Prevalence, risk factors and vaccination efficacy of contagious ovine ecthyma (orf) in England

Orf is a viral disease found in English sheep flocks which can cause economic losses. It is a zoonosis with little epidemiological research available in the UK. In 2012, 3000 questionnaires were sent to English sheep farms in order to investigate the prevalence of orf, determine vaccination efficacy and to identify some of the potential risk factors. The usable response rate was 25.4 per cent. The usable farms (N=762 in the years 2011 and 2012) were used to model the percentage of animals affected on the farm, and the probability of a farm being found with the disease. The disease prevalence (DP) was standardised for the year and calculated as 1.88 per cent for ewes and 19.53 per cent for lambs. The disease risk ratio (RR) for the use of the vaccine was calculated as 2.04 for ewes and 0.75 for lambs, and therefore, the study found that lamb vaccination was beneficial (RR <1). Weed infestation and an increased number of orphan lambs were associated with increased cases of orf. We conclude that the DP in ewes and lambs affect each other, though the impact is higher for lambs in the presence of increasing prevalence in ewes. A short lambing season lowers the probability of a farm experiencing cases of orf. Vaccination was effective in lambs but not in ewes, though lambs benefitted when ewes were vaccinated (reduced orf prevalence in lambs born from vaccinated ewes), probably because any unvaccinated ewes may have been carriers that could spread the virus to the new-born lambs.