This patient is infested with Pediculus humanus capitis otherwise known as head lice.
Pediculus humanus capitis are insects belonging to the family Pediculidae. The family also includes Pediculus humanus humanus (body louse) and Pthirus pubis (crab louse). Pediculus humanus capitis is most closely related to Pediculus humanus humanus with the two being morphologically identical. Pediculus humanus humanus however has a predilection for infesting the body and attaching its eggs on the clothing of the host. Pediculus humanus capitis will instead have a predilection for attaching its eggs on the hair shafts of the host and thus is found in scalp hair. It can also be found in other areas of the body that may have hair but it is predominantly found as an infestation of the head. Another difference between the two closely related lice is that Pediculus humanus capitis is not known to be a vector of disease while Pediculus humanus humanus is a vector of a variety of bacterial diseases (epidemic typhus, trench fever, and relapsing fever). Pediculus humanus humanus is also the larger of the three lice at 2-4 mm in length.
The eggs of lice are called “nits” and have a glue produced by the females reproductive organ that affixes them to either the hair shaft or, in the case of Pediculus humanus humanus, the fibers of clothing. Though the names eggs and nits are used interchangeably it is important to note that “nits” actually is a term used to describe the empty casing of a louse egg. The photo in this case study is that of a nit attached to a hair shaft (see below).
These nits are about 0.8 mm in length. In individuals that are heavily infested there may be many nits present. The photo below is another photo taken of nits seen on two separate hair shafts.
Below is another photo taken where we can actually see the hair louse secured tightly on the hair shaft of the host with it’s claw-like legs.
Pediculus humanus humanus and Pediculus humanus capitis are ectoparasites found to exclusively infest humans. They can not survive away from their human host and feed on the blood of their host. These two are mentioned together because of their similarity in morphology. Pthirus pubis is distinctly different morphologically in that it has a more round “crab-like” appearance and has a predilection for coarse hair such as pubic hair, armpit hair, and occasionally beards. Pthirus pubis is also considered a sexually transmitted louse, while Pediculus humanus humanus and Pediculus humanus capitis are transmitted via the sharing of personal items. In the case of Pediculus humanus capitis, hair brush, combs, and hats can be sources of transmission. Morphologically the lice have fused thoracic segments that are heavily segmented, they are wingless, and have three pairs of legs that generally have a hook-like appearance giving them the ability to securely hold fast to the host hair or clothing fibers. Pediculus humanus capitis are 2.5-3mm in length.
Symptoms of infestation include itching around the scalp, ear, and neck. The itching is an allergic reaction to the saliva of biting lice. Though the lice do not transmit any diseases, the scratching can lead to secondary bacterial infections with Staphylococcus aureus being the most common cause. Pediculus humanus capitis are difficult to spot because they are small, shy away from light, and are relatively fast. Nits, being small are even more difficult to spot. Though heavy infestations are relatively easy to diagnosis visually especially around the hairline of the neck.
Pediculus humanus capitis treatment consists of disposing of or washing in hot water contaminated sources such as hats, scarves, pillows, blankets, brushes, etc. The use of over-the-counter medications such as permethrin (Nix); pyrethrins (RID) are effective. Ivermectin or Spinosad as prescription lotions are also effective treatments. Wet-combing is a method used to remove nits and involves the combing of wet conditioned hair with a fine-toothed comb.
Though generally considered more of a problem in children where the sharing of scarves, hats, and brushes is most likely, Pediculus humanus capitis can occur in the homeless and in overcrowded environments such as prisons.
Hope you enjoyed this case study!
REFERENCES:
CDC. About Body Lice. Centers for Disease Control and Prevention. Available at https://www.cdc.gov/lice/about/body-lice.html. June 24, 2024; Accessed: April 7, 2025.
Leung AKC, Lam JM, Leong KF, Barankin B, Hon KL. Paediatrics: how to manage pediculosis capitis. Drugs Context. 2022 Mar 14;11:2021-11-3. doi: 10.7573/dic.2021-11-3. PMID: 35371269; PMCID: PMC8932250.
[Guideline] Cummings C, Finlay JC, MacDonald NE. Head lice infestations: A clinical update. Paediatr Child Health. 2018 Feb. 23 (1):e18-e24.
*Disclaimer: The information contained in this case study is to be used only as a case study example for teaching purposes. The information in the case study is both factual and fictional. Opinions formulated by the author are intended to stimulate discussion.