Horses are attacked by two kinds of lice: a biting and a sucking louse. The biting louse moves freely over the animal, chewing at the dry skin and hairs, while the sucking louse pierces the skin and sucks the blood. The bites are painful, and the blood loss can be a severe drain on the vitality of the host when the lice become abundant. With both types of lice, the coat about the horse's head, the withers, and the base of the tail become unkempt and full of scurf. Parts of the horse's body may be rubbed raw because of the irritation. Lice generally become noticeable during winter and early spring.
The biting louse is about 1/10 inch long and is chestnut brown except for the abdomen, which is yellowish with dark crossbands. The head is broad and rounded in front and forms a full semicircle in front of the antennae. Eggs are glued to horses' hairs close to the skin, especially around the angle of the jaw and on the flanks. The eggs hatch in 5 to 10 days, into small lice of the same general shape as the adult. They are fully grown in 3 or 4 weeks. Breeding is continuous throughout the year, but the numbers become fewer during the summer.
The sucking louse is a dirty grayish or yellowish-brown color, and is about 1/8 inch long. The thorax is only half as wide as the abdomen, and the head, which is distinctly narrowed toward the front, is less than one-third the width of the abdomen. The lice generally are found about the horse's head and neck, and at the base of its tail. The egg stage normally lasts from 11 to 20 days, but eggs may hatch as long as a month after they are laid. The young lice are similar in shape to the adults, except paler in color, and reach maturity in 2 to 4 weeks. There are several generations in a year.
Producers can save on the cost of insecticide treatments for lice by adopting cultural control practices. New horses brought to the farm should be isolated and carefully inspected for lice before they are allowed to mingle with the rest of the animals. Careful and regular monitoring for lice can detect problems before an infestation gets out of control.
Many insecticides and application procedures are effective for managing lice. As with any insecticide application, it is essential to consult the label to ensure the insecticide is registered for use on horses. Several application methods are available.
Insecticides must be used properly to achieve satisfactory control of lice. Many louse-control products require two treatments, 10 to 14 days apart. The second treatment is essential to kill newly hatched lice that were present as eggs at the time of the first treatment and therefore were not killed. Failure to make the second treatment in a timely manner will create problems requiring many more subsequent treatments.