Transmission of Mycoplasma dispar among a succession of newborn calves on a dairy farm
Tanskanen, R.
Acta Veterinaria Scandinavica 28(3-4): 349-360
1987
ISSN/ISBN: 0044-605X PMID: 3454547 Document Number: 303999
Transmission of Mycoplasma dispar among 33 newborn calves (nos. 1-33) on a dairy farm, transferred in order of birth into individual pens in a separate calf rearing room during a period of 50 days, was investigated. The first calf (no. 1) was deliberately infected. Weekly nasal swabs were taken 9 times. Twenty-eight susceptibles (data for 4 calves are excluded because of omissions in sampling) were found to be positive for M. dispar on average 8.6 days (the range 5 to 15 days) after the start of exposure. Among the first 7 (nos. 2-8) and the latter 21 susceptibles (nos. 9-10 and 15-33), the infection was first detected on average after 12.1 and 7.4 days, respectively. The internal measured (from the start of exposure to the first detection of infection) comprised 3 parts: the transmission time proper, the latent period and the delay in detection of positivity due to the sampling interval. The variation affecting the first 2 parts of the measure, and especially the transmission time, clearly must account for the above difference between the 2 study phases; an association with the density of infective calves was apparent. The last component of the measure was distributed randomly and its lengthening effect on the figures was estimated at 2 to 3 days. By substraction, the average period preceding detectable colonisation in the latter part of the study was approximated as 4 to 5 days. Through another approach, estimation by Reed-Muench calculation, the figure was 4.7 days. The existing knowledge of the usual lengths of latency-with the present limited variation among the intervals measured-indicates that the lag in transmission, i.e. the transmission time proper, in the latter phase of the study was unlikely to exceed a few hours; the major part of the average 4 to 5 days interval was thus accounted for by latency. The principal mode of transmission was inferred to have been air-born, sometimes over several meters. Forty-five adult cows, sampled after the delivery, including most dams of the calves studied, were all negative for M. dispar; M. bovirhinis and Acholeplasma laidlawii were isolated from 8 and 3 cows, respectively.