Ectoparasites include arthropods and creatures from other phyla that infest the skin or hair of animals; the host animals provide them with sustenance and shelter. The ectoparasites may penetrate within or beneath the surface of the host or may attach by mouthparts and specialized claws. These organisms may inflict direct mechanical injury, consume blood or nutrients, induce hypersensitivity reactions, inoculate toxins, transmit pathogens, and incite fear or disgust. Humans are the sole or obligate hosts for many kinds of ectoparasites and serve as facultative or paratenic (accidental) hosts for many others.
Arthropods that are ectoparasitic or otherwise cause injury include insects (such as lice, fleas, bedbugs, wasps, ants, bees, and flies), arachnids (spiders, scorpions, mites, and ticks), millipedes, and centipedes. Certain nematodes (helminths), such as the hookworms (Chap. 256), are ectoparasitic in that they penetrate and migrate through the skin. Infrequently encountered ectoparasites in other phyla include the pentastomes (tongue worms) and leeches.
Arthropods may also cause injury when they attempt to take a blood meal or as they defend themselves by biting, stinging, or exuding venoms. Various arachnids (spiders and scorpions), insects (bees, hornets, wasps, ants, flies, true bugs, caterpillars, and beetles), millipedes, and centipedes produce ill effects during these behaviors. Similarly, certain ectoparasites (e.g., ticks, biting mites, and fleas) that typically infest nonhuman animals can be medically significant. In the United States, lesions caused by arthropod bites and stings are so diverse and variable that it is rarely possible to identify the precise causative organism without a bona fide specimen and taxonomic expertise.
The human itch mite, Sarcoptes scabiei var. hominis, is a common cause of itching dermatosis, infesting ~300 million persons worldwide at any one time. Gravid female mites (~0.3 mm in length) burrow superficially within the stratum corneum, depositing three or fewer eggs per day. Six-legged larvae mature to eight-legged nymphs and then to adults. Gravid adult females emerge to the surface of the skin about 8 days later and then (re)invade the skin of the same or another host. Newly fertilized female mites are transferred from person to person mainly by direct skin-to-skin contact; transfer is facilitated by crowding, poor hygiene, and sex with multiple partners. Generally, these mites die within a day or so in the absence of host contact. Transmission via sharing of contaminated bedding or clothing occurs far less frequently than is often thought. In the United States, scabies may account for up to 5% of visits to dermatologists. Outbreaks occur in preschools, hospitals, nursing homes, and other residential institutions.
The itching and rash associated with scabies derive from a sensitization reaction to the mites and their secretions/excretions. A person’s initial infestation remains asymptomatic for up to 6 weeks before the onset of intense pruritus, but a reinfestation produces a hypersensitivity reaction without delay. Burrows become surrounded by inflammatory infiltrates composed of eosinophils, lymphocytes, ...