Pediculosis is caused by two species of lice: Pediculus humanus and Phthirus pubis. P. humanus has two subspecies: P. humanus capitus (head louse), which primarily affects the scalp, and P. humanus corporis (body louse), which primarily affects the trunk. P. pubis (pubic louse) primarily affects the genital area, but the axilla and eyebrows can be involved as well.
Note that the body louse is the vector for several human pathogens, notably Rickettsia prowazekii, the cause of epidemic typhus, whereas the head louse and the pubic louse are not vectors of human disease.
Lice are easily visible, being roughly 2 to 4 mm long. They have six legs armed with claws by which they attach to the hair and skin (Figure 69–1). Pediculus has an elongated body, whereas Phthirus has a short body and resembles a crab, and hence its nickname, the crab louse (Figure 69–2). People infected with Phthirus are said to have “crabs.”
Pediculus corporis—body louse. Note the elongated abdomen of Pediculus corporis. In contrast, the pubic louse has a short “crab-like” abdomen. (Used with permission from Dr. F. Collins, Public Health Image Library, Centers for Disease Control and Prevention.)
Phthirus pubis—pubic louse. Note the short, rounded “crab-like” abdomen of Phthirus pubis. In contrast, the body louse has an elongated abdomen. (Reproduced with permission from Public Health Image Library, Centers for Disease Control and Prevention. Original content provider is the World Health Organization.)
Nits are the eggs of the louse and are typically found attached to the hair shaft (Figure 69–3). They are white and can be seen with the naked eye. Nits of the body louse are often attached to the fibers of clothing.
Pediculus capitis—egg case (nit). Arrow points to an egg case (also known as a nit) attached to hair shaft. Note embryo within egg. (Used with permission from Dr. D. Juranek, Public Health Image Library, Centers for Disease Control and Prevention.)
Head lice are transmitted primarily by fomites such as hats, combs, and towels. These are especially common in school children. Body lice live primarily on clothing and are transmitted either by clothing or by personal contact. Body lice leave the clothing when they require a blood meal. Pubic lice are transmitted primarily by sexual contact.
Widespread infestations of body lice occur when personal hygiene is poor (e.g., during wartime or in crowded refugee camps).
Adult lice feed on blood and, in the process, inject saliva into the skin, which induces a hypersensitivity reaction and, as a consequence, pruritus.
Pruritus is the main symptom. Excoriations may result from scratching, and secondary bacterial infections may occur. In pediculosis capitis, the adult lice are often difficult to see, but the nits are easily visualized. In pediculosis corporis, the adult lice are primarily in the clothing rather than on the body. In pediculosis pubis, the adult lice and nits can be seen attached to the pubic hair.
The laboratory is not involved in diagnosis. Nits fluoresce under ultraviolet light of a Wood’s lamp, which can be used to screen the hair of large numbers of people.
Permethrin (Nix, RID) is the treatment of choice, as it is both pediculicidal and ovicidal. However, resistance to permethrin is increasing. Ivermectin (Sklice) is also effective, and resistance has not been reported. Nits are removed using a fine-toothed (nit) comb. Patients with body lice often do not need to be treated, but the clothing should be either discarded or treated.
Children should not share articles of clothing. Many schools have a policy that children cannot attend school until they are nit-free, but the need for this exclusionary approach is under review. The personal items of affected individuals, such as towels, combs, hair brushes, clothing, and bedding, should be treated. Sexual partners of those infested with pubic lice should be treated and tested for other sexually transmitted diseases.
Myiasis is caused by the larva of many species of flies, but the one best known is the botfly, Dermatobia hominis. Fly larvae are also known as maggots. Note that maggots are occasionally used to debride nonhealing wounds, but these maggots do not cause myiasis.
The flies that cause myiasis are found worldwide and infest many animals as well as humans. Human infestation occurs most often in tropical areas. Dermatobia is common in Central and South America.
The precise route of transmission varies depending on the species of fly. In one scenario, the adult fly deposits its egg in a wound and the egg hatches to produce the larva. In another, the fly deposits the egg in the nostrils, in the conjunctiva, or on the lips. In yet another, the fly deposits the egg on unbroken skin and the larva invades the skin.
Dermatobia is especially interesting in that it deposits its egg on a mosquito. When the mosquito bites a human, the warmth of the skin induces the egg to hatch and the larva enters the skin at the site of the mosquito bite.
The presence of the larva in tissue induces an inflammatory response.
The characteristic lesion is a painful, erythematous papule resembling a furuncle (Figure 69–4). The lesion may also be pruritic. The larva can often be seen within a central pore. Some patients report a sense of movement within the lesion. A history of travel to tropical regions is commonly elicited. Cutaneous myiasis is the most common form, but ocular, intestinal, genitourinary, and cerebral forms occur.
Dermatobia hominis—myiasis. Botfly larva emerging from the center of erythematous nodular skin lesion. Insert shows intact larva. (Reproduced with permission from Kang S, Amagai M, Bruckner AL, et al. Fitzpatrick’s Dermatology in General Medicine. 9th ed. New York, NY: McGraw-Hill Education; 2019.)
The laboratory is not involved in diagnosis except when identification of the larva is needed.
Surgical removal of the larva is the most common mode of treatment. If the larva is visible, manual extraction can be performed. If the larva is not visible, the central pore can be covered with petroleum jelly, thus causing anoxia in the larva. This induces the larva to migrate to the surface.
Prevention involves limiting exposure to flies, especially in tropical areas. General measures, such as wearing clothing that covers the extremities, mosquito netting, and insect repellant, are recommended.
Cimex lectularius is the most common bedbug found in the United States. It has an oval, brownish body and is about 5 mm long (Figure 69–5). Bedbugs reside in mattresses and in the crevices of wooden beds. At night, they emerge to take a blood meal from sleeping humans. The main symptom of a bedbug bite is a pruritic wheal caused by a histamine-related hypersensitivity reaction to proteins in the bug saliva (Figure 69–6). Some individuals show little reaction. The bite of a bedbug is not known to transmit any human disease. Calamine lotion can be used to relieve the itching. Malathion or lindane can be used to treat mattresses and beds.
Cimex lenticularis—bedbug. Bedbug in the process of ingesting blood from skin. They are wingless, reddish-brown, and about 5 mm long. (Reproduced with permission from Public Health Image Library, Centers for Disease Control and Prevention.)
Bedbug bites—several urticarial wheals surrounded by erythema on the patient’s back. (Reproduced with permission from Kang S, Amagai M, Bruckner AL, et al. Fitzpatrick’s Dermatology in General Medicine. 9th ed. New York, NY: McGraw-Hill Education; 2019.)