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This article was published in 1957
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
The Differential Diagnosis and Treatment of Footrot and the Cultural Requirements of F. Nodosus
D. F. STEWART, D.V.Sc., Dip. Bact., Officer-in-charge, McMaster Laboratory, C.S.I.R.O., Sydney, N.S.W.
As a result of unusually high rainfall in recent years contagious footrot of sheep has spread widely in Australia and has appeared in many areas which are usually free of the disease. Other foot diseases have also become rife and have led to some confusion; particularly when true footrot is present, or recently has been present, in the same flocks. It is relatively easy to differentiate footrot from foot abscess and anyone familiar with these conditions should not be in doubt. There has appeared recently, however, a condition which resembles footrot much more closely but which appears to be a distinct entity. It has occurred on properties where footrot has never been known to occur and where no sheep have been introduced, as well as in flocks from which footrot had been eradicated. This disease has been termed "scald".
In the early stages it is almost indistinguishable from footrot, except for the absence of the distinctive odour. The characteristic slimy exudate between the claws is present and the sheep are acutely lame. However, the lesions do not become as extensive as those of true footrot; separation of the sole may occur, but not of the hard horn on the wall of the hoof. It responds rapidly to foot-bathing in 5% formalin, without paring of the hoof, but may recur if pastures remain wet. It is difficult to differentiate this condition from early cases of footrot when individual sheep are examined but on a flock basis the differentiation is usually clear.
Unfortunately, the examination of smears is not always conclusive because occasionally organisms which resemble Fusiformis nodosus, as well as spirochaetes, are seen in smear from cases of scald, although the general bacterial flora is not typical of that seen in true footrot as more cocci and diptheroid organisms usually are present. Nevertheless the presence of an organism closely resembling F. nodosus has led, at times, to a hasty and inaccurate diagnosis of footrot.
The method for culturing F. nodosus described by Beveridge (1941) has not proved reliable in our hands but the recent development at this laboratory of a medium prepared from prekeratin extracted with urea and then digested, has proved much more dependable. F. nodosus has been cultured regularly on this medium from cases of true footrot but to date it has not been cultured from "scald". Furthermore, a condition resembling "scald", although not so persistent, has been set up at the laboratory after inoculating the motile fusiform and Spirochaeta penortha, without F. nodosus, into the scarified feet of sheep.
There is no evidence to suggest that F. nodosus is not the primary organism of contagious footrot or that true footrot can arise spontaneously without the introduction of infected sheep. On the other hand,"scald" does arise spontaneously, in that sense, and rapidly affects the majority of the flock.
It is well established that correct paring is a pre-requisite for the efficient treatment of footrot. Stewart (1954a and b) showed that 10 per cent. chloromycetin in methylated spirits was a highly efficient agent and subsequent experience has endorsed its value. It has been criticised because of its apparent high cost. It was estimated originally that one pint of the solution would treat 300 feet but some graziers have treated successfully as many as 600 feet with one pint. In any case the cost of the medicament is small compared with the total costs involved. For example, one grazier who kept careful accounts of the costs involved in the eradication of footrot from a flock of some 3,000 sheep stated that the cost of the chloromycetin used was £29. The cost of this medicament is not important, therefore, and it is highly efficient. It obviates continual rechecking and paring of the affected sheep. Pryor (1956) in an eradication campaign in Victoria with a flock of 1,076 sheep in which formalin was mainly used, calculated the cost involved to be 2/2d. per head, including labour. There are, no doubt, many bactericidal agents which will yield a percentage of cures when applied to the properly pared foot, but the chloromycetin treatment was developed to enable an outbreak to be brought under control rapidly. In our hands, it has proved capable or doing so.
In addition to those already reported (Stewart, 1954), the eficiency of several other chemicals has been tested at this laboratory, but none of them has shown any particular virtue. Among them were mercurio acetate, tincture of iodine, creosote, zephiran (acationio detergent), copper pentachlorphenate, colloidal sulphur and high-octane petrol, which has been widely used by graziers. Petrol dried affected feet up very rapidly but four of six feet which we treated relapsed within two or three weeks.
