of lice. However, chemical control is not usually necessary when hygiene is maintained and infested items are washed appropriately no less than once a week.

Pthirus pubis

Chemical treatments are effective when combined with practices that remove eggs. A lice comb can be used to remove eggs from hair and dipped in hot soapy water to kill eggs and lice between each pass. Avoiding contact with infested individuals is highly recommended. Frequent changing and dry-cleaning or machine washing of clothing, especially underwear, nightwear, and bedding, with hot water before placing in a hot dryer cycle is also recommended. Medicated creams or shampoos are the main treatments for killing P. pubis, and often must be applied to the whole body and left for a few hours before washing. Reapplication is necessary one week after the initial treatment. It is recommended that any current or recent sexual partners undergo treatment, even if they are not displaying symptoms of an infestation.

References

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CDC. 2019. Parasites. Centers for Disease Control and Prevention. Available https://www.cdc.gov/parasites/lice/body/index.html

CDC. 2018. Pthiriasis. Centers for Disease Control and Prevention. Available https://www.cdc.gov/dpdx/pthiriasis/index.html

CDC (n.d.). Head Lice. Centers for Disease Control and Prevention. Available http://publichealth.lacounty.gov/acd/docs/HeadLiceLifeCycle.pdf

CDC. (n.d.). How to Examine for Head Lice. Centers for Disease Control and Prevention. Available https://www.cdc.gov/parasites/resources/pdf/headliceflyer.pdf

Hodgson, E. 2008. Human lice. Utah State University – Extension. Available https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1914&context=extension_curall

Koch et al. 2016. Management of Head Louse Infestations in the United States – A Literature Review. Pediatric Dermatology. Available https://onlinelibrary.wiley.com/doi/full/10.1111/pde.12982

Naddaf, S. 2018. Lice, Humans, and Microbes. Iranian Biomedical Journal. 22(5): 292-293. Available https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6058184/

Patel, P., A. Tan, and N. Levell. 2021. A clinical review and history of pubic lice. Clinical and Experimental Dermatology. 46(7):1181-1188. Available https://academic.oup.com/ced/article/46/7/1181/6598616

Weems, H. 2020. Featured Creatures: Body Louse. University of Florida – Entomology and Nematology. Available https://entnemdept.ufl.edu/creatures/urban/human_lice.htm