Some grow 20 times their normal size and can drink up to two milliliters of blood. A heavy tick infestation can irritate the host horse enough to interfere with grazing and can lead to loss of body condition or anemia. Depending on the species, ticks live for two years or more. Adult females feed on warm-blooded animals before they produce eggs, which they lay from late spring to midsummer. The resulting larvae do not feed on warm-blooded hosts until the following spring, when they feed, molt, develop into nymphs, feed again, and then become adults, ready to mate and lay eggs.
Ticks climb onto vegetation to wait for a host to pass by so they can grab on.
Once solidly attached, with mouth parts and head buried in the skin, they can be hard to dislodge. Ticks are the main carriers of many protozoan diseases, including piroplasmosis. The protozoa in most cases survive from one generation of ticks to successive generations by infecting their eggs. Some ticks also spread diseases caused by bacteria, viruses, and rickettsia. A viral disease sometimes spread by ticks is the Western stain of equine encephalomyelitis.