Harris (1955) claimed success, in a limited trial, with the application to pared affected feet of an ointment which contained 5mg. terramycin and 10,000 units of polymyxin B sulphate per gram. The treated feet were protected subsequently by boots. Thomas (1957) showed that F. nodosus was sensitive to terramycin but not to polymyxin B. As preliminary tests at the laboratory showed that terramycin was promising it was tested extensively in the field. As an ointment is extremely difficult to apply satisfactorily to a raw, bleeding surface, the terramycin was dissolved in methylated spirits. Unfortunately, it is unstable in methylated spirits and loses its potency within about 24 hours. A more satisfactory formulation must be found, therefore, before it is likely to be used widely in the field.
The first field experiments with terramycin were conducted during a dry summer period and remarkably good results were obtained. With 5% terramycin in methylated spirits one treatment cured all of 105 affected feet after appropriate preparation, one treatment with 2% terramycin cured 99.2% of 123 feet, 1% cured 87.2% of 47 feet and 0.5% terramycin cured 88.3% of 94 feet. For comparison, 104 feet were treated with 5% chloromycetin in methylated spirits and 92.3% were cured by one treatment. These results under hot, dry conditions were far superior to those obtained by Stewart (1954a) under wet, cold conditions; when 5% chloromycetin cured only 50% of 22 affected feet. Therefore, a further experiment was conducted during the following winter on another property. One treatment with 5% terramycin then cured 75% of 200 feet, and 86% were cured with two treatments; whereas one treatment with 5% chloromycetin cured 67% of 51 affected feet. Thirty-three affected feet were treated once with an ointment which contained 5mg. terramycin and 10,000 units of polymyxin B sulphate per gram and 61% were cured. The feet remaining affected after this experiment were cured by one treatment with 10% chloromycetin.
In our experience one of the main reasons for failure in an eradication campaign is the too-ready acceptance of apparent cures, so that treated sheep are put back into the flock too soon. It is more dimcult to be certain that sheep are, in fact, cured after treatment with formalin than after treatment with chloromycetin. In one experiment, six affected sheep were suitably pared and foot-bathed in 5% formalin. After four treatments at weekly intervals the sheep were apparently cured and there was no odour or obvious sign of the disease. At this stage they might well have been regarded as cured. However, the horn on two feet of one sheep did not appear to be growing smoothly and, within three weeks, both had relapsed into active footrot. No matter what treatment is applied to the feet it is important to hold apparently cured sheep, preferably through an autumn or spring, before regarding them as cured.
It is curious that so vulnerable an organism as F. nodosus, which is strictly anaerobic, non-sporing and dies rapidly away from the lesion, causes such a peculiarly localised and persistent disease as footroot. It will not give rise to a detectable lesion in any other part of the sheep except around the base of the horns and it does not naturally produce a lesion of the feet of any other animal except the goat.
Recently, the growth requirements of F. nodosus have been studied at the McMaster Laboratory in the hope that this might throw further light on the pathogenesis of the disease. As stated earlier, the V.F. horse serum-cystein medium described by Beveridge (loc. cit.) has not been reliable in our hands. Whether this has been due to the absence of an essential nutrient or to the presence of inhibitory factors is not clear. However, Mr. J. H. Thomas, B.V.Sc., of this laboratory, has developed recently a medium which has been found to support the growth of F. nodosus regularly. It is prepared by the proteolytic digestion of a urea extract of prekeratin which has been collected from sheep's feet. On this medium the organism forms the characteristic moated colonies, and fresh isolations can be made readily from the lesions of footrot. It has been found, also, that the organism will grow readily on a tryptic digest of wool keratin, with added yeast extract. It does not appear that the amino-acid content of these digests differs markedly from that of digests of other proteins but, somewhat surprisingly, they contain little cystein. Further work is required before the growth requirements of F. nodosus can be defined
It is hoped that this work will lead to the development of a reliable medium for the culture of F. nodosus. Such a medium would be of great assistance in the diagnosis of the disease and in the study of its pathogenesis and immunology. A knowledge of the growth requirements of F. nodosus may well lead to a new approach to the control of the disease.
References:
Beveridge, W. I. B., 1941.-Bull. Comm. sci. industr. Res, Aust., No. 140.
Harris, S. T., 1955.-Br. vet. J. 111 : 212.
Pryor, J. J., 1956.-J. Dept. Agric. Vic. 54 : 14.
Stewart, D. F., 1954a.-Aust. vet. J. 30 : 209.
Stewart. D. F., 1954b-Aust. vet. J. 30 : 380.
Thomas, J. H., 1957.-Aust. vet. J. (in Press)