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This article was published in 1938
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.

SOME ASPECTS OF THE SHEEP BLOW-FLY PROBLEM

DR H. G. BELSCHNER, D.V.Sc.
(Senior Veterinary Surgeon, Dept. of Agriculture).

Although a considerable amount of research work has been carried out by numerous investigators, the fact remains that the sheep blow-fly problem still constitutes a major problem and is responsible for serious economic loss throughout Australia.

From the information which one can gather from the older graziers, it would appear, however, that the losses as a result of sheep blow-fly strike are not as severe now as they were 30 years ago. Nevertheless, they are severe enough in certain seasons and cause for much concern.

Although fly strike is more or less seasonal, more cases occurring in Spring or Autumn, this is by no means regular, and it is not uncommon, in the absence of normal rainfall, for eighteen months to elapse without any severe wave of strike, only odd sheep being struck during this period.

The extent and relative importance of blow-fly strike are influenced by a variety of factors, of which the most important are:—

(1) Climatic factors, eg., rainfall, temperature and humidity, which affect not only the activity of the adult files but the development on the sheep of an area attractive to the fly and favourable for strike;

(2) Type of sheep, with which is associated not only the variations in predisposition of individuals within the flock but also the differences in predisposition of different breeds, to fly strike, e.g., Merino as compared with British breeds and crossbred sheep.

It is the interaction of these factors which determine the extent and relative importance of fly strike in the various sheep-raising districts.

The susceptibility of the sheep itself to fly strike is largely influenced by seasonal conditions and particularly does this apply to "body strike."

Rainfall, temperature and humidity are important factors also in rendering sheep susceptible to the more common form of fly strike in the region of the breech of the sheep, known as "crutch strike." During prolonged showery weather, and especially when the atmospheric humidity is high, as commonly occurs during the warmer periods of the year, the breech of the sheep may be kept continuously moist, especially when the fleece is long, and an area on the skin of the breech is rendered susceptible to fly strike. If, following rain, however, there is a drying wind, the breech tends to lose its moisture and is then less susceptible to strike.

Further, during a season of good rainfall there is much green grass and herbage, the ingestion of which has its effect on the sheep by causing softer faeces and increased voiding of urine, thus intensifying the soiling of the breech and contributing to its attraction for the fly and the maintenance of strike.

Apart from these general considerations of climatic factors, there is the question of variation in predisposition of sheep to fly strike, and this will form the main subject of my address to you this afternoon.

Predisposition of Individual Sheep to Blow-fly Strike.

Before dealing more specifically with the results of veterinary investigations of the blow-fly problem, it would be as well at this stage to refer to the main forms of strike met with. They are:—

1. Crutch strike.

2. Body strike.

3. Head Strike.

4. Pizzle Strike.

1. Crutch Strike—As the crutch is obviously only part of the breech, the term "breech strike" would be preferable, but as the term is in such common use it will he retained.

This, except for periodical waves of body strike, is the commonest form of strike met with. It includes all those cases where sheep are struck about the breech, and although not confined to them, is more common in ewes than in wethers or rams.

Ewes are more susceptible to crutch strike than rams on account of the situation of the urethral opening of the ewe, this leading to frequent wetting of the breech by urine. With all forms of fly strike, moisture is an essential factor.

It has been shown experimentally in the Insectaries at Canberra that artificial strike may be produced by simply wetting the fleece and skin thoroughly and applying newly-hatched maggots and keeping the area moist for a few days.

The production of artificial and experimental breech strikes was found to be more difficult, this development being greatly influenced by the amount of wrinkling of skin on the breech. As a general rule, under field conditions, it is not the wetness alone which attracts the fly to oviposit, but also the exudate resulting from the inflammation which follows the continual wetting by urine or water of an area of skin such as a crutch wrinkle. The further decomposition of urine, wool grease and exudate, as a result of bacterial activity, leads to an intensification of this inflammatory process.

Where these conditions occur on the breech of a sheep, an odour attractive to the fly is given off, and if flies are prevalent and active, strike readily follows. Naturally, when the wool is long around the breech or the decomposition processes occur in a fold or wrinkle of skin, the action is aggravated. Between such wrinkles there is commonly an accumulation of skin secretions and epithelial debris, and these becoming involved in the bacterial decomposition an area still more attractive to the fly is readily produced in such a location.

