Topical treatment of ovine footrot
Dichelobacter nodosus, the infectious agent of footrot, is easily destroyed in the laboratory by air and water. Yet it is an alarming survivor in the ruminant hoof, its natural habitat. In under-run and deep covert (hidden) footrot lesions, D. nodosus may survive five days of exposure to zinc sulphate, a powerful disinfectant that also dries the surface of the interdigital skin. Although topical treatments may ‘cure’ many infected sheep, they will seldom achieve 100% eradication of D. nodosus bacteria from the feet of all sheep.
The destruction of D. nodosus in footrot lesions by repeated foot bathing appears to be cumulative. Zinc sulphate plus wetting agent penetrates a short distance into skin tissue and gradually reduces the number of viable D. nodosus bacteria in superficial lesions.
D. nodosus is inaccessible to surface treatments in deep under-run lesions. Research has shown that there is a significant risk that a treatment of five daily bathings of feet with under-run and deep footrot lesions (lesion score 3 or 4) will be unsuccessful. Five daily bathings are needed to ensure complete destruction of D. nodosus in feet with lesions of maximum score 1 or 2. In early summer, the five bathings may be spread over ten days.
What is covert footrot?
Sheep with covert footrot carry D. nodosus without showing obvious lesions. In general, covert footrot has two forms:
Superficial covert footrot
This is the most common form in Western Australia, where outbreaks are invariably associated with mild virulent footrot. D. nodosus is present on or near exposed surfaces of the skin and is susceptible to external factors. Five-day foot bathing is very effective against this type of footrot.
Deep covert footrot
This form of footrot lesion is where D. nodosus has invaded the deeper tissues of the hoof. The skin or horn surface may appear to have healed and scar tissue may be evident but it is dry. Foot bathing will not cure footrot in these circumstances. This form of covert footrot is usually associated with highly virulent organisms. However, it is also a potential cause of treatment failure in susceptible sheep that are chronically infected with lower virulence organisms, particularly in flocks located in high rainfall environments.
Why five-day foot bathing?
Five-day bathing will increase the probability of eradicating superficial footrot lesions that may otherwise progress to deep lesions. It can also minimise the spread and progress of minor lesions during the ‘spread period.’ However, in practice, foot bathing during mid to late summer increases the risk of creating under-run footrot lesions surrounded by scar tissue that then acts as a source of infection after the next break of season. For this reason, foot bathing is not recommended for use in mid to late summer or autumn.