A longitudinal study was performed in 20 Dutch dairy ferms experiencing acute clinical Mycoplasma bovis outbreaks. The farms were visited five times at three-week intervals, with collection of blood, milk and eye swab samples from clinically diseasedand randomly selected healthy animals in three age groups: calves (1-6 months), young stock (6-24 months) and dairy cows, as well as bulk tank milk (BTM) samples. M. bovis bacteria were detected by culture of milk and polymerase chain reaction (PCR) of eye swabs, and antibodies against M. bovis were tested using an enzyme-linked immunosorbent assay (ELISA) on serum samples. Data were analysed using logistic regression models correcting for repeated measurements and within-herd correlations. M. bovis waspresent in all age groups within a few weeks after the onset of clinical signs. Most M. bovis test-positive cattle did not exhibit clinical signs and thus constitute hidden sources of infection. All farms continued to be infected for the duration of thethree-month study period.
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