Sarcoptes scabiei crustosæ, which is a distinct variety, if not species, of the human itch mite, is the cause of so-called Norwegian itch. This disease is very contagious, and is much more resistant than the ordinary scabies. Unlike the latter, it may occur on the face and scalp.
Sarcoptes scabiei not only attacks man but also occurs on a large number of mammals. Many species, based on choice of host, and minute differences in size and secondary characters, have been established, but most students of the subject relegate these to varietal rank. Many of them readily attack man, but they have become sufficiently adapted to their normal host so that they are usually less persistent on man.
Notoedres cati (usually known as Sarcoptes minor) is a species of itch mites which produce an often fatal disease of cats. The body is rounded and it is considerably smaller than Sarcoptes scabiei, the female ([fig. 61]) measuring 215-230µ long and 165-175µ wide; the males 145-150µ by 120-125µ. The most important character separating Notoedres from Sarcoptes is the position of the anus, which is dorsal instead of terminal. The mite readily transfers to man but does not persist, the infestation usually disappearing spontaneously in about two weeks. Infested cats are very difficult to cure, unless treatment is begun at the very inception of the outbreak, and under ordinary circumstances it is better to kill them promptly, to avoid spread of the disease to children and others who may be exposed.
Demodecidæ
The Demodecidæ are small, elongate, vermiform mites which live in the hair follicles of mammals. The family characteristics will be brought out in the discussion of the species infesting man, Demodex folliculorum.
Demodex folliculorum ([fig. 62]) is to be found very commonly in the hair follicles and sebaceous glands of man. It is vermiform in appearance, and with the elongate abdomen transversely striated so as to give it the appearance of segmentation. The female is 380-400µ long by 45µ; the male 300µ by 40µ. The three-jointed legs, eight in number, are reduced to mere stubs in the adult. The larval form is hexopod. These mites thus show in their form a striking adaptation to their environment. In the sebaceous glands and hair follicles they lie with their heads down ([fig. 63]). Usually there are only a few in a gland, but Gruby has counted as many as two hundred.