In short-woolled, or recently crutched sheep, or on a plain (nonwrinkled) breech, the surface is more freely exposed to the air and consequently to drying, and so is rendered unattractive to the fly.

Various writers have referred to the importance of urine soiling in relation to crutch strike in ewes. Whilst I agree, as the result of the examination of many thousands of sheep in the course of field studies, that urine is chiefly responsible for producing a continually moist condition of the breech, especially on a wrinkle. I have also observed moist areas in the crutch or in an infold of skin on the breech of the sheep which were not due to urine. It has further been observed that sheep which have narrow hindquarters and their hocks approaching each other, are more prone to exhibit these moist areas.

Dr. L. Bull has suggested that an albuminous exudate on the skin or into the fleece, following upon a dermatitis set up as a result of the liberation of free ammonia from the urine by bacterial decomposition, may be important in attracting the blow-fly and even be essential to the development of the larvae in the earliest stages. Field observations tend to support this contention.

Other factors rendering sheep susceptible to crutch strike by keeping the breech moist and contributing to skin exudation and bacterial activity are diarrhoea, especially if associated with urine-soiling, prolonged rain, and also heavy dews. As a result of the latter, there is continual wetting of the breech through the sheep resting on wet pastures.

2. Body Strike.—This includes those cases in which the sheep are struck on any part of the body apart from the breech, head or pizzle, the strikes occurring commonly about the withers. back, loins, or even along the sides. Ewes and wethers are equally prone to body strike. This form of strike occurs only in wet seasons and may be absent for two or three consecutive years. A bad wave of this type of strike may be anticipated about every five years, perhaps less. The vast majority of body strikes have been shown to be associated with fleece rot. This latter condition is brought about by the fleece being kept continuously wet by prolonged rain, accompanied by a warm, humid atmosphere, favourable to bacterial growth on the skin and in the fleece. Fleece rot when active is attractive to the fly, probably because of the odour which is given off as a result of bacterial decomposition of the surface skin scales, wool scales, and yolk. Strike develops because of the moisture and readily available food for the maggots. I intend to deal more fully with fleece rot later.

3. Head Strike.—Occurs usually in rams. Although it sometimes follows wounds sustained in fighting, the strike usually occurs on the wool and skin at the base of the horns, especially if the horns are set close to the poll. At the base of the horns conditions exist which are similar to those found in an infold of skin on the breech of the sheep. Here is to be found an accumulation of skin secretions and much debris of epithelial origin (kemp, cells from surface layers of skin) and dust. There is frequently sufficient moisture here for strike to occur, but the susceptibility of the sheep to be struck in this situation is increased by external conditions such as rain or high atmospheric humidity. The cause of head strike has not been studied extensively. It is probable that the increased vascularity of the skin and its gradual differentiation into horn at this point plays a part. In this situation there appears to be increased activity of the skin glands. Bacterial decomposition of keratin may occur and the combination of these, bringing about a soggy condition attractive to the fly, provides a suitable environment for the maggots to develop.

4. Pizzle Strike.–Likely to occur in wethers and young rams and follows urine scalding and bacterial decomposition in the wool around the prepuce, or the presence of balanitis.

The Results of Veterinary Investigation of the Blow-fly Problem.

Prior to 1924 most of the investigational work carried out into the blow-fly problem had been on the entomological side. In that year the Nyngan Experiment Farm was set aside for research on sheep diseases, but chiefly for the purpose of studying the veterinary aspect of the sheep blow-fly problem. I have been associated with this research since its inception and have worked in cooperation with Professor Seddon and also with other veterinarians who were for varying periods stationed at the Nyngan Experiment Farm. The work is now being continued at the Trangie Experiment Farm. In addition, much time has been spent classifying sheep on private properties for predisposition to blow-fly strike and making general observations.

At Nyngan the blowfly problem was attacked from several points of view, but primarily to find out why sheep were struck. Thus, studies were made of the changes which occurred in affected areas on the sheep and particularly of processes which might have occurred in them prior to their becoming struck. By such means it was hoped to detect and distinguish those factors which lead to flies striking certain sheep.

