Contagious equine metritis is an inflammatory disease of the proximal and distal reproductive tract of the mare caused by the bacteria Taylorella equigenitalis, which usually results in temporary infertility. It is a nonsystemic infection, the effects of which are restricted to the reproductive tract of the mare. When present, clinical signs include endometritis, cervicitis and vaginitis of variable severity and a slight to copious mucopurulent vaginal discharge. Recovery is uneventful, but prolonged asymptomatic or symptomatic carriage is established in a proportion of infected mares. Direct venereal contact during natural mating presents the highest risk for the transmission of T. equigenitalis from a contaminated stallion or an infected mare. Direct venereal transmission can also take place by artificial insemination using infective raw, chilled and possibly frozen semen. Indirectly, infection may be acquired through fomite transmission, manual contamination, inadequate observance of appropriate biosecurity measures at the time of breeding and at semen-collection centres. Stallions can become asymptomatic carriers of T. equigenitalis. The principal sites of colonisation by the bacterium are the urogenital membranes (urethral fossa, urethral sinus, terminal urethra and penile sheath). The sites of persistence of T. equigenitalis in the majority of carrier mares are the clitoral sinuses and fossa and infrequently the uterus. Foals born of carrier mares may also become carriers. The organism can infect equid species other than horses, e.g. donkeys. Effective vaccines are not yet available. Taylorella equigenitalis is not known to infect humans.