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, create openings in the skin for secondary bacterial infection, and incite fear or disgust. Human beings are the sole or obligate hosts for only a few kinds of ectoparasites but serve as facultative, dead-end, 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. 226), 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 cause injury when they attempt to take a blood meal or as they defend themselves by biting, stinging, or exuding venoms. Papular urticaria and other lesions caused by arthropod bites and stings are so diverse and variable (depending upon the host’s health status and prior exposure to the arthropod’s saliva, venom, or other exudates) that it is difficult to identify the precise causative organism without a bona fide specimen and taxonomic expertise.
The human itch mite, Sarcoptes scabiei var. hominis, is an obligate human ectoparasite and a common cause of itchy dermatosis, infesting ~300 million persons worldwide. Gravid female mites (~0.3 mm in length) burrow superficially within the stratum corneum, depositing several 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, scabies mites die within a day or so in the absence of a suitable host. Transmission via sharing of contaminated bedding or clothing occurs less frequently than is often thought. In the United States, scabies may account for up to 5% of visits to dermatologists. Outbreaks are known to occur in preschools, hospitals, nursing homes, and other institutional residences.
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 re-infestation produces a hypersensitivity reaction without delay. Burrows become surrounded by inflammatory infiltrates composed of eosinophils, lymphocytes, and histiocytes, and a generalized hypersensitivity rash later develops in remote sites. ...