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Bovine besnoitiosis is a parasitic disease caused by the protozoan Besnoitia besnoiti. Described many decades ago, recent epidemiological studies reveal its important spread within Europe in the last years. To date, many epidemiological aspects related to life cycle, routes of transmission, incidence rates and associated risk factors are lacking; hence, the establishment of appropriate disease control programmes poses an important challenge. Thus, the aim of the present study was to determine the epidemiological pattern of the disease in an endemic herd reared under extensive conditions (Spanish Pyrenees) by identifying main factors associated with infection and clinical disease dynamics. The study population consisted of 276 Brown Swiss and Pirenaica adult animals and 145 calves born and weaned at the farm during the study. Three sampling time frames were used: January 2010, September 2010 and February 2011, which allowed us to differentiate two periods designated as mountain and valley periods. The data related to animals (breed, sex and age) and herd management (animal grouping and time in housing) were recorded. The data collection methodology was mainly based on clinical examinations and defining the serological status against bovine besnoitiosis by the immunofluorescent antibody testing of blood samples. The total prevalence among adult animals was 38.34% (CI95%: 34.53–42.07), with 18.54% of seropositive animals showing clinical signs. In regard to the cumulative incidence, 34.57% of new infections were detected during the mountain period, in contrast to the 24.59% observed in the valley period. The incidence density was 0.058 and 0.061 new infections per animal-month for the mountain and valley periods, respectively. According to the seroepidemiological study, the seroconversion probability of B. besnoiti infection was directly associated with the number of seropositive cows with whom an animal had been stabled as well as the housing period duration, supporting horizontal transmission by close contact as one of the most important methods of disease spread. In addition, the risk of developing the clinical course increased with age, and the presence of clinical signs was related to higher antibody responses. Among calves (from 3.1 to 7.1 months old) sampled once at weaning, the total seroprevalence was 15.17% (CI95%: 9.36–21.04), and the chronic stage was observed in three animals, supporting the ability of B. besnoiti to infect and even cause disease in animals less than 6 months old. Finally, the risk of calf seroconversion was positively related to the serological status of the cows, suggesting postnatal transmission between dams and offspring by contact during the suckling period. [ABSTRACT FROM AUTHOR]