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This article was published in 1961
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.

The Differential Diagnosis and Control of Foot Diseases of Sheep

N. P. H. GRAHAM, B.V.Sc., Division of Animal Health, C.S.I.R.O., McMaster Laboratory, Glebe, N.S.W.

Introduction

To the veterinarian in the field, the differential diagnosis of foot diseases in sheep may present a very perplexing problem. This is particularly so when only one or a few affected sheep are seen, for there is a variety of foot lesions which may be variations of the same disease and different diseases may be present concurrently in the same flock. The elucidation of this complex problem is further hampered by the fact that insufficient is known of the aetiology of some of these conditions.

Some of these foot conditions are due to various types of trauma, sometimes complicated by infection by various types of bacteria which usually seem to be secondary invaders. These bacterial infections are most active and spread most rapidly when sheep are depastured on lush wet pastures when the weather is warm, but they may persist in a chronic form under more adverse environmental conditions.

All these conditions have been described from time to time in scientific literature, and graziers have been familiar with them for many years. Nevertheless, difficulties in differential diagnosis have persisted, as the line of demarcation between several of the conditions is not clear cut. In an endeavour to clarify the position, Gregory (1949) discussed the separate entities of footrot, digital suppuration or foot abscess, suppurative cellulitis and contagious pustular dermatitis. Beveridge (1941) reviewed the literature on a number of different foot diseases of sheep and described "scald". Harriss (1948) described strawberry footrot and differentiated it from contagious pustular dermatitis.

Diseases of the feet may be divided into those which affect the skin and soft structures of the foot and those which involve the horny structure of the hoof. It is usually a fairly easy matter to distinguish between these two groups, but it is often very difficult to differentiate the various disease processes within each group, particularly when only a few affected sheep are available for examination.

DISEASES WHICH INVOLVE THE SKIN AND SOFT TISSUES

These may cause lameness, are of comparatively short duration and some are restricted to certain age groups. They are listed below:—

(a) Contagious Ecthyma (Scabby Mouth): Scabby lesions may occur on the skin of the coronet. The condition starts with an acute inflammatory vesicular stage during which the sheep may show lameness. The vesicles rupture and a rough heavy scab forms over the area. The course of the disease is short and the presence of scabs or vesicles on other sites such as the lips may help diagnosis. sheep are affected and the presence of the virus in the scabs can be confirmed by transmission tests.

(b) Mycotic Dermatitis (Lumpy Wool): Lesions of mycotic dermatitis are occasionally found on the coronet in the form of hard scabs firmly attached to the skin. The condition can persist for long periods. The condition is usually found in young sheep, it does not cause lameness and typical lesions are usually found elsewhere on the body. The causal organism Dermatophilus dermatonomus is present in the scabs.

(c) Proliferative Dermatitis (Strawberry Footrot): This is caused by Dermatophilus pedis which produces areas of proliferating granulation tissue and heaped up scabs on the skin of the coronet or interdigital space. The disease, which was described by Harriss (1948), has not been recorded in this country but could possibly be present.

(d) Suppurative Cellulitis: This was described by Gregory (1939). It is a severe cellulitis, caused by Fusiformis necrophorus, which involves the coronet and may extend well up the leg, especially in sheep wooled down to the coronet. It is most commonly seen in young sheep. The causal organism is present in large numbers in the pus associated with the lesions.

(e) Post Dipping Lameness: This is an acute transient lameness caused by Erysipelothrix rhusiopathiae. It was first diagnosed in New Zealand by Whitten, Harbour and Allan (1948) when sheep which had been dipped in a foul BHC dip became lame. The infection is believed to take place through small wounds and abrasions on the feet. During the acute stage the foot is hot with very slight swelling and is very painful. The sheep recover in about 4 days. As most dips now contain a suitable bacteriostatic agent to control this organism, the condition is rarely seen.

