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This article was published in 1981
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Studies of the Body Strike Problem of Sheep
J.E. Watts, Veterinary Research Station, NSW Department of Agriculture, Glenfield NSW and G.C. Merritt, McMaster Laboratory, Division of Animal Health, CSIRO, Glebe NSW
INTRODUCTION
The control of breech strike depends on the use of management procedures including the use of the Mules operation, correct tail docking, crutching, worm control and insecticidal application. The correct and timely use of selected control measures can often eliminate or greatly reduce the need for recourse to the use of chemicals.
However, control of body strike remains highly dependent on the application of insecticides by jetting and showering with its associated loss of effectiveness as the blowflies become resistant to the insecticide. Both the grazing industry and scientists have recognised the intractable nature of the body strike problem and accorded it a high priority for investigation.
In high rainfall areas where fleece-rot, the main precursor, and body strike may occur in most years, woolgrowers usually run sheep such as fine wool merinos which possess a high degree of natural resistance to fleece-rot. They usually also cull any sheep affected with either fleece-rot or body strike: a method of direct selection which appears to have reduced the body strike problem in subsequent generations of young sheep. However, the selection pressure placed on eliminating fleece-rot from these flocks usually outweighs that given to maintaining or increasing greasy fleece weights. Consequently, there has been a tendency for small-framed and light-cutting merino sheep to have evolved in these areas. To counter this uneconomic situation some wool growers have introduced large-framed and heavier cutting merino rams.
In eastern Australia most merino studs are located in the drier inland areas, particularly the Riverina, and Macquarie regions of New South Wales where the occurrences of fleece-rot and body strike are infrequent. It is therefore unnecessary to consider breeding for reduced incidence of fleece-rot in this environment in sheep which are destined to produce commercially in similar environments. However, when rams bred in these drier areas are sold to commercial producers located in the wetter tableland areas (as indicated above), these animals are usually of unknown, but often high susceptibility to fleece-rot. The result has been greatly increased losses from body strike in high rainfall environments among the progeny of these introduced sires. Thus, practical experience has emphasised the need for a method of choosing highly productive animals whose fleece-rot incidence will be low when transferred to the wet areas.
The aim of our studies has been to provide new insights into the mechanisms of the initiation of fleece-rot and body strike and in so doing, develop effective control measures that eschew the use of insecticides. Short-term solutions, such as vaccination of sheep against fleece-rot, are being explored together with long-term solutions, that is, genetic selection of sheep for reduced susceptibility to body strike. The latter approach has been followed by a number of astute woolgrowers for some years as it offers more or less permanent protection for sheep against body strike. To date, we have examined many fleece and skin properties of sheep in relation to fleece-rot susceptibility in a relatively large number of flocks. This approach has been adopted as a forerunner to examining potentially useful predictor traits of body strike resistance at the genetic level.
FLEECE-ROT FORMATION
Previous Studies
Although there have been many reports concerning microorganisms isolated from the fleeces of sheep exhibiting fleece-rot and adventitious colourations associated with prolonged wet weather (Stuart 1894, Seddon and McGrath 1929, Waters 1932, Seddon 1937, Botha 1945, Wilson 1946, Fraser and Mulcock 1956, Fraser and Mulcock 1958, Henderson 1968, van Tonder et al. 1976), their precise significance in the development of fleece-rot has not been reported previously. Some investigators (Mackerras 1936, Belschner 1937) postulated that fleece-rot is induced by bacterial activity whilst others (Bull 1931, Hayman 1953) considered that moisture alone is the cause and that the effects of bacterial activity are of a secondary nature.
Furthermore, considerable conjecture has existed as to the nature and origin of the crusted material which accumulates on the wet skin surface to form the fleece-rot lesion. Both Belschner (1937) and Hayman (1953) have described the crusted material as 'serous exudate'. Whilst this is likely, no supportive evidence is given for this nor the manner in which the 'exudate' is formed. On the other hand, Nay and Watts (1977) suggested that vascular elements may leak onto the skin surface via damaged wool follicles ('plugs') during the development of fleece-rot.
The present studies have examined, therefore, the changes in the bacterial flora, and soluble protein levels on the skin surface of sheep during the development of fleece-rot; and the nature and origin of soluble protein in fleece-rot lesions.
Present Studies
Specific details of individual experiments are reported by Merritt and Watts (1978a,b), and Watts and Merritt (1981). The main conclusions of these studies are:
(i) The sequence of events in the development of fleece-rot and body strike has been elucidated. It was shown that when sheep are wetted by heavy rains, vast increases in the numbers of bacteria on the fleece and skin surface occur. Frequently, only one species of bacteria, Pseudonomas aeruginosa, proliferates and produces a bacteriostat which inhibits the growth of other resident micro-flora. The development of fleece-rot is characterised by a sudden and substantial increase in the amount of soluble protein in small areas on the skin surface. Preliminary examination of this proteinaceous material by immunoelectrophoresis and double diffusion has shown that plasma proteins can be readily detected in skin washings from fleece-rot lesions. Ps. aeruginosa bacteria flourish on the skin surface in these discreet fleece-rot areas of high soluble protein concentration ('hotspots') (see Table 1) often producing pyocyanin, which usually colours the wet wool green.
TABLE 1
Increase in soluble protein and bacteria on the wet skin of a sheep susceptible to fleece-rot
| Site sampled | Concentration of soluble protein (mg%) | Number of bacteria present (per ml. washing) |
|---|---|---|
| Fleece-rot 'hotspot' | 53.8 | 700 million |
| No fleece-rot (only 1 cm from fleece-rot site) | 0.5 | 3,000 |
(ii) Using radiotracer techniques, the transfer of I125 - labelled albumin from the bloodstream onto the wet skin surface of 3 sheep affected with fleece-rot was studied. Skin washings from fleece-rot areas which were sufficiently severe to predispose sheep to body strike ('hotspots', as described in Table 1) were characterised by marked increases in the concentrations of soluble protein and counts of radiolabelled albumin. This type of lesion occurred at small, discreet sites and blended imperceptibly with surrounding areas of fleece-rot (Table 2). However, the primary green blowfly, Lucilia cuprina, was able to locate these 'hotspots' with repeatable ease, laying its egg mass at this moist site and thereby ensuring that the newly hatched larvae would survive and feed on the sheep's skin.
TABLE 2
Counts of radioactivity and concentrations of soluble protein in skin washings 48 to 168h after the intravenous administration of I125-albumin to sheep affected with fleece-rot and body strike.
| Skin site sampled | Number of sites | Mean reproductivity count (cpm/ml) | Mean concentration of soluble protein (mg%) |
|---|---|---|---|
| Sheep A | |||
| Bodystrike | 3 | 1452 ± 114* | 110.6 ± 13.5 |
| Fleece-rot | |||
| 'Hotspots' | 8 | 248 ± 33 | 28.6 ± 3.6 |
| Adjacent sites | 20 | 64 ± 3 | 13.1 ± 1.1 |
| Dry Skin | 1 | 55 | 1.2 |
| Sheep B | |||
| Fleece-rot | |||
| 'Hotspots' | 7 | 480 ± 40 | 26.0 ± 2.8 |
| Adjacent sites | 15 | 69 ± 1 | 6.2 ± 0.4 |
| Dry Skin | 6 | 66 + 1 | 1.9 + 0.2 |
* mean
+ standard error
By precipitating the skin washings of fleece-rot sites with 10% trichloracetic acid or rabbit anti-sheep albumin it was shown that nearly all of the radioactivity resided in the protein precipitate. In other words, these fleece-rot 'hotspots' were formed as the result of plