Scabies needs no introduction. It is an extremely contagious skin condition caused by the mite Sarcoptes scabiei var. hominis; about 1 million new cases are reported each year in the US. It’s a condition in which the mites burrow into human skin to lay their eggs that later hatches and grow into adults.
Sarcoptes scabiei or scabies mite is a parasite that lives within the subcutaneous tissues of human skin. Similar mites cause "mange" in wild and domestic animals, which is entirely different than human scabies.
Scabies mites are prevalent globally, and tend to affect all socioeconomic groups. Scabies mites are usually host specific and are dependent on host for their life cycle. These oval shaped, straw colored mites are very tiny, measuring just 0.2-0.4mm in length. Their bodies are covered with fine lines and some long hairs; these mites are impossible to see with naked eyes. The female mite has some short blunt spines distributed on the dorsal surface that aid her in maintaining her position within the burrow. The mites have no eyes, and they have short and thick legs, with the first two pair of legs stalked.
History of Discovery
Scabies is an ancient disease. Scabies is estimated to date back over 2,500 years according to archeological evidences from Egypt and the Middle East. The first recorded incidence of scabies is believed to be from the Bible (Leviticus, the third book of Moses) ca. 1200 BCE.
A Roman physician Celsus is credited with giving the term “scabies” to the disease and describing its characteristic features. However, the parasitic etiology of scabies was later documented by the Italian physician Giovanni Cosimo Bonomo (1663-1969 ADE) in his popular 1687 letter, “Observations concerning the fleshworms of the human body”. With this discovery, scabies became one of the first diseases with a known cause.
The scabies mite goes through four stages in its lifecycle: egg, larva, nymph, and adult.
Upon invading a human host, the female mite burrows into the epidermis, outermost layer of skin, where she lays two or three eggs each day. These eggs are oval in shape and 0.1–0.15 mm long and hatch as larvae in three to four days. A female mite can lay up to 30 eggs and dies. Upon hatching, the six-legged larvae drift to the skin surface and burrow into peeling skin, generally into hair follicles, where vesicles form. After three to four days, the larvae shed that turns into eight-legged nymphs. The molt turns a second time into larger nymphs, before a final transformation into adult. Adult mites then mate when the male mite penetrates the molting pouch of the female. Mating occurs once in the entire lifetime, as that one event leaves the female fertile for the rest of her life. The impregnated female drops the molting pouch in search of appropriate location for a permanent burrow. Once a location is found, the female creates her characteristic S-shaped burrow, laying eggs in the process. They will continue lengthening her burrow and laying eggs for the duration of her life.
How long can scabies live outside the body?
Scabies mites can live about 72 hours without human contact. However on a person, the mites can live up to two months. Scabies Mites can survive longer in colder conditions with higher humidity. On a human, mites can burrow into the skin, and symptoms usually begin three to six weeks after infestation.