Sarcoptes Scabiei - Clinical Significance

Clinical Significance

The action of the mites moving within the skin and on the skin itself produces an intense itch that may resemble an allergic reaction in appearance. The presence of the eggs produces a massive allergic response that, 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 at 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, especially at night, and severe damage to the skin often comes as a result of scratching, in particular 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 percent of the British population is infected with these mites, which take 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 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 in typical farm conditions. However, once in a herd, the mites are very difficult to eliminate without great measures taken.