One had heard a great deal about the general health of the sheep being a very important factor in the causation of strike. This was, of course by many prominent graziers. Naturally the health of the sheep on the Nyngan Farm was given very close attention by us, especially in the first four years of our observations. As veterinarians I think we were able to say whether the struck sheep were healthy or not.

A considerable amount of time was also spent on measuring the distance between the tuber ischii of the sheep; studying malformation or injury to the vulva; observing excoriation and granulomatous conditions of the vulva and adjacent areas; the influence of lambing; the amount of rendition in the wool; and lastly, the amount of wrinkling of the skin at the breech area.

After several years of observation it appeared fairly definite that predisposition to fly attack of the breech region was determined chiefly by the conformation of the breech. Sheep most commonly struck all showed markedly wrinkled breeches, and conversely sheep which had not been struck showed plain breeches.

I might say that every sheep on the farm was ear-tagged and a card was kept for each sheep. The Veterinary Officer in residence, or the shepherd, made a note of the ear-tag of every sheep that was struck, and these were later entered on the cards, together with any other information about the sheep. We thus had a complete history of each sheep, including the number of times it was struck by blow-flies in any particular season. At the end of the year we were able to sort up the cards and ascertain the sheep which had been struck one or more times and those which had not been struck at all.

One of the earliest observations was that which was made on 1,000 Merino sheep. These sheep had been running under similar conditions, and the season 1927-28 being a good one, the strike Incidence had been high. By the aid of the cards the sheep were drafted into groups according to the number of times they had been struck. There were five groups, ranging from those which had not been struck at all to the group in which all the sheep had been struck more than six times. An endeavour was then made to find out the main differences between the sheep which had not been struck and those which had been repeatedly struck. All the sheep were individually examined.

The conclusions reached were as follows:—

(1) The sheep most commonly struck all showed markedly wrinkled breeches and this appeared to be the constant factor in predisposing sheep to crutch strike. Conversely, sheep which had not been struck showed plain breeches.

(2) Ill-health was a negligible factor in predisposing sheep to fly strike.

(3) Malformation of the vulva, deflecting the urinary stream on to the wool, or a wrinkle, rendered certain sheep susceptible to fly strike.

(4) Sheep exhibiting bad conformation of the hindquarters, with hocks approaching each other, were prone to crutch strike. The next step was an attempt to classify sheep according to our views on susceptibility. Various lots of sheep were classified into three groups—

A.—Relatively insusceptible.

B.—Partially susceptible.

C.—Definitely predisposed to fly strike.

The results of the strikes in the various groups have been shown in tabular form and published.

You will have observed that the classified sheep behaved consistently in much the same manner. It will be noted, however, that in the season 1930/31—a period of high rainfall—a greater proportion of A. class sheep were struck than in the earlier years of moderate rainfall. The reason for this I have previously pointed out. i.e., the breech area being maintained in a continuously moist condition, thereby favouring the development of a dermatitis, the subsequent excoriation forming an area attractive to the fly. There is also the possibility of the occurrence of fleece rot in the wool on the breech.

Following upon the results obtained at the Nyngan Farm. the classification of Merino sheep on private properties was undertaken, the owners subsequently recording the strikes. The figures obtained again revealed the marked predisposition of C. class sheep, and conversely the low predisposition of A. class sheep to crutch strike.

It has been further observed that the severity of the crutch strike is greater in C. class sheep than in A. class sheep. This may be due to several causes, namely, the more favourable environment between infolds of skin for the maggots to survive and establish themselves, and to the greater tendency for C. class sheep to be re-struck.

A very important advance in our knowledge of the sheep blow-fly problem was thus made, and it established in crutch strike—

(1) That sheep are not struck by chance.

(2) That they are not struck because the sheep are unhealthy in the ordinary accepted sense. ie., not because they are "wormy," suffering debility from privation or other cause, or affected with any of the common diseases which affect sheep. (During nine years' observations at the Nyngan Farm, no treatment of any sheep for internal parasites was necessary).

(3) That the predisposition of sheep to crutch strike is due to definite physical character of the sheep itself.

