Ectoparasiticides Used in Large Animals: Overview

Arthropod parasites (ectoparasites) are a major cause of production losses in livestock throughout the world. In addition, many arthropod species act as vectors of disease for both animals and humans. Treatment with various drugs to reduce or eliminate ectoparasites is therefore often required to maintain health and to prevent economic loss in food animals. The choice and use of ectoparasiticides depends to a large extent on husbandry and management practices, as well as on the type of ectoparasite causing the infestation. Accurate identification of the parasite or correct diagnosis based on clinical signs is necessary for selection of the appropriate drug. The selected agent can be administered or applied directly to the animal, or introduced into the environment to reduce the arthropod population to a level that is no longer of economic or health consequence.

Parasites that live permanently on the skin, such as lice, keds, and mites, are controlled by directly treating the host. Some mange mites burrow into the skin and are therefore more difficult to control with sprays or dips than are lice and keds, which are found on the surface of the skin. However, once these obligate parasites are eradicated, reinfection occurs only from contact with other infected animals.

Nonpermanent parasites (ticks, flies, etc) are less easily controlled because only a small proportion of the population can be treated at any one time, and other hosts may maintain them. Some tick and mite species stay on the host only long enough to feed, which may be as short as 30 min, or as long as 21 days. Biting flies, such as the horn fly, can be found continuously on the backs and undersides of cattle, where they suck blood up to 20 times a day; other biting flies (such as stable flies and horse flies) and mosquitos feed to repletion, then leave the animal to lay eggs. Nonbiting flies, such as the face fly or the house fly, may visit infrequently but can be very annoying and may transmit disease agents. Larvae of certain blowflies live on the skin or in tissues of sheep and other animals and cause cutaneous myiasis. Larvae of other flies spend several months inside animals, eg, nasal bots in the nasal passages of sheep and goats, bots in the stomach of horses, and cattle grubs or warbles in the spinal canal, back, or esophageal tissues. (See also flies, Flies: Introduction.)

Many ectoparasite infestations are seasonal and predictable and can be countered by prophylactic use of ectoparasiticides. For example, in temperate countries flies are seen predominantly from late spring to early autumn, tick populations increase in the spring and autumn, and lice and mites during the autumn and winter months. Treatments can therefore be targeted at anticipated times of peak activity as a means of limiting disease and parasite populations.