As she burrows into the skin the female deposits her eggs, which measure about 150 × 100µ. Fürstenberg says that each deposits an average of twenty-two to twenty-four eggs, though Gudden reports a single burrow as containing fifty-one. From these there develop after about seven days, the hexapod larvæ. These molt on the sixteenth day to form an octopod nymph, which molts again the twenty-first day. At the end of the fourth week the nymphs molt to form the sexually mature males and the so-called pubescent females. These pair, the males die, and the females again cast their skin, and become the oviparous females. Thus the life cycle is completed in about twenty-eight days.
The external temperature exercises a great influence on the development of the mites and thus, during the winter, the areas of infestation not only do not spread, but they become restricted. As soon as the temperature rises, the mites increase and the infestation becomes much more extensive.
In considering the possible sources of infestation, and the chances of reinfestation after treatment, the question of the ability of the mite to live apart from its host is a very important one. Unfortunately there are few reliable data on this subject. Gerlach found that, exposed in the dry, warm air of a room they became very inactive within twenty-four hours, that after two days they showed only slight movement, and that after three or four days they could not be revived by moisture and warming. The important fact was brought out that in moist air, in folded soiled underwear, they survived as long as ten days. Bourguignon found that under the most favorable conditions the mites of Sarcoptes scabiei equi would live for sixteen days.
The disease designated the "itch" or "scabies," in man has been known from time immemorial, but until within less than a hundred years it was almost universally attributed to malnutrition, errors of diet, or "bad blood." This was in spite of the fact that the mite was known to Mouffet and that Bonomo had figured both the adult and the egg and had declared the mite the sole cause of the disease. In 1834 the Corsican medical student, Francis Renucci, demonstrated the mite before a clinic in Saint Louis Hospital in Paris and soon thereafter there followed detailed studies of the life history of the various itch mites of man and animals.
The disease is a cosmopolitan one, being exceedingly abundant in some localities. Its spread is much favored where large numbers of people are crowded together under insanitary conditions and hence it increases greatly during wars and is widely disseminated and abundant immediately afterwards. Though more commonly to be met with among the lower classes, it not infrequently appears among those of the most cleanly, careful habits, and it is such cases that are most liable to wrong diagnosis by the physician.
Infection occurs solely through the passage, direct or indirect, of the young fertilized females to the skin of a healthy individual. The adult, oviparous females do not quit their galleries and hence do not serve to spread the disease. The young females move about more or less at night and thus the principal source of infestation is through sleeping in the same bed with an infested person, or indirectly through bedclothes, or even towels or clothing. Diurnal infestation through contact or clothing is exceptional. Many cases are known of the disease being contracted from animals suffering from scabies, or mange.