(f) Strongyloides papillosa Invasion: When large numbers of S. papillosa larvae are allowed to invade the skin experimentally, they can produce a severe inflammatory reaction with serious exudation and pus formation. The foot becomes very swollen and the sheep shows severe lameness. The reaction lasts 2 or 3 days. Possibly a similar invasion by these larvae occurs in the field.

(g) Traumatic Lameness: may be due to a wide variety of injuries. Some injuries such as thistle pricks, stubble abrasions or grass seed penetration may cause lameness in a large number of sheep at the same time. Stone bruises, occluded interdigital glands,etc., tend to cause isolated cases of lameness.

FOOT DISEASES WHICH INVOLVE THE HORN OF THE HOOF

These are usually of greater economic importance. The conditions are generally long lasting, they often become chronic and some of them permanently damage the foot. Sheep of all ages are affected, but the incidence is generally higher in older sheep. Lameness is a very common symptom. They are considered below.

(a) Footrot: This is a specific contagious disease caused by Fusiformis nodosus. It usually starts as a mild inflammation of the skin-horn junction on the axial aspect of the claws. This is followed first by a lifting of the horn in this area, then by separation of extensive areas of the horn of the hoof from the underlying epithelium. This separation involves first the axial and then the plantar aspects of the heel and extends later to the sole and abaxial walls. In moderately extensive lesions there is a considerable amount of necrosis and a little pus. It is common to find both claws of the same foot affected and a severe inflammation of the interdigital skin. Lameness is usually evident once the separation involves the sole of the foot. Smears of material collected from an active lesion at the margin of the healthy tissue generally show F. nodosus, Sp. penortha and F. necrophorus. As F. nodosus is often present in small numbers. It may be necessary to examine a number of smears in order to find the organism. Moreover, an organism morphologically similar to F. nodosus is seen in smears from "scald" lesions. The similarity of the flora from footrot and "scald" makes the microscope examination of smears of little value in differentiating between these two conditions. There appear to be some cultural characteristics of F. nodosus that are not shared by morphologically similar organisms, but the difficulty of transporting viable material from the field to the laboratory presents one of the problems that restrict the value of cultural examinations as an aid to differential diagnosis. The rapidity with which a footrot lesion extends is a considerable help in diagnosis, and under favourable conditions a high transmission rate is characteristic of footrot. Under dry conditions, however, the lesions often show little tendency to spread and may even regress, and there is often no transmission of the disease from sheep to sheep.

(b) "Scald”: Although "scald" has been recognised for very many years, very little is known about it. Beveridge (1941) wrote "It was generally described as an inflamed scalded appearance of the skin between the digits with some exudation of serous fluid and often some separation of the horn round the heels and did not extend very far. There is often lameness, which may be severe and is usually worse in early morning and wears off during the day. Scald occurs on lush pastures during warm weather, but sometimes after frosts, and may appear suddenly in a large number of the flock". When under-running of the horn does occur, the lesion is usually drier that that of footrot.

"Scald" has been transmitted experimentally to the scarified feet of sheep by applying material from infected feet. Although in many cases in the field "scald" lesions have healed spontaneously in a short time, in other instances a chronic form of the condition has persisted for some months. "Scald" has usually been associated with warm wet conditions but cases have been seen which have persisted in dry summer weather.

The similarity in the appearance of early cases of footrot and "scald" may make it difficult to differentiate between these two conditions in the field if sheep are seen only on a single occasion, and especially if only a few sheep are available for examination. As stained smears from footrot and from "scald" are very similar, they are of no value in making a differential diagnosis between these two diseases. Affected sheep should be kept under observation and the way in which the lesions progress noted. Provided environmental conditions are suitable, the lesions of footrot will progress so that the hard horn becomes separated from the foot. On the other hand, in "scald" any separation which occurs will usually be confined to the sole of the foot.