(4) That this predisposition in the large majority of animals is due to the conformation of the breech, and in a small minority to distortion of the vulva from accident or disease.

(5) That under seasonal conditions of high rainfall, sheep not normally predisposed to crutch strike may be rendered susceptible.

(6) That those physical factors which determine predisposition to crutch strike can be recognised by visual and manual examination of the sheep, so that sheep can be assigned to various groups according to their degree of predisposition,

(7) Lastly, on putting these ideas into practice by classifying sheep and, recording subsequent strikes, it has been found that the anticipated and the recorded predisposition have been consistent.

Whilst graziers commonly recognised that an excessively wrinkly sheep was more prone to fly attack than a plainer bodied sheep, they had no definite ideas as to why this win so, and had not imagined, as has now been shown, that sheep could by classified according to their predisposition to fly strike, simply on the extent of wrinkling about the breech. Sheepmen have leaned very strongly to the idea that individual sheep were struck because of the ill-health of the animal. Although there is no evidence in support of it, some still believe that the solution of the blow-fly problem will be found in the provision of some medicated lick.

One still hears the statement that "flies will not attack a perfectly healthy sheep." Whilst it is admitted that an obviously sick sheep might be particularly prone to fly-strike, especially if it be exhibiting diarrhoea, or be in as particularly weak state. I am firmly of the opinion that the reasons that have already been indicated that, as a general rule, ill-health is not a factor in predisposing sheep to strike.

It has been interesting to note in the course of travelling in the State, that since attention was first directed in 1931 to the experimental work which had been carried out until then, many owners have accepted our conclusions regarding crutch strike, and have paid considerable attention to the culling of wrinkly-breeched sheep. They are aiming at the gradual elimination from their flocks of all C. class sheep.

Certain professional sheep classers and Government sheep and wool instructors, are paying increased attention to the breech or the sheep. On questioning some of the sheep-owners as to what extent they considered the breeding of plain-breeched sheep reduced the fly trouble the answers ranged from 50% to 85%.

Some sheepmen accept the idea for flock sheep, but consider that to attempt to eliminate breech wrinkles in stud sheep would be detrimental to the type of fleece produced. Generally, however, the tendency in recent years has been to breed a plainer bodied type of sheep for reasons other than blow-fly insusceptibility, but still more recently this has more than ever been kept in mind and the breech of the sheep is receiving particular attention.

More and more owners are coming to realise that predisposition of individual sheep is a very real factor indeed, and the only reason that some of them are loath to put these conclusions into practice is the fear that by so doing they may reduce wool "quality," and through anticipated reduction in density, reduce their total wool clip.

As there appears to be a tendency for C. class sheep in general to be more dense in the wool than A. or B. class eheep, one can understand the fears of the sheep-breeder that reduced density of the fleeces might result if breeding towards an A. type sheep is carried out. I suggest, however, that this should not occur to any great extent if proper culling of open, slack-woolled sheep is adopted, as should be done in any case, irrespective of the type of breech. Further, by the careful selection of ewes free or comparatively free from breech wrinkles, and by mating these with plain-breeched rams, density, as far as it is normally required, should be retained in the A. class sheep.

With certain stud flocks which are of the wrinkly type, the elimination of the wrinkly breech will be slower than in those studs where the sheep are already comparatively plain, both as to body and breech conformation.

The matter of wool production and commercical value of wool produced by A. and C. type sheep has already received some attention, and I refer you to recent articles which have appeared in the Australian Veterinary Journal on some experimental work carried out with my colleagues, Miss H. Newton Turner and Mr. H. B. Carter. Breeding trials are at present in progress at the Trangie Experiment farm, the result of which should give us more definite information on the subject. We can also look forward with considerable interest to the results of the work which Mr. Carter is carrying out on fleece density.

I should just like to mention one point in connection with the breeding away from breech wrinkles, and that is that professional sheep classers have informed me that already greater uniformity has been attained in those flocks where attention has been paid to the elimination of C. class sheep.

