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This article was published in 1967
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
The Control of Footrot
N. P. H. GRAHAM, B.V.Sc., Senior Research Scientist, McMaster Laboratory, C.S.I.R.O.
For a number of years we have been studying the epidemiology of footrot at the McMaster Laboratory and some of our findings have indicated ways in which efficient footrot control systems can be planned. One of the biggest problems in the control of footrot is the evolution of a method of managing infected flocks, bearing in mind that whatever is done to these flocks has to be integrated into the management of the property as a whole and we often find agricultural practices such as hay-making or cereal-cropping have a prior call on the resources available. Because of the multiplicity of factors involved, each property should be considered individually and what may provide a satisfactory answer in one case may be quite uneconomic in another.
A number of experimental findings have influenced our thinking in this respect.
The first of these is that the footrot organism always gains entry through a small superficial lesion on the skin between the heels. Within a few days the infection spreads down under the horn, invading the tissue of the corium. The organism does not penetrate through the horn or live entirely in the horn tissue. These findings showed that the early lesions before the infection spread under the horn would be susceptible to topical applications of a suitable bactericide. Secondly, antibiotics administered parenterally could be transported to the site of infection under the horn.
Another finding was that footrot would only spread under very specific environmental conditions. Prolonged wetting of the feet was necessary. Isolated rainy periods of four or five days in an otherwise dry period do not induce the spread of footrot. Temperature also exerts a controlling influence and at low temperature the spread of footrot is inhibited. Mean ambient temperatures of about 50°F. provide the dividing line. Below this temperature footrot will not spread, in wetness of the environment. Above this temperature adequate moisture is the limiting factor. Because of this interaction between temperature and moisture the spread of footrot tends to become highly seasonal and periods favourable for spread are usually of only a few weeks duration.
It is possible therefore to regard footrot as being present in two phases.
The active phase, characterised by the rapid spread of the disease, occurs in warm weather when the pasture is wet for long periods. In this phase the lesions show an acute ulcerative interdigital dermatitis and active invasion of the hoof. F. nodosus is comparatively common in smears taken from lesions during this stage.
The chronic phase is characterised by the fact that the disease is not spreading. The weather is cold or the environment is dry or only intermittently wet. In this stage the lesion may spread under the horn but the interdigital dermatitis regresses and may be absent. F. nodosus is often difficult to find in smears taken at this stage or may apparently be absent.
The existence of these two phases imposes important considerations in the control of footrot.
The control of footrot requires the same sequence of procedures as the control of any other epizootic. First, the disease must be accurately diagnosed. Second, the spread of the disease must be stopped and finally, infected animals must be cured or slaughtered.
1. The diagnosis of footrot is often difficult. The constant location of the initial lesion and the invasion under the horn are helpful characteristics, but it largely depends on clinical experience.
2. The spread of the disease is confined to the active phase and measures to reduce the spread of infection are only of value during this phase.
3. The treatment or elimination of infected sheep is most effectively carried out during the chronic phase. These measures are much less effective in the active phase.
Control programmes can therefore be organised along the following lines:
ACTIVE PHASE
Control of spread. Speed in the initiation of control measures is important. The movement of sheep on the property should be limited to prevent the infection being spread from one flock to another.
Infected flocks should be treated as soon as possible to limit the spread of the disease in the infected flocks. Regular footbathing is the only treatment so far devised which supplies the required control. The interval between treatments is important: three or four days is the most desirable period because, as the interval is increased, the degree of control decreases and at fortnightly intervals no worthwhile control is achieved. There is no advantage in holding sheep in the bath, a simple walk-through treatment being sufficient. This treatment has the advantage that it can be quickly organised. It can be carried out in the paddock in which the sheep are confined. Large numbers of sheep can be treated quickly. Driving infected sheep is reduced to a minimum and so ewes can be treated during pregnancy or when they have lambs at foot.
Attempts to examine and treat infected sheep in this phase are contraindicated as it is a slow process requiring the sheep to be held in yards for long periods and so facilitating the spread of the disease; moreover, some lesions are in a very early or incubation stage and would be missed. Treatments of individual feet carried out under these conditions are less effective than when they are done during drier seasons and many may be quite ineffective.
