Arthropods damage their hosts in a variety of ways. Their feeding on the host's skin may cause intense itching, especially characteristic of lice and mites. Damage by mites causes the skin disease called mange, characterised by raw, thickened skin that has lost its hair. Much of the damage is actually inflicted as a result of the pigs responding to the very intense itching caused by mites. Rubbing, licking, and scratching irritate the skin, which becomes raw. Fluid oozes onto the surface, coagulates, and dries to form a crust. This damaged area expands as mites travel to the edges of the lesion seeking fresh skin upon which to lay their eggs.

The entire lesion is susceptible to secondary bacterial infections which cause even more damage to the skin. Infestations of lice are less harmful than are those of mites, but are serious in some geographical locations.

Animals plagued by external parasites become anxious and do not feed or rest well. The result of parasite worry may be reduced gains and possibly even weight loss.

Life cycle of 1 host tick

Life cycle of 2 host tick

Note: in any stage of the multi-host tick life cycle, the host may be the same animal in the same species, or a member of anther species.

Life cycle of 3 host tick

Sucking of blood by lice and ticks can be a serious drain on animals, which must continually replace the lost blood. When the louse population is sufficiently high, iron stores, which are used for blood elements, become depleted and anaemia occurs. Heavily infested pigs may be forced to use ingested nutrients to replace losses rather than the build lean body mass or muscle, thereby reducing feed efficiency.

One of the damaging characteristics of ectoparasites is their ability to transmit disease-carrying microorganisms to their hosts. The hog louse (Haematopinus) can carry the virus of swinepox from one pig to another. Ticks may transmit protozoans of the genus Babesia and Eperythrozoonwhich can cause acute disease and even death in swine of all ages and abortion in pregnant sows.

The annoyance from the severe itching caused by mites and lice distracts swine from eating thereby reducing weight gain.