This study described the dynamics of natural infection in cattle when grazing on tick-infested pastures in an oceanic climate. Four groups of 5 cows or heifers were randomly selected within a dairy herd known to have been infected by Anaplasma phagocytophilum. Blood and serum samples were collected every 15 days and the cattles were subjected to clinical evaluation in order to determine the moment of infection. An indirect immunofluorescence assay was performed to titrate anti-A. phagocytophilum IgG in bovine sera. Nineteen out of 20 animals were seropositive at least once during the grazing period. Titres increased and decreased in parallel with the seroprevalence. This study was able to confirm the seasonal variations in seroprevalence and serum titres of IgG to A. phagocytophilum. Two peaks in seroprevalence are demonstrated specifically the number of seropositive Breton cattle increased in spring, decreased during summer and increased in spring, and then dropped after the middle of November. A PCR positivity to A. phagocytophilum was detected in 4 animals but none were apparently ill. Lack of tick-borne fever signs in infected cattle could be due to immunity acquired in the previous pasture season. However, 1 PCR-positive heifer had not been in the pasture before. Reasons for the inconsistent expression of the disease might be due to the low infective dose inoculated by the vector or to genetic variants of A. phagocytophilum which could differ in pathogenicity.