Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

Three species commonly attack man. Strangely enough, there are very few accurate data regarding their life history.

Pediculus humanus ([fig. 65]), the head louse, is the most widely distributed. It is usually referred to in medical literature as Pediculus capitis, but the Linnean specific name has priority. In color it is of a pale gray, blackish on the margins. It is claimed by some authors that the color varies according to the color of the skin of the host. The abdomen is composed of seven distinct segments, bearing spiracles laterally. There is considerable variation in size. The males average 1.8 mm. and the females 2.7 mm. in length.

The eggs, fifty to sixty in number, stick firmly to the hairs of the host and are known as nits. They are large and conspicuous, especially on dark hair and are provided with an operculum, or cap, at the free end, where the nymphs emerge. They hatch in about six days and about the eighteenth day the young lice are sexually mature.

The head lice live by preference on the scalp of their host but occasionally they are found on the eyelashes and beard, or in the pubic region. They may also occur elsewhere on the body. The penetration of the rostrum into the skin and the discharge of an irritating saliva produce a severe itching, accompanied by the formation of an eczema-like eruption ([fig. 66]). When the infestation is severe, the discharge from the pustules mats down the hair, and scabs are formed, under which the insects swarm. "If allowed to run, a regular carapace may form, called trichoma, and the head exudes a fœtid odor. Various low plants may grow in the trichoma, the whole being known as plica palonica."—Stiles.

Sources of infestation are various. School children may obtain the lice from seatmates, by wearing the hats or caps of infested mates, or by the use, in common, of brushes and combs. They may be obtained from infested beds or sleeper berths. Stiles reports an instance in which a large number of girls in a fashionable boarding school developed lousiness a short time after traveling in a sleeping car.

Treatment is simple, for the parasites may readily be controlled by cleanliness and washing the head with a two per cent solution of carbolic acid or even kerosene. The latter is better used mixed with equal parts of olive oil, to avoid irritation. The treatment should be applied at night and followed the next morning by a shampoo with soap and warm water. It is necessary to repeat the operation in a few days. Xylol, used pure, or with the addition of five per cent of vaseline, is also very efficacious. Of course, the patient must be cautioned to stay away from a lighted lamp or fire while using either the kerosene or xylol. While these treatments will kill the eggs or nits, they will not remove them from the hairs. Pusey recommends repeated washings with vinegar or 25 per cent of acetic acid in water, for the purpose of loosening and removing the nits.

Treatment of severe infestations in females is often troublesome on account of long hair. For such cases the following method recommended by Whitfield (1912) is especially applicable:

The patient is laid on her back on the bed with her head over the edge, and beneath the head is placed a basin on a chair so that the hair lies in the basin. A solution of 1 in 40 carbolic acid is then poured over the hair into the basin and sluiced backwards and forwards until the whole of the hair is thoroughly soaked with it. It is especially necessary that care should be taken to secure thorough saturation of the hair over the ears and at the nape of the neck, since these parts are not only the sites of predilection of the parasites but they are apt to escape the solution. This sluicing is carried out for ten minutes by the clock. At the end of ten minutes the hair is lifted from the basin and allowed to drain, but is not dried or even thoroughly wrung out. The whole head is then swathed with a thick towel or better, a large piece of common house flannel, which is fastened up to form a sort of turban, and is allowed to remain thus for an hour. It can then be washed or simply allowed to dry, as the carbolic quickly disperses. At the end of this period every pediculus and what is better, every ovum is dead and no relapse will occur unless there is exposure to fresh contagion. Whitfield states that there seem to be no disadvantages in this method, which he has used for years. He has never seen carboluria result from it, but would advise first cutting the hair of children under five years of age.