Changes in prevalence of ovine paratuberculosis following vaccination with Gudair®: Results of a longitudinal study conducted over a decade.

Since its registration in 2002, Gudair® vaccine has become the key tool for managing paratuberculosis in sheep in Australia. This study was conducted to evaluate its effectiveness in reducing paratuberculosis prevalence in 12 flocks over more than one decade. Flocks with variable initial prevalence (5, 4 and 3 with low, medium, and high prevalence, respectively) were enrolled in the study in 2003-04. Six biennial faecal samplings were conducted in these flocks over a 10year period. At each sampling, faeces from four age groups (3, 4, 5 and 6year-old sheep) were collected from each flock. The aim was to select 7 or 14 pools of faeces constituted with pellets from 50 or 25 sheep from each age group, respectively, but the numbers and sizes of pools did vary due to logistical issues. Samples were cultured using pooled faecal culture approach and sheep level paratuberculosis prevalence was estimated. Changes in probability of a pool being positive and in paratuberculosis prevalence over time were evaluated by fitting generalised-linear and linear mixed models. The proportion of positive faecal pools significantly declined over time from 50.3% at the first sampling in 2003-04 to only 3.1% at the last sampling in 2013-14, suggestive of a 30-fold reduction in the odds of a pooled faecal sample to be positive (p<0.001). Similarly, the average animal level prevalence in flocks dropped from 7.64% at the first sampling to 0.12% at the last sampling. However, faecal shedding persisted and was still present in 3 of the 8 flocks (37.5%) that remained in the study at the last sampling conducted in 2013-14. This is the longest study ever conducted to evaluate paratuberculosis vaccine effectiveness. The results will enable sheep producers to make informed decisions on managing paratuberculosis, and in evaluating the risks of purchasing and trading vaccinated sheep.