Transmission of Mycoplasma dispar among a succession of newborn calves on a dairy farm.

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 interval 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 lenghtening effect on the figures was estimated at 2 to 3 days. By subtraction, 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-borne, 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.