from Hany M. Elsheikha writing in Essentials of Veterinary Parasitology:
Ticks are giant acarids (phylum Arthropoda), which have a major veterinary and public health impact. They represent an obstacle in economic growth especially in developing countries. Due to their feeding behaviour ticks inflict considerable physical damage and irritation which disrupt the foraging of livestock, thereby reducing productivity and fitness, and lowering defences against other diseases (e.g. tick-borne fever predisposes lambs to tick pyaemia). Wounds induced by tick bites are open to invasion by secondary bacterial and fungal, and other opportunistic infections. Tick infestation may also cause tick paralysis, thought to be due to a neurotoxin elaborated by the tick's ovaries, and introduced into the host with saliva while the tick is feeding. This condition is generally characterized by progressive, ascending, flaccid motor paralysis with muscle in-coordination and ataxia.
Further reading: Essentials of Veterinary Parasitology