LICE - pediagenosis
Article Update

Wednesday, February 10, 2021



Lice are nonflying insects that live off the blood meal from a human host. They have been human pathogens for thousands of years and continue to cause millions of cases of disease annually. Three variants of the louse exist: the head louse, the body louse, and the pubic louse. For the most part, lice cause localized skin disease from the biting they do to secure their blood meal. However, some lice have been known to transmit other diseases to humans. The most important infectious agents transmitted by body lice are the bacteria that cause epidemic typhus, relapsing fever, and trench fever. These infections are uncommon in the United States and North America but are still seen, and one should be aware of their causes and vectors.

Clinical Findings: Lice are capable of infesting any human, independent of age, sex, or race. Body lice are seen more frequently in patients of low socioeconomic status and especially in homeless individuals. Underlying mental health issues in this subset may also predispose one to conditions that are opportune for infestation. Pubic lice, or “crabs,” is a sexually transmitted disease that is seen in younger adults more frequently than in other age groups; however, it has been reported to occur in people all ages.


Pediculosis capitis (head lice infestation) is probably the most common louse infestation in North America and Europe. The louse, Pediculus humanus capitis, preferentially locates to the scalp and lives between the hair shafts. These lice are transmitted by close contact and from fomites such as combs, pillows, and head rests. Patients complain of severe itching on the scalp and neck. On inspection, small (1-2 mm), red, excoriated papules are seen. Evidence of scratching becomes prominent as time goes on without a diagnosis. The diagnosis is confirmed by finding a louse, which is typically 2 to 4 mm long and light brown in color. On occasion, the abdomen of the louse can appear red, which is the case directly after a blood meal. These insects are not particularly fast moving, nor can they fly or jump; as a result, they are easy to capture and identify. Egg sacks (nits) are firmly adhered to the hair. This is in contrast to the common hair cast, which can easily be moved up and down the hair shaft with minimal effort. The nits are laid in close proximity to the scalp, usually within 0.5 mm. The nits hatch within 2 weeks. Therefore, nits found more than 2 cm from the scalp are often nonviable, and the larva has already emerged from the nit. Persistent infections can lead to bacterial superinfection and pyoderma with cervical adenopathy.

Pediculosis pubis (pubic lice infestation) is a commonly acquired sexually transmitted disease. The pubic louse, Phthirus pubis, is structurally different from the body or head louse and can easily be distinguished. Patients complain of itching and often note pinpoint drops of blood in their undergarments. This is caused by small amounts of bleeding after the pubic lice feed. These lice have specialized arms that allow them to climb around the entire human body, and they may be seen at any location. They have a tendency to affect the eyelashes and eyebrows. This is important to look for clinically, to appropriately treat all affected regions.

Pediculosis corporis (body lice infestation) is commonly seen in homeless individuals and in those with poor hygiene. Historically, body lice have been associated with epidemics during times of war, because close contact for extended periods leads to easy transfer from one host to another. The body louse, Pediculus humanus corporis, is indistinguishable from the head louse on inspection with the naked eye. Entomologists trained in differentiating the species are capable of discerning the two. Body lice live on the clothing and leave it to feast on human blood. Patients present with multiple pruritic, red to pink, excoriated papules anywhere on the body. On inspection of the skin, one typically will not find lice. It is only with close inspection of the clothing or bedding material that the infestation becomes apparent. Hundreds to thousands of lice may be present on the clothing, particularly in small hiding spaces such as the seams. Along with the lice, many eggs and larvae may be seen.

The body louse has been shown to be a carrier of the bacterial agents that cause relapsing fever, trench fever, and epidemic typhus: Borrelia recurrentis, Bartonella quintana, and Rickettsia prowazekii, respectively. The louse carries the bacteria within its gut.

B. recurrentis is responsible for causing the disease relapsing fever. It is transmitted from one human to another when the fecal material of a human body louse gains entry into the bloodstream. This bacterium is unique in that it can rearrange its surface proteins. This is believed to be the reason for the relapsing and recurrent fevers: The host immune system reacts in a periodic manner to the changing surface of the bacteria.

B. quintana is a bacterium that is transmitted through the feces of the louse. After a louse defecates on a patient’s skin and the patient scratches, the stool and the bacteria are implanted into the skin, which causes infection. Also, the louse often bites after defecating and causes skin trauma that transfers the bacteria into the skin. B. quintana is the etiologic agent of trench fever, bacillary angiomatosis, and peliosis and has also been shown to cause endocarditis. B. quintana infections are most commonly seen in patients who are infected with the human immunodeficiency virus and in homeless individuals.

R. prowazekii is an obligate intracellular parasite that is transmitted to humans through the feces of the human body louse. The natural environmental reservoir for this bacterium is the flying squirrel (Glaucomys volans). The infected louse feeds on the human, and the fecal material that contains the R. prowazekii bacteria is deposited into the fresh wound, allowing for infectious transfer. This infection is most frequently seen during times of war, when individuals are in close contact with one another for significant periods. Signs and symptoms of epidemic typhus include fever, rash, pain, delirium, and other constitutional symptoms.

Pathogenesis: P. humanus capitis affects humans and has a high propensity to infest the scalp. These lice live on the host and periodically take a blood meal from the scalp or neck area. In patients with very long hair, the blood meal may be taken from the back or any area of skin that is in contact with the hair. The lice are able to reproduce rapidly. The females, which are a bit larger than the males, lay eggs that hatch and develop into adults capable of reproducing within 4 weeks.

Histology: The histological findings on skin biopsy are similar among all forms of louse bites. Histological evaluation cannot differentiate a louse bite from any other insect bite with certainty. Skin biopsies are rarely performed in these cases, because the diagnosis is made clinically. Biopsy specimens show a nonspecific, mixed superficial and deep inflammatory infiltrate with eosinophils. This may suggest a bite reaction. Unlike tick bites or scabies, in which occasionally tick parts or scabies mites are seen in a biopsy specimen, a biopsy from a patient with a lice infestation will never show mouth parts or other elements of the louse.


 Treatment: Therapeutic agents to treat lice are similar among all species of human lice. The most commonly used therapies are based on permethrin; when used appropriately, they show good cure rates. These treatments should be used in conjunction with an agent that helps remove the nits from the hair shafts, and physical removal with a lice hair comb is a must. Therapy should be repeated on a weekly basis. Bedding and clothing need to be disinfected. The use of lindane has decreased because of its potential neurotoxicity. Malathion and oral ivermectin show excellent efficacy. Oral ivermectin needs to be repeated in 1 week, because it does not kill the developing larvae within the nits.

Therapy for body lice also requires complete disinfection of the household or living areas. Overtly infested clothing should be thrown away. Professional fumigation should be considered.

Share with your friends

Give us your opinion

Note: Only a member of this blog may post a comment.

This is just an example, you can fill it later with your own note.