Lice which infest the skin of the horse are of two kinds: one which, by its sharp-pointed mouth, is able to puncture the skin and live on the blood, hence called Haematopinus, and another the head and mouth of which are not adapted for puncturing the skin; this variety feeds on the loosened cuticle and clings to the hair, hence the name, Trichodectes, which is given to it. The two varieties are commonly found together. In very young animals lice are frequently extremely abundant, and by the constant irritation which they produce they prevent the animal from feeding or resting in any comfort. It is consequently important to employ some dressing which will have the effect of killing the parasites. Various agents are in favour for this purpose; the most simple is any kind of fat oil which will destroy the lice by stopping their breathing pores, as train-oil and linseed-oil. Mercurial ointment is a very effective remedy, but it is also very dangerous. Tobacco water, made by boiling an ounce of tobacco in a quart of water, is also a very good dressing, or, instead, a solution of carbolic acid 1 part to 50 parts of soft soap and water may be used.

In consequence of the invariable presence of numerous eggs on the skin, which will in all probability escape the action of the dressing, it is necessary to repeat it in a week. Afterwards the animal must be examined occasionally in order to ascertain if any more parasites are present. The illustrations in Plate XXXVI, figs. 1 and 2 show the different varieties of lice found on the skin of the horse.