In most flocks as now constituted there are animals which might be classed as A. B. and C. and there is wool of varying qualities and types. This latter factor—lack of uniformity in the wool—is frequently commented on by wool classers and wool buyers. Type of country, seasonal conditions, and health, are important factors in the type of wool produced by sheep, both collectively and individually, but under the best conditions there still exists considerable unevenness of wool, due to variation of quality, length, softness. etc. Lack of uniformity of conformation is equally marked, when one considers predisposition to fly strike, and after the examination of many flocks I have been impressed with the variation in the extent of wrinkling of the breech which occurs.

Wrinkliness of the skin is responsible for unevenness of the wool, and therefore to a degree unevenness of wool is associated with predisposition to fly strike.

Whilst appreciating the high state of excellence which certain flocks have attained in the matter of type and bulk of wool, I feel that if more consideration were given at sheep-classing time to the question of predisposition to strike, both in ewes and rams, owners would build up flocks of better constitutioned sheep and more even as regards type of wool.

Further, it as is suggested by recent experiments, there is a tendency for the A. type sheep to produce a higher proportion of top line wool than the C. class sheep, it would seem that the result would be greater uniformity, better wool and lessened suriceptibility to blowfly strike.

It is significant to note that invariably there is a greater percentage of C. class sheep than A. class in the culls from any flock, and that these sheep are usually culled on account of faulty wool. Time will not permit of my dealing with this aspect further.

Broadly, from the observations which have so far been made, it would appear that predisposition to blow-fly strike may be largely bred away from, and experimental work suggests that this may be done without sacrificing bulkiness of fleece, length of staple, or general "quality" of wool. I suggest that the tendency should be to breed towards the A. class rather than the C. class type of breech. It would not be practical to cull immediately all wrinkly-breeched sheep, but by gradually eliminating sheep of the C. type and then later of the B. type, and concentrating on A. type sheep with fairly dense fleeces, it should be possible to build up a flock of plain-breeched sheep without sacrificing fleece weight. By doing this, greater uniformity should be obtained in the flock, the wool clip should be of increased commercial value, and there will definitely be a considerably reduced amount of blow-fly strike.

Body Strike—Its Association with Fleece Rot and Type of Sheep Susceptible to the Latter Condition.

It was originally shown by Professor Seddon that body strike follows the development in the fleece of a condition described by him as "Water-rot" and which he considered to be due to bacterial decomposition in the wool and on the skin. The condition has also been known by other names, i.e., "Weather Stain," "Water Stain," "Dead Yolk," etc. It was on the suggestion of the late Dr. Gilruth that the condition has been termed "Fleece Rot."

The organisms responsible for the condition are probably present in the fleece as a result of contamination by dust, etc. In a dry fleece these bacteria do not produce any ill-effect, but when the fleece is kept wet for a prolonged period, as happens in continued wet weather, the moisture so present, together with the warmth of the body, provides the necessary requirements for bacterial growth. The moisture, moreover, leads to a partial maceration of the surface of the skin, so that on these damaged cells the multiplying bacteria thrive. As a result of this bacterial activity, a certain degree of superficial dermatitis occurs with consequent exudation from the skin, leading to the formation of a crust and matting of the fibres. With the growth of the wool the matted material becomes raised from the skin and appears in a band across the proximal end of the staples of the affected parts of the fleece. Such a band may be from one-eighth to half-inch in breadth, according to the period during which the condition has been active. With the advent of dry weather, the wool dries out and the conditions no longer being favourable to the multiplication of bacteria, the subsequent growth of the wool is unaffected by the fleece rot. With further wet conditions, however, fleece rot may occur, causing another band of matting. This is due to bacteria from the first band being carried down to the skin, the process being readily recognisable in certain staples.

The most common sites for fleece rot to occur are the wither, neck, back or loin, though it may occur on the shoulder or sides. The fly strikes the wool of those parts where the condition is active, i.e., whilst bacteria are actively multiplying and where there is evolved an odour which is attractive to the blow-fly.

It is noticeable that on certain properties in the same district the condition of fleece rot is much less prevalent than on others, and again, that in a single flock some sheep are badly affected and others are quite free.

