Onset and remission of ovine footrot.

Sheep exposed to uncontrolled outbreaks of footrot were observed on one farm for 3 successive years and on another for one year. Observations were made from mid-winter to early summer. In each outbreak about 70% or more of sheep exposed to footrot acquired the disease. Natural remission occurred in each outbreak and some cases healed while new cases continued to develop. A feature of all outbreaks observed was that a much higher proportion of late onset cases healed than cases which developed early in the outbreaks. By contrast, 90% of sheep already affected before spring had lesions which persisted into next summer. It is suggested that there is a spectrum of resistance to footrot among sheep. This spectrum includes animals which remain chronically infected and those that do not become infected at all. Intermediate levels of resistance are reflected firstly in the time taken by different groups of sheep, to become affected after exposure and secondly, to heal after becoming affected.