(c) Digital Suppuration (Foot Abscess): This almost invariably occurs as a result of a lengthy exposure of the feet to mud contaminated with faecal material. These conditions apparently facilitate the entrance of F. necrophorus. It may enter through cracks or channels in the hoof and may multiply under the horn causing suppuration and severe lameness. There is generally some swelling of the tissues underlying the horn so that lameness is evident even when the lesion is small. There are three or four fairly common sites at which the organism appears to gain entry into the hoof. The axial surfaces of the heels often chafe and the organism under-runs the heel area. The vertical groove at the junction of the heel and the toe on the axial surface of the digit is another common site of infection. Sinuses may extend upwards from shelly toe lesions and sometimes the small ducts in the horn of the toe become infected. Foot abscess lesions are generally associated with large amounts of pus. However, it is sometimes difficult to differentiate clinically between small chronicles ons of foot abscess and small chronic lesions of footrot.

(d) Shelly Toe (Dry Separation): This is quite common in many flocks. The horn of the antero-abaxial wall of the hoof becomes separated from the underlying epithelium. The lesion appears to start at the bottom of the wall and spreads upwards. The cavity beneath the horn may be packed with soil and faecal matter or may be partially empty. If the contents are wet there may be some evidence of bacterial activity such as inflammation of the lamellae or the presence of a sinus extending beyond the general area of the cavity. Material packed in the cavity may cause lameness by mechanical pressure. Sheep of all ages are affected. The cause of the condition is unknown.

CONTROL OF FOOT DISEASES

There is very little information available on the control or economic importance of any diseases of sheep's feet with the exception of footrot.

Foot Abscess caused by F. necrophorus infection, is usually self limiting, but can often cause so much damage to the soft tissues, tendons and skeletal structures of the foot that the foot becomes permanently malformed and there may be permanent lameness. Healing of these infections may be assisted by providing drainage for sinuses which may develop, but often the lesions are diffuse with no definite pockets of pus. Various sulpha drugs have been tried from time to time with little success. Antibiotics have also failed to provide a certain cure. One possible reason for this is that the lesions are generally well established before they are detected and therefore, before treatment is commenced. Prophylactic vaccinations with a F. necrophorus vaccine have been tried on a number of occasions but the results of these trials have never been published. The scanty information available suggests the results have been inconclusive.

Footrot can be effectively cured by paring and dressing the feet with any one of a number of therapeutic agents, of which 10 per cent Chloromycetin in alcohol seems to be the most reliable. However, in evaluating any material for the control of footrot, the curative results are determined not only by the therapeutic agent under test but also by the thoroughness with which the foot is prepared by paring.

While the control of footrot in individual sheep depends on the skill with which the feet are treated, the eradication of footrot from a flock or property depends not so much on the treating of infected sheep as on the grazier's ability to detect and remove all infected sheep from his flocks and to prevent the re-introduction of the disease. This is illustrated by the fact that sheep were treated for footrot in only 15 of the 75 flocks from which footrot was eradicated in West Australia 1958 (Anon. 1959). In the other flocks, infected sheep or even whole flocks were sent in for slaughter.

A footrot eradication campaign on a property is costly and time-consuming, and can easily fail through lack of proper flock management. As the essential part of the campaign is the segregation of infected and/or clean sheep, adequate and secure fencing is necessary. As all sheep must be examined on a number of occasions, adequate labour and time must be available. This is particularly important as footrot treatments or inspections have to be integrated with many other procedures such as haymaking, harvesting or shearing, which normally employ all the available labour. Also, there are periods such as lambing when some sheep cannot be handled.

The time required to examine an infected flock depends very largely on the amount of treatment which has to be carried out. Pryor (1956) found that 30 to 50 sheep per man per day was the average rate of paring, examination and treatment in an eradication campaign he conducted in Victoria, while Ensor (1957) in New Zealand gave a figure of 44 to 56 sheep per man per day. When owners have attempted to eradicate footrot when the incidence of infected sheep has been high, it is very common to find they fail at the first attempt but may be successful the following season. In many cases, this is due to their lack of experience in the managerial procedures associated with eradication but often it is due to the complications which follow a high initial infection. Beveridge (1941) in discussing this wrote "efforts should first be made to reduce the number of affected animals by intensive treatment. Unless the number infected is reduced to 5 per cent at the most. the subsequent procedure is not only more difficult but also more liable to failure". Fitzpatrick (1958) in a series of field tests found that walking sheep through a footbath containing 5 per cent formalin at weekly intervals would reduce the incidence of footrot, even in periods favourable to its spread to the levels indicated by Beveridge as being essential for the commencement of eradication.