During an extensive field investigation which I was privileged to carry out during 1934, 5O properties were visited in representative areas of the State, and information was obtained from the inspection of 61,000 sheep in yards for evidence of fleece rot and body strike. At the same time a systematic examination was made of a dozen sheep of approximately the same age and taken at random on each of 43 of these properties. The idea was, in addition to broad field cbservations made by inspecting sheep in races and yards, to carefully examine in detail a total of about 500 sheep on properties well distributed over the State.

The object of the investigation was, amongst other things, to find out if possible, whether sheep carrying a certain type of wool, or of a certain conformation of wither or back, were more prone than others; whether it was a question of the amount of condition in the wool; or a question of density, count, character, colour or softness of the wool, and so on.

It was thought that such an inquiry might throw considerable light upon the occurrence of the complaint, and further that it might be revealed as in the case of crutch strike, that there was a definite type of sheep (or type of wool) susceptible to fleece rot, so predisposing it to body strike.

Time will not permit of my dealing with the economic importance of Fleece Rot, its distribution, or location. I should like to point out, however, that microscopic examinations of wool affected with fleece rot carried out at Glenfield have shown that whilst some fibres of the affected wool had definitely deteriorated, others were not so affected. This point has been discussed with wool buyers, who agree that this might be so, but they would not take a risk when buying wool, and that any wool affected such as the representative samples shown, would be regarded as inferior wool.

In the earlier stages of fleece rot it would appear that the bacterial activity is confined to the yolk between the cells of the wool fibre, but the fibre itself is not affected. At this stage the soundness of the fibre is not interfered with. Later, however, the bacterial activity may extend to the cells themselves and thus affect the fibre. This question of the damage to the wool fibre is of considerable importance and will require close laboratory study. The internal formation of the wool fibre is composed of numerous cells. It may be found, on further investigation, that the unsoundness of certain wool affected with fleece rot, is due to a breaking down of the fibre as a result of bacterial actions. This is, of course, only a postulation and did not form part of my study.

Professor Seddon has shown that certain of the colourations associated with fleece rot scour out, whilst others do not. Woolbuyers are not familiar with those types of fleece rot which scour out, and at present, they regard all wool affected with fleece rot as coloured wool. As all wool must be able to be scoured pure white to have its full market value, the value of affected wool is considerably depreciated.

The main economic importance of fleece rot, however, is the fact that it renders the sheep susceptible to fly strike. The factors influencing the occurrence of the condition, such as season, country, altitude, humidity and temperature, were inquired into but cannot be dealt with here. One thing, however, I might mention: in view of the popular but quite erroneous idea that one type of fleece rot. viz., "green colouration," is due to the drippings of trees in heavily timbered country, attention was paid to the extent and nature at the trees on the properties visited. It was found that this and other types of fleece rot are as prone to occur on open, treeless or lightly timbered as they are in heavily timbered country.

FACTORS INFLUENCING SHEEP SUSCEPTIBILITY.

What are the factors which influence sheep susceptibility to fleece rot? Firstly my observations have shown the age of the sheep to be most important. On all properties visited it was found that the young sheep, six to twelve months were those most severly affected, and generally showed the highest incidence of body strike. The explanation of the greater susceptibility of young sheep to fleece rot (apart from the fact that frequently they have not been culled) seemed to be the openness of the wool, particularly in the case of the unshorn hoggett, and the softness and mobility of the skin, due to immaturity.

It is noted that when weaners or year old sheep are exposed to heavy rain, the fleeces are all parted down the middle of the back, allowing the water right in on the skin. The presence of the lamb "tip" on the wool of the sheep which have not been shorn undoubtedly plays a part in favouring the entrance of water.

Breed Incidence.—Whilst the majority of the sheep examined were merinos. every opportunity was taken of inspecting British breeds and crossbreds. A fairly high incidence of fleece rot was found in certain flocks of these sheep. On one property at Cooma, where 110 two and three years old Corriedale ewes were hand examined, 63 per cent. were found to be affected with fleece rot. (These sheep were cull sheep from a property near Forbes). On another property at Crook well, 40 per cent, affection was seen in a small mob of 50 Corriedale X merino sheep, 15 months old.

This observation is important from another standpoint in view of an opinion held in certain quarters that fleece rot is chiefly to be found in dense woolled sheep.

The sex or general bodily health of the sheep was found not to influence the incidence of fleece rot.

