Poll evil

Poll evil is a traditional term for a painful condition in a horse or other equid, that starts as an inflamed bursa at the anterior end of the neck between vertebrae and the nuchal ligament, and swells until it presents as an acute swelling at the poll, on the top of the back of the animal's head. The swelling can increase until it ruptures and drains. It can be caused by infection from Actinomyces bovis or Brucella abortus organisms, but may also occur due to parasite infestation, skin trauma, or badly fitting horse tack. Because of modern efforts to reduce the incidence of brucellosis in livestock, horses are less exposed to the Brucella abortus organism, and hence most modern cases of poll evil arise from trauma linked to a horse striking its head against poorly designed or low-clearance structures, or to improper use of equipment, particularly leaving a halter on the horse around the clock.