Ectoparasites are organisms that live on the external surface of their host and derive nourishment by feeding on blood, skin, or secretions. The most clinically significant human ectoparasites include lice, scabies mites, and ticks, each causing distinct dermatological conditions and, in the case of ticks, transmitting serious infectious diseases.
Pediculosis (louse infestation) is caused by three species: Pediculus humanus capitis (head louse), Pediculus humanus corporis (body louse), and Phthirus pubis (pubic louse or crab louse). Lice are wingless, dorsoventrally flattened insects with clawed legs adapted for grasping hair or clothing fibers. They feed exclusively on blood and deposit eggs (nits) that are cemented to hair shafts or clothing fibers. Head lice infestations cause intense pruritus and are most common in school-age children. Body lice are vectors for epidemic typhus, trench fever, and relapsing fever. Diagnosis is by visual identification of live lice or nits using a fine-toothed comb.
Scabies is caused by the mite Sarcoptes scabiei var. hominis. The female mite burrows into the stratum corneum, creating serpentine tunnels where she deposits eggs. The resulting inflammatory response causes intense pruritus, particularly at night. Characteristic lesions include burrows, papules, and vesicles in the interdigital spaces, wrists, axillae, and genital region. Crusted (Norwegian) scabies occurs in immunocompromised patients, with extensive hyperkeratotic lesions containing thousands of mites. Diagnosis is by microscopic identification of mites, eggs, or fecal pellets in skin scrapings. Treatment involves antiparasitic drugs such as permethrin or ivermectin.
Ticks are arachnids belonging to the order Ixodida. Hard ticks (Ixodidae) have a scutum and include Ixodes scapularis (deer tick, vector of Lyme disease), Dermacentor variabilis (American dog tick, vector of Rocky Mountain spotted fever), and Amblyomma americanum (lone star tick). Soft ticks (Argasidae) lack a scutum and transmit relapsing fever. Ticks feed by inserting barbed mouthparts into the host and secreting cement-like saliva. Attachment periods range from hours to days.