Conformation of Sheep.

(1) Withers. Three types of withers were found to render sheep susceptible to fleece rot. Firstly, the sheep with high shoulder blades; secondly the broad withered sheep; and thirdly, those sheep showing a "pinch" behind the wither, i.e., which are unduly narrow over the fourth to sixth ribs, just behind the posterior dorsal angle of the scapula.

In the first, the constant movement of the high shoulder blades has the effect of opening up the fleece on the withers and thus allowing the rain to enter. In the second, i.e., broad withered sheep, there is frequently a distinct depression between the tops of the shoulder blades from which water does not drain away readily. In the third, the animal is "pinched" behind the wither or exhibits a definite "grip" the animal frequently also having high shoulder blades. Of the three faults, the third is the one most prone to lead to fleece rot. In the depression caused by the "pinch" or "grip" moisture is retained, bacterial multiplication proceeds and fleece rot develops. (See graph).

(2) Body Generally.—A classification of the breeches of those sheep specially examined showed that sheep with wrinkly breeches were no more prone to be affected with fleece rot than those with plain breeches, Also, the degree of wrinkling of the body generally, apart from the breech, was not parallelled by the occurrence of fleece rot, and therefore wrinkliness is not a factor predisposing to fleece rot (See graph).

TYPE OF WOOL.

This received very close attention. On some properties it was possible to examine large numbers of sheep for wool "quality," using the term in its widest sense and on 43 properties the wool of about a dozen sheep was examined in detail. The data tabulated reveal important results which are illustrated by graphs.

(1) Density.—Somp sheep breeders have stated that the slightly wrinkled, dense woolled type of sheep is the one most prone to be affected with fleece rot. Providing the sheep was not of the cull type, this was not found to be so. The highest percentage of fleece rot was found in slack woolled sheep and decreased as density increased.

Considering the question of density of the wool over the withers, it was found that the difference was even more marked. Openness of the wool on the wither was found to be a definite factor in predisposing sheep to fleece rot, not only in the specially examined sheep but on large numbers of other sheep inspected.

(2) Condition.—Little differences was found between the light conditioned and heavy conditioned fleeces, although there appeared to be a tendency for the lighter conditioned fleeces to be more commonly affected. (See graph).

(3) Spinning Counts—All the wools were classed as to spinning count—crossbreds and British breeds included. The results showed that fleece rot is not confined to the coarse woolled British breeds or to the relatively fine woolled merinos. The figures suggest, however, that the finer wools are less affected. (See graph).

(4) Colour Handle and Character.—These play the most important part in the predisposition to fleece rot. Considering them in their order, it was found that in connection with colour, the brighter wools were least susceptible and there was a progressive susceptibility through fairly bright, light cream, cream to yellow, wools. (See graph).

Wool handle (softness of the wool to the touch). This was determined by examination of healthy unaffected wool generally over the shoulder or ribs. Harshness predisposes to fleece rot (see graph).

Lastly, it was observed that the higher grade, better character, more stylish wools, were the least susceptible to fleece rot.

The term "character" is difficult to define. It is really a breeders' term and signifies that the wool possesses strongly marked distinctive qualities of its breed and type.

The observations made on wool character on the specially examined sheep were supported by the inspection of still larger numbers of sheep on the same properties. Further, the very high incidence of fleece rot in cull sheep was particularly noticeable. The following is an example:—A small lot of two year old ewes were being classed into flock sheep (breeding ewes, culls for sale, and rejects for immediate slaughter. These sheep were examined for fleece rot, with this result:—

Flocks .. 130 examined, 27 per cent. affected fleece rot.

Culls .. 110 examined, 63 per cent. affected fleece rot.

Rejects .. 49 examined, 92 per cent. affected fleece rot.

As the sheep were classed by the classer chiefly for wool "quality" and density, this observation shows the high incidence of fleece rot in open and inferior woolled sheep.

Reviewing the observations made, therefore, it will be seen that in a season favourable to fleece rot, there is evidence of a definite susceptibility factor in individual sheep. Close investigation of this aspect have shown that relative susceptibility of individual sheep of the same age is determined chiefly by one or a combination of any or all of the following:—

(a) "Quality" of wool;

(b) Conformation of withers;

(c) Density of fleece, particularly over the withers.

