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.
To reduce the rate of culling
Left uncontrolled, the prevalence of sheep with lesions may be over 50 per cent during early summer, often in cases of mixed infection with ‘S’ (virulent) and ‘U’ (benign) strains. Five-day foot bathing can minimise score 1 and 2 footrot lesions associated with these strains, with little or no culling. When applying five-day bathing in this situation, allow enough time between treatment and the summer eradication inspection, according to procedures recommended by your stock inspector.
Detection of lesions after treatment
Five-day foot bathing at pasture senescence does not interfere with the detection of lesions six weeks after treatment. Lesions were observed within two weeks after five-day bathing in an experimental flock infected with a highly virulent ‘S’ strain. However, it was impossible to predict which individual sheep would have lesions after the foot bathing. Flocks infected with low virulence ‘S’ strains of D. nodosus showed no lesions in the six weeks after bathing in early summer or in the following spring.
Factors affecting topical treatment of footrot
Environment, host resistance and virulence of D. nodosus strains are the major factors affecting the severity of lesions and the effectiveness of topical treatments. The deeper the lesions, the lower the chance of successful foot bathing. D. nodosus can persist in deep covert lesions for many months.
Environmental features that help summer eradication also increase the success of five-day foot bathing.
- Footrot can self-cure on dry or sandy, well-drained soils.
- The chance of footrot eradication increases in hot, dry summers.
- Footrot was not eradicated from a sheep flock chronically infected with mild ‘S’ strains of D. nodosus when the sheep were placed on wet, lush pasture after five-day bathing.
Sheep with foot abscesses can harbour the footrot bacterium without showing typical footrot lesions. There is no guarantee that D. nodosus harboured in foot abscesses will be successfully treated by five-day bathing. Abscesses surrounded by thick scar tissue interfere with the footrot treatment.
Other aspects to consider:
- Placing treated sheep on wet or lush pastured environments will greatly reduce the success of five-day foot bathing. Allow feet to dry out on grating or similar overnight between each treatment.
- Foot bathing solutions must be topped up daily and be free of mud.
- Mis-mustering can undo all the benefits of foot bathing treatments.
- Failure to adhere to the treatment protocol increases the risk of breakdown.
Protocol for five-day foot bathing
The feet of all sheep are examined by an experienced inspector and lesion scores recorded in the week before five-day foot bathing.
- All sheep with greater than score 2 lesions, foot abscess or abnormalities as listed in the 'Farmer’s Guide to Eradication’ are culled from the property before bathing.
- Treat only sheep with score 0, 1 or 2 lesions.
- Prepare the foot bathing solution by slurrying 20 percent zinc sulphate heptahydrate plus wetting agent in a drum for 24 hours to dissolve.
- Bathe the sheep in the solution, covering the coronet of the foot, for ten minutes each day.
- Monitor the strength of the solution daily with a Zinc Sulphate Footbath Hydrometer (Nufarm, Victoria).
- Use the same solution for five days, if it is not too dirty, topped up daily with extra chemical to maintain concentration and volume.
- Carry out the foot bathing in a dry environment, such as on grating, to maximise uptake of zinc in the horn. Mud overloads the bath with organic matter that binds and inactivates zinc sulphate.
- Time the daily foot bathing of the mob of sheep so they are on the slats of a shearing shed overnight or a pen with dry sandy soil, or graze a pasture paddock that is completely hay-off. This action prolongs the desiccation effect of zinc to dry the surface of the interdigital skin. The triple action (disinfection, short and longer-term desiccation) is how the daily foot treatment eventually overcomes the D. nodous bacteria’s ability to survive in the ruminant hoof.
- Dispose of the foot bathing solution according to local regulations.
- After foot bathing, keep the sheep on dryland conditions.
Biosecurity
Livestock, machinery, fodder and people can introduce animal and plant diseases, weed seeds and pests. Develop a biosecurity plan for your farm to reduce the risk of these problems.