Generally, infections of cows/heifers are transient, often lasting only a few months. Further, the recovered females develop short-lived immunity to re-infection. After an early abortion peaking at about 7–10 weeks of pregnancy and an additional 2–4 weeks for a new oestrus cycle, these cows/heifers become pregnant again if bulls are still available, resulting in an extended breeding season being observed in many T. foetus-positive herds. However, some infected females carry the pathogen for a much longer period of time. Skirrow (1987) found two of seven infected cows maintained infections for 6 and 9 weeks, respectively, after parturition. Furthermore, infected cows/heifers pass the pathogen to virgin or uninfected bulls by mating and the infections are successfully established in these males (Clark et al., 1974a, b).