Epidemiology of ovine Johne's disease 2 - pasture contamination level and age susceptibility.

Abstract

To develop pasture management strategies for control of ovine Johne’s disease, research was conducted in a flock of 840 sheep to determine whether age of sheep and pasture contamination levels affect ovine Johne’s disease. Lambs (median age 5.5 months at the start of the trial) were highly susceptible to infection and should not be exposed to high levels of contamination or a proportion will develop severe infection leading to death. Of great benefit to industry was the finding that hoggets and adult sheep were relatively resistant to the clinical effects of OJD. Even though infection occurred, ewes rarely succumbed to the disease while hoggets were more resistant than lambs. This means that hoggets (median age in this study 22.5 months) and adult sheep can be used to graze contaminated pasture with less risk of clinical impact compared to lambs, and so these classes of sheep can be used to prepare pasture for lambs. Pasture spelling should also be useful. Conventional wire strand fences do not prevent spread of infection so disease control needs to be based on an area-wide approach.