Control of Mycobacterium avium subsp. paratuberculosis infection on a New Zealand pastoral dairy farm.

Background: Johne's disease is a major production limiting disease of dairy cows caused by infection with Mycobacterium avium subsp. paratuberculosis in calf-hood. The disease is chronic, progressive, contagious and widespread with no treatment and no cure. Economic losses arise from decreased productivity through reduced growth, milk yield, fertility and also capital losses due to premature culling or death. Control chiefly centers upon removing those animals which actively shed bacteria and protecting calves from infection. A prolonged pre-clinical shedding phase, lack of test sensitivity, organism persistence and abundance in the environment as well as management systems that expose susceptible calves to infection make control challenging, particularly in pastoral, seasonal dairy systems. Combining a novel testing strategy to remove infected cows along with limited measures to protect vulnerable calves at pasture, this study reports the successful reduction over a four-year period of seroprevalence of cows testing positive for MAP infection in a New Zealand pastoral dairy herd.

Results: For all age groups considered the apparent seroprevalence of cows testing positive decreased from 297 / 1,122 (26%) in 2013-2014, to 24 / 1,030 (2.3%) in 2016-2017. Over the same period, the apparent seroprevalence in primiparous cows decreased from 39 / 260 (15%) to 7 / 275 (2.5%) and in multiparous cows from 258 / 862 (29.9%) to 17 / 755 (2.3%). The reported proportion of calved cows culled annually from suspected clinical Johne's disease fell from 55 / 1,201 (5%) in the year preceding the control program to 5 / 1,283 (0.4%) in the final year of the study.

Conclusions: On this farm, reduction in the prevalence of infection was achieved by reducing the infectious pressure through targeted culling of heavily shedding animals together with limited measures to protect calves at pasture from exposure to Mycobacterium avium subsp. paratuberculosis. Whilst greater protection of young animals through separation from infected cows and their colostrum and milk would have reduced the risk of neonatal infection, this study demonstrates, in this case, that these management measures while prudent were not essential for effective reduction in the prevalence of MAP infection.