Animal Health: Immunity, Probiotics and Health Status

The study evaluated possible transmission of Mycoplasma bovis from sand bedding to naive dairy calves. Screening of a closed herd showed 99% probability that the herd was free of mycoplasma in calves. Neonatal calves (n = 12) from the herd were blocked by weight and height and randomly assigned as controls (n = 6) bedded with quarry sand, or exposed (n = 6) bedded with M. bovis-positive bedding sand (confirmed by PCR) from another farm. Calves were housed at Utah State University in calf hutches, fed commercial milk replacer and calf starter, with strict biosecurity and separation between groups. Exposed group sand cultured positive for Mycoplasma spp. during weeks 1, 5, 6, 7, 11 and negative for the rest of the 15 week study; control group sand was always mycoplasma-negative. Exposed group calves were bedded on mycoplasmal bedding for 138 total calf-days. All 94 sera tested for antibody against M. bovis were negative. All 16 tracheal swabs and all 67 nasal and ear swabs collected from all calves were mycoplasma culture-negative. Two calves died and 3 were euthanized before the end of the study; the remaining 7 calves were euthanized after 15 weeks. All calves were necropsied and full diagnostic testing was performed. No exposed or control calves had any gross lesions of mycoplasma infection. All post-mortem culture (n = 60) and PCR (n= 48) tests on trachea (cultured only), deep lung, peri-bronchial lung, retropharyngeal lymph node, and carpal or tarsal joint fluid from all 12 calves were negative for Mycoplasma spp. The PCR could differentiate M. bovis if positive. Using test sensitivity and sequential probability, the probability of each calf being detected positive at least once if they had become infected with mycoplasma following 4 weeks of exposure was calculated. For the 9 calves that survived beyond 25 days of age, probabilities of detection were between 96.5% and 99.3%. There was no evidence that Mycoplasma bovis-positive bedding sand was a source of infection to naive dairy calves. Further studies in lactating cows are still indicated because of the possibility of infection through teat ends.