This disease is by no means so frequent in this country as in some others. It is said to constitute about one-fourth of all the cases of skin disease in Glasgow. The disease, as is now well known, is caused by the presence of an animal parasite, the acarus scabiei. This little insect which is barely visible to the unaided eye, burrows in the skin, making a somewhat crooked channel in which it deposits its eggs (Fig. 339), which in due time are hatched, and rapidly develop into full grown acari. The female is the cause of all the mischief, as she alone burrows, the purpose being to deposit the eggs just under the surface of the epidermis. The track left by the insect in burrowing can be readily seen by the aid of a small magnifying glass as a little dotted line about one-fourth of an inch in length. The eruption varies much in different cases, sometimes being very scanty, in other cases resembling a bad case of eczema.

Fig. 339. Female Scabiei Itch Mite Laying Eggs in a Burrow.

Fig. 339. Female Scabiei Itch Mite Laying Eggs in a Burrow.

It is most often found between the fingers, in the bend of the elbows and knees, and upon the front of the body. The itching in some cases is most intolerable, in others slight. It is most severe at night. It is in some cases very difficult to decide whether or not a patient has scabies or some other skin disease.

A variety of the disease known as grocer's itch is sometimes produced by the irritation of an insect known as the acarus sacchari.