Epidemiological study of risk factors associated with Mycoplasma bovis infection in fattening calves

To establish the role of Mycoplasma bovis as an agent of respiratory diseas e in fattening calves, an epidemiologic study was undertaken. A recently,va lidated commercially available ELISA was used to diagnose M. bovis infectio n by seroconversion in paired sera obtained for each animal at entry in the fattening herd and at follow-up seven weeks later. Management data as well as relevant clinical and epidemiological variables were prospectively reco rded. The overall seroconversion rate observed among the 415 calves in 23 f attening herds on 13 farms was 54.7%. Significant factors for seroconversio n were the mixing of fattening herds of different age groups (risk ratio RR 1.70, 95% confidence interval (CI) 1.48 to 1.96), and the presence of at l east one seropositive animal in the fattening herd (RR: 2.02; CI: 1.69 to 2 .40). The proportion of clinical episodes of respiratory disease attributab le to M. bovis infection was 50.3%. The average weight gain during the obse rvation period was reduced by 7.6% in seroconverting calves and these anima ls had about 2 times more antibiotics prescribed by a veterinarian than cal ves remaining negative for M. bovis throughout follow-up (RR 1.83). Materna l antibodies against M. bovis were detected in 39% of new-born calves born from seronegative cows and had a half-life of 20 days, potentially limiting the usefulness of vaccines against M. bovis in this age group.