Foot bathing should be carried on as long as the active season lasts. This may be no more than four to six weeks. Once the environment dries out footrot will not spread and so there is no object in foot bathing except that it will cure some feet.
THE CHRONIC PHASE
In the chronic phase the disease is no longer spreading in the flock and so there is more latitude in timing treatments. In this phase a choice must be made of a number of possible lines of action and plans must be drawn up to implement the decision.
The first choice is one of a rather negative nature in that the owner decides he is unable to eliminate all infected sheep from the property and will have to rely on active phase control measures and the treatment of a limited number of infected sheep. Such a choice would be rational if clean musters are impossible or the enterprise involves continually buying sheep. In some areas this choice is forbidden by legislation.
The second choice involves the selling of all sheep and restocking with clean sheep. As this relieves the owner of any further handling of infected flocks it can be economically attractive, the loss on the sale of the sheep being less than the cost of treatment especially if it releases labour for more profitable farming practices. However, it requires an available reservoir of clean sheep which can be purchased.
The third choice involves examining the feet of all sheep and segregating them into clean and infected flocks. This involves quite a lot of time and labour. Some paring is usually necessary to establish the absence of infection, however, to attempt to pare sheep for treatment at this stage usually slows down the whole procedure to an undesirable extent. The establishment of the clean flock is the prime object. Subsequently the clean sheep must be checked and rechecked to ensure no infected sheep have been missed and this work must have priority over treatment of infected sheep. To establish this clean flock is expensive and it must therefore be guarded against the possible reintroduction of the disease. This is always a danger if infected sheep are retained on the property.
The infected sheep can be handled in one of two ways. They can be sold for slaughter or an attempt can be made to cure the majority of them. Which course of action is most desirable depends on a number of factors such as the composition of the flock, the number of sheep involved, etc. It may be worth while trying to cure breeding stock if they have special genetic value but it is doubtful if it is worth trying to cure wethers. The number of sheep involved will depend very largely on the success or otherwise of the treatment carried out during the active phase. Curative treatments are, however, expensive and saving the cost of these treatments may more than compensate for any loss incurred by selling the infected sheep. If it is decided to cure infected sheep it must be remembered that a lot of them usually fail to respond and these eventually have to be sold for slaughter. The question which has to be resolved is how many treatments should be attempted before a sheep is classed as incurable. It is probably economic to attempt one treatment but serious consideration should be given to the desirability of re-treatments.
Two methods of treatment are available, the topical application of a germicide and the parenteral administration of an antibiotic.
TOPICAL APPLICATION
To ensure a high proportion of cures very careful and thorough paring is essential prior to applying the medicament and requires a considerable amount of skill by the operator. This operation is best done in an atmosphere free from pressure and this does not exist at the initial segregation where large numbers of sheep must be handled expeditiously. Labour accounts for most of the cost of treatment and it is therefore uneconomic to apply any bactericide which does not give a high percentage of cures.
PARENTERAL ADMINISTRATION
The use of the parenteral administration of antibiotics is the result of recent work on the histopathology of footrot. The administration of drugs by this means is relatively simple and as only sufficient paring is necessary to establish that the sheep is infected it is not subject to the errors which occur with paring for topical applications. This method of treatment is rapid and could easily be integrated into the initial segregation procedures. The cost of the drugs is high but this may well be offset by savings in time and labour.
Whichever method of treatment is used these sheep have to be checked and rechecked to ensure that no infected sheep remain in the group.
In conclusion, to obtain effective control of footrot it is necessary to fully understand the objective and economic implications of each particular phase of the control measures. Careful advanced planning is essential to ensure that the suggested programme is within the managerial and financial capabilities of the owner. Once a line of action has been chosen planning is necessary to integrate the necessary measures into the overall managerial programme of the property. Failure to thoroughly plan the control measures usually leads to a waste of money and effort and often to failure of the venture.