Introduction
The prevalence of Mycobacterium avium subsp. paratuberculosis (MAP) in domestic sheep worldwide is often unknown as the disease is ‘non-notifiable’ in many countries, even when it may be notifiable in cattle (Whittington et al., 2019).
Studying MAP in sheep has had a relatively low priority compared with MAP in cattle or the more politically and therefore strategically important mycobacterial diseases such as tuberculosis. There are huge populations of sheep at risk globally; in a recent survey, 50% of countries had more than 1 million sheep, half of these more than 10 million sheep and there were reports of paratuberculosis in North and South America, Australasia, the Middle East, Asia, Africa and Europe (Whittington et al., 2019).The difficulty of determining the extent of disease has been compounded by the lack of diagnostic tests with high levels of sensitivity and specificity, and the high costs involved with identifying the disease compared with the low value of a sheep. However, the prevalence of MAP appears to differ markedly between countries and geographical regions within countries (Sergeant, 2001; Sevilla et al., 2005). Published data on the prevalence of affected sheep flocks range from 2-32% between different countries (Bakker et al., 2000; Sergeant, 2001; Nielsen and Toft, 2009; Kumthekar et al., 2013; Gurung et al., 2018; Ma et al., 2019). In Spain, the estimated prevalence within sheep flocks is 8-10% (Sevilla et al., 2002). In Australia, MAP is thought to infect 2.4-4.4% of the flocks, based on data from abattoir surveillance. Dairy sheep flocks in Canada and Cyprus have had estimates of 66.8 and 60.8% flocks infected, although these findings may be limited to the farming enterprise examined (Liapi et al., 2011; Bauman et al., 2016b). In New South Wales, Australia, 6-10% of the flocks had ovine paratuberculosis, while in Western Australia less than 1% of the flocks were thought to be infected (Sergeant, 2001; Sergeant and Baldock, 2002). Within- flock prevalence has not been examined in detail but mortalities of 5-15% per year have been seen in high-prevalence flocks (Reddacliff et al., 2006). In 2019, almost half of 11 countries with estimates of the prevalence of MAP in sheep based on objective criteria had greater than 10% of flocks affected, while almost one in five countries had greater than 40% flock-level prevalence (Whittington et al., 2019).
12.2
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