A handful of mites can make you itch for weeks after they die
Scabies—from Latin scabere, "to scratch"—is infestation by the human mite Sarcoptes scabiei var. hominis, only 0.2–0.45 mm long. Often just 10 to 15 mites drive the misery. The itch is an allergic reaction, so symptoms can lag weeks behind the first burrows and linger after treatment.
Classic signs are severe night-worsened itch and a pimple-like rash, sometimes with linear or S-shaped burrows in finger webs, wrists, waist, elbows, buttocks, or genitals. First infections usually declare within about three weeks (sources also cite two to six weeks); reinfection can itch within 24 hours or a few days. Heads are usually spared except in young children, the elderly, or immunocompromised hosts. Scratching can break skin and invite bacteria. As of 2015 about 204 million people (2.8% of the world) were affected, equally by sex, more often young and old, and more in crowded tropical and developing settings—care homes, schools, camps, prisons, hospitals. Other animals do not spread human scabies; related mange mites cause sarcoptic mange in animals.
Female mites tunnel into the stratum corneum to lay eggs that hatch in three to ten days; adults live three to four weeks on the host. A healthy person averages about 11 burrowing females. Spread usually needs prolonged skin contact of at least about ten minutes—shared living or sex—not brief brushes; fomites matter less because mites die within two to three days off skin at room temperature. Crusted (Norwegian) scabies, in weak immune systems, can host up to two million mites under thick scales, shedding easily via short contact or objects and resisting topical killers under crust. Nodular scabies—raised itchy oblong bumps in 5–10% of cases—may take months to fade even after mites are gone. Eighteenth-century Italians Bonomo and Cestoni identified the mite; adults have eight legs like other arachnids.
Treatment options named in sources include oral or topical ivermectin, permethrin, crotamiton, and lindane. Household members and sexual contacts from the prior month should be treated together. Bedding and clothes from the last three days need hot wash and dry; more laundry is unnecessary once the three-day off-host limit is respected. Itch may continue two to four weeks after successful killing; retreatment is considered if symptoms persist beyond that. IgE responses to mite proteins—some cross-reacting with house-dust mites—explain both delayed first reactions and rapid reinfection flares. Rates rise with humidity and fall with temperature. The old nickname "seven-year itch," common in 1800s documents, reflected how stubborn the condition once was before modern regimens.
Source: Scabies
