Head lice infestation (also known as “Pediculosis capitis”) is a human medical condition caused by the colonization of the hair and skin by the parasitic  insect Pediculus humanus capitis—the head louse.

Typically, only the head or scalp of the host is infested, although the disease can occur in other hairy parts of the body, like leg hairs.

Head lice feed on human blood (hematophagy), and itching from louse bites is a common symptom of this condition.

Lice infestation in general is known as pediculosis, and occurs in many mammalian and bird species. The term pediculosis capitis, or simply “pediculosis”, is sometimes used to refer to the specific human pediculosis due to P. humanus capitis (i.e., head-louse infestation).

Humans are hosts for two other lice as well—the body louse and the crab louse.

Head-louse infestation is widely endemic, especially in children.

It is a cause of some concern in public health, although, unlike human body lice, head lice are not carriers of other infectious diseases.

Signs and symptoms

The most common symptom of infestation is pruritus (itching) on the head which normally intensifies 3 to 4 weeks after the initial infestation.

The bite reaction is very mild and it can be rarely seen between the hairs.

Bites can be seen, especially in the neck of long-haired individuals when the hair is pushed aside.

In rare cases, the itch scratch cycle can lead to secondary infection with impetigo and pyoderma.

Swelling of the local lymph nodes and fever are rare.

Cause

Head lice are generally spread through direct head-to-head contact with an infested person; transmission by sharing bedding or clothing such as head wear is much less common. Body lice are spread through direct contact with the body, clothing or other personal items or such,of a person already carrying lice. Pubic lice are most often spread by intimate contact with an infested person. Head lice occur on the head hair, body lice on the clothing, and pubic lice mainly on the hair near the groin. Human lice do not occur on pets or other animals. Lice do not have wings and cannot jump.

From each egg or “nit” may hatch one nymph that will grow and develop to the adult louse. Full-grown lice are about the size of a sesame seed. Lice feed on blood once or more often each day by piercing the skin with their tiny needle-like mouth-parts. Lice cannot burrow into the skin.

Head lice and body lice (Pediculus humanus) are similar in appearance, although the head louse is often smaller. Pubic lice (Pthirus pubis), on the other hand, are quite distinctive. They have shorter bodies and pincer-like claws, and are colloquially known as “crabs”.

Vectorial capacity

Head lice (Pediculus humanus capitis) are not known to be vectors of diseases, unlike body lice (Pediculus humanus humanus), which are known vectors of epidemic or louse-borne typhus (Rickettsia prowazeki), trench fever (Rochalimaea quintana) and louse-borne relapsing fever (Borrellia recurrentis)

Diagnosis

The condition is diagnosed by the presence of lice or eggs in the hair, which is facilitated by using a magnifying glass or running a comb through the child’s hair.

In questionable cases, a child can be referred to a health professional.

The use of a louse comb is the most effective way to detect living lice. In cases of children with dirty, long and/or curly/frizzy hair, an alternative method of diagnosis is examination by parting the hair at 2 cm intervals to look for moving lice near the scalp.

With both methods, special attention should be paid to the area near the ears and the nape of the neck.

The examiner should examine the scalp for at least 5 minutes.

The use of a magnifying glass to examine the material collected between the teeth of the comb could prevent misdiagnosis.

The presence of nits alone however, is not an accurate indicator of an active head louse infestation. Children with nits on their hair have a 35-40% chance of also being infested with living lice and eggs. If lice are detected, the entire family needs to be checked (especially children up to the age of 13 years).