The Italian biologist Diacinto Cestoni showed in the 18th century that scabies is caused by the mite Sarcoptes scabiei, variety hominis. The disease produces intense, itchy skin rashes when the impregnated female tunnels into the stratum corneum of the skin and deposits eggs in the burrow. The larvae, which hatch in 3–10 days, move about on the skin, moult into a nymphal stage, and then mature into adult mites. The adult mites live 3–4 weeks in the host's skin.
The action of the mites moving within the skin and on the skin itself produces an intense itch which may resemble an allergic reaction in appearance. The presence of the eggs produces a massive allergic response which, in turn, produces more itching.
Sarcoptes is a genus of skin parasites, and part of the larger family of mites collectively known as “scab mites”; they are also related to the scab mite Psoroptes, also a mite that infests the skin of domestic animals. Sarcoptic mange affects domestic animals and similar infestations in domestic fowls causes the disease known as “scaly leg”. The effects of Sarcoptes scabiei are the most well known, causing “scabies”, or “the itch”. The adult female mite, having been fertilised, burrows into the skin, usually the hands or wrists, however other parts of the body may also be affected, and lays its eggs.
The burrowing is carried out using the mouthparts and special cutting surfaces on the front legs. While these are being used, the Sarcoptes scabiei anchors itself with suckers on its feet. Eggs are laid in small numbers as the mite burrows, and as these hatch, six-legged larvae climb out on to the skin and search for hair follicles, where they feed and moult (discard old cuticles to grow). It is in the hair follicles that the larvae show the first nymphic stages, with eight legs. In the nymphic stages, the creature feeds and moults and, if male, gives rise to the adult. In the case of females, another moult occurs before adulthood. The female has more moults than a male and therefore takes longer – seventeen days to the other nine to eleven days for a male to reach adulthood. The female is about twice the size of the male.
Although the life cycle is only about two weeks, individual patients are seldom found to have more than about a dozen mites on them. Even so, this number can cause agonising itching, in particular at night, and severe damage to the skin often comes as a result of scratching, particularly by the introduction of infective bacteria, which may lead to impetigo or eczema.
The eggs are laid by the female at a rate of about two to three eggs a day for about two months. It is considered that about two per cent of the British population is infected with these mites, it taking about twenty-five minutes to an hour to burrow into the skin.
The best conditions in which to harbour Sarcoptes scabiei is in areas where there is frequent skin to skin contact, therefore (as also mentioned previously) the hands and wrists, as the mites are transmitted by skin contact with carriers, and they very easily spread. Infestations of Sarcoptes scabiei are commonly found in pigs. They significantly depress growth and feeding rate, but usually die out in around five days from in typical farm conditions. However, once in a herd, the mites are very difficult to eliminate without great measures being taken.