Antibodies to T. gondii have been found in sheep worldwide (Table 4.1). The prevalence of antibodies in ewes was more than twice that in lambs, but results were dependent on the age of lambs sampled.441,504,601,874,1077,1120 Seroprevalence was shown to increase with age, reaching 95% in 6-year-old ewes in some ocks,306 suggesting that animals acquire infection postnatally. In general, most sheep acquired infection before 4 years of age, but one-third of old ewes were still seronegative in highly endemic ocks.306 Prevalence was also higher in ewes on farms where epizootics of abortions were reported.306 Seroprevalence in intensively managed sheep was lower than in semiintensive management.3,1077,1114,1148

There is little information on the rate of seroconversion in sheep under different management systems, except in a study reported by Lundèn et al.,874 in which a ock of sheep in Sweden was followed serologically for 6 years. Most sheep became infected in autumn. An interesting nding was that 39% of 136 ewes became seropositive while only 6% of lambs seroconverted.