The seasonal patterns, however, differ between outbreaks, and in a manner which does not reflect differences in the timing of lambing.
Within 1 week of lambing, the ewes and lambs were moved onto 1 of 3 fields close to the farmhouse.
However, the observed mean lambing date was 7 days later than expected.
The ewes grazed this pasture until 82 days before the expected lambing date (day k82), at which time their litter size were assessed via ultrasonic scanning.
The outbreak was probably due to exposure to dust contaminated by a passing flock of sheep, which at the time of the outbreak was engaged in lambing.
This date was, on average, 21 days before lambing (day k21) and the observations during the 6-week mid-pregnancy feeding period have been presented relative to this mean day.
The vaccine was less effective in lambs from farms where ewes were vaccinated prior to lambing, reducing infection from 3-5% to 2-5%, a reduction of 29%.
The non-availability of the discretion for weekends and bank holidays means that many sheep farmers must apply for exclusions to cover lambing periods.