In conducting an eradication campaign. preference should always be given to adequate inspections of the clean portion of the flock, even if it necessitates some neglect of the sheep undergoing treatment. They must also be rigorously protected from the re-introduction of footrot by sheep which are thought to be cured. Just how much effort should be expended on attempting to cure every infected sheep depends largely on the numbers involved. At least one attempt is probably justified, but it is doubtful if it is an economic procedure to persevere with those cases which do not respond readily to treatment. Their continued presence on the property is always a danger to the clean sheep, and it is probably more economic to have them killed off than to spend excessive amounts of time and money trying to cure them.

It is generally suggested that an eradication campaign should be carried out in the summer months when the environment is unfavourable for the spread of footrot. This system is based on footrot control in the winter rainfall zones and it would be better to just refer to the dry season. It is not envisaged that nothing should be done to check footrot during the wet season. It is during this period that regular footbathing should be carried out to reduce the number of infected sheep to the low level Beveridge claims is necessary for eradication. While a dry season eradication campaign has many advantages, it is not always possible to fit it in with other aspects of flock management, and in some areas eradication has been carried out successfully during the wet season. In the dry season footrot does not spread and it is possible to restrict the handling of "clean" sheep to routine examinations at fairly infrequent intervals, secure in the knowledge that even if infected feet have been missed they will do no harm. The disadvantages of a campaign at this season are that the feet are generally very hard and, unless they have been soaked, are difficult to pare effectively, and that small pockets of infection do not develop rapidly and may be difficult to find. In the wet season the spread of infection in the clean sheep can be controlled by regular foot bathing at intervals of not more than 7 days. The feet are generally softer and more easily pared and any latent infections develop more rapidly. In the wet season sheep can be concentrated on a small area and so time is saved in mustering and the provision of spelled paddocks is easier.

One other aspect of the planning of an eradication campaign requires consideration, that is, whether the treatments should be carried out at a central point such as the wool shed where quite elaborate facilities can be provided or whether they should be done in temporary yards erected in or near the paddock in which the sheep are running. The latter arrangement limits the facilities which can be provided, but it has the big advantage that stock movements across the property can be avoided and so one possible way of spreading infection is eliminated.

"Scald" often occurs on properties from which footrot has recently been eradicated. A similar condition has been seen in calves where no sheep have been run for many years. Presumably the organism responsible for "scald" is not so specific in its growth requirements as is F. nodosus. Although there is little experience yet with the treatment of "scald", some graziers claim that it can be cured by a single treatment in a formalin footbath.

SUMMARY

There is a number of diseases affecting the feet of sheep. The aetiology of some of them is not fully determined, so that laboratory diagnosis is not precise. Because of clinical resemblance between some stages of some foot diseases, differential diagnosis can be difficult or uncertain.

With the exception of footrot, little is known about appropriate methods for the control of foot conditions. Individual sheep can be cured of footrot, but the eradication of footrot from some properties poses many managerial problems which are influenced by local circumstances.

References:

Anon (1959)—Rep. Dep. Agric. W. Aust. 1959: 22
Beveridge, W. I. (1941)—C.S.I.R. Aust. Bull. No. 140
Ensor, C. R. (1957)—N. Z. J. Agric. 94: 218
Fitzpatrick, D. H. (1958)—Livestock Digest 4 (3): 8
Gregory, T. S. (1939)—Aust. Vet. J. 15: 160
Harriss, S. T. (1948)—J. Comp. Path. 58: 314
Pryor, W. J. (1956)—J. Agric. Vict. 54: 14.