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This study was designed to examine (i) the seroprevalence of Mycobacterium avium subs paratuberculosis (MAP) in a high-producing dairy herd with clinical symptoms of bovine paratuberculosis, (ii) MAP seroconversion and seronegativation dynamics in the herd and (iii) possible relationships between MAP infection status and herd reproductive performance. One single blood test per cow was performed early post-partum on a monthly basis from day 10–40 post-partum during the first year of the study in 519 cows belonging to a commercial dairy herd. A subset of 111 cows that became pregnant during the study was tested again 60–200 days later during the early foetal period, immediately after the first confirmation of gestation at 58–64 days post-AI. Logistic regression analysis indicated no effect of any independent variable on MAP seropositivity and conception rate 28–34 days post-AI. MAP seropositivity was not a factor affecting the anoestrous, subfertility and early foetal loss rates. In the subset of 111 cows, animals that seroconverted had a 3.9 times greater risk of suffering from early foetal loss (30.3%, 10/33) than the remaining pregnant animals (10.3%, 8/78), (95% confidence interval: 1.11–13.4; p = 0.003). In conclusion, early foetal loss was positively correlated with seroconversion to MAP. Reproductive performance was not impaired by MAP infection.