These are placed in order of importance. Wherever sheep were of outstanding merit as regards "quality" of wool (character, handle and colour), only a low incidence of fleece rot was found, whereas a high incidence was invariably noted in other sheep which were not so well bred and which carried wool lacking in those features which constitute "quality."

Of these wool features, that known as "character" is considered the most important, but bundle and colour also play a considerable part. The insusceptible type of fleece is that with good character, soft handling, bright wool whilst the susceptible type lacks character, is harsh, and exhibits a good deal of yellow colouration in the yolk.

Next in importance comes faulty conformation of the withers. When the withers were of such conformation that they favoured the retention of moisture, the sheep was usually affected. A "pinch" behind the withers forms a convenient depression for the retention of moisture, and among sheep exhibiting this fault there was a high incidence. When the depression was so exaggerated as to form a recognisable "grip" the sheep wits invariably affected. Sheep showing other faults of the withers, such as the presence of high shoulder blades or broad withers, were also commonly affected, the former favouring the entrance, and the latter the retention of moisture. Sheep with well formed withers, however, were comparatively free, unless the wool was of poor "quality."

The relationship of these two features—wool "quality" and wither conformation—was found to have a bearing upon the incidence of fleece rot. Thus, sheep with poor wool "quality" and lacking in good wither conformation were more commonly affected than those which manifested only one of these faults.

Density is also important. Particularly prone to fleece rot are those sheep lacking in density of wool on the withers, as well as on other parts of the body. Whilst slackness of wool, especially over the withers, generally denotes susceptibility, all such fleeces did not exhibit fleece rot. If the wither be of good conformation, moisture is not retained, and if in addition the wool be of good "quality" and so of the insusceptible type, it does not readily become effected. Conversely, sheep with fleeces of poor wool "quality" are less liable to fleece rot, if the fleece be dense, for then the effect of the density tends to keep out the water. Even a dense-woolled sheep may be affected if the conformation of the withers is bad, though the liability for it to be affected will be modified by the type of wool the sheep is carrying.

One may wonder why fleeces with the higher character, softer, and brighter wool, or broadly, the higher "quality" wools are less susceptible than fleeces lacking to some extent at least these desirable attributes. In my opinion the explanation is to be found in the growth, development and structure of the wool fibre. The best "quality" wool can be grown only on carefully bred, properly selected, adequately nourished sheep, and sheep of this type do not have harsh wool. As it is recognised that the harsher the wool the less compact is the cellular structure of the fibre, one may postulate that the softer handling wools, with the more compact cellular structure are less likely to admit moisture.

Again as the evenness of fibre is dependent on the cellular structure. it may be that regular serrations hold the natural grease evenly all round the fibre, and this adequately protects it front moisture. One does not know, however, whether it is a question of the internal or external cellular structure or both which determines the protection that is apparently afforded. The answer to such a question can come only from laboratory study.

Although these investigations have shown fleece rot to be a common condition, it is noteworthy that sheep on a number of properties showed little fleece rot indeed, notwithstanding that the conditions in that district were favourable to its development. The reduction of susceptibility depends greatly on breeding and selection. A discussion of the breeding of many of the sheep examined during this investigation would be out of place here, but it may he stated that a great many inferior woolled sheep were seen. Wherever the sheep were of outstanding merit in character, handle and colour of the wool, a very low incidence of fleece rot was found. Without exception, when fleece rot was noticed in a sheep carrying a high grade fleece, some fault in the conformation of the animal was present.

Body strike in sheep depends almost entirely upon the pre-existence of fleece rot and it is obvious that there exists a type of sheep definitely predisposed to this latter condition. Whether that sheep will suffer front body strike or not depends upon two conditions. viz., that the presence of those external factors conducive to the development of fleece rot; and (b) the prevalence of blow-flies.

The prevention of body strike, therefore, depends principally on reducing the susceptibility of our flocks to fleece rot by selective breeding.

It is evident from these studies that the question of whether sheep are struck or not whether it be on the body or on the breech, depends very largely upon the susceptibility of individual sheep.

 


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