Ovine Johne's disease

Johne's disease is an infectious enteric disease caused by the bacterium Mycobacterium avium subspecies paratuberculosis (MAP). The disease is insidious with a long incubation period which results in chronic inflammation, leading to a progressive granulomatous enteritis. Subsequent poor absorption of nutrients and reduced metabolic efficiency lead to reduced reproductive performance, progressive weight loss, impaired immunocompetence and ultimately reduced productivity.

The disease deleteriously impacts animal welfare and is responsible for economic losses because of decreased productivity, increased replacement costs associated with premature culling and increased mortality. Paratuberculosis also arouses concerns regarding public health; ongoing contention exists around the potential zoonotic link between Johne's disease and Crohn's disease in humans.

The two main groups of MAP strains are ‘type S’ and ‘type C’, named after the host species from which they were originally isolated, sheep and cattle respectively (Collins et al, 1990). This nomenclature used for the S- and C-type strain classification implies an exclusive correlation between strain type and host species of origin, which is not the case. C-type strains are capable of infecting a broad range of host species, including sheep, and a wide range of wild and domestic species, including non-ruminants globally (Stevenson, 2015; Bryant et al, 2016). C-type usually predominate in cattle isolates (Stevenson, 2015), although Verdugo et al (2014) reported that S-type strains were more common in New Zealand beef cattle than C-type indicating that S-type also has the capacity to infect cattle and other species as well as sheep (Whittington et al, 2001; de Juan et al, 2006; Ghosh et al, 2012; Verdugo et al, 2014).