Within-flock transmission of sheep scab in naive sheep housed with single infested sheep.

The within flock transmission of sheep scab was studied in two trials, which lasted 14 and 12 weeks, respectively. A total of 40 non-infested, scab-naive sheep were divided into six groups of between 6 and 20 individuals. Each group was subsequently housed with a single infested index case, carrying scab lesions of different ages. The groups of sheep in both trials were video recorded continuously and all the sheep were individually examined on a weekly basis. The occurrence of transmission was measured in two ways: first, by the detection of early scab lesions during clinical examination and, second, by an increase in the rubbing of individual sheep as noted from the analysis of the video recordings. These two methods complemented each other in predicting the week on which transmission occurred. Overall, transmission was detected in 34 of the 40 scab-naive sheep. Transmission was shown to occur at about the time when the population of adult mites in lesions on index cases reached their peak numbers. The highest number of mites at this time, however, differed markedly between index sheep. There was no obvious relationship between the incidence of transmission and the time index sheep spent rubbing each day or other index sheep variables, including the age of the lesion or the time since the introduction to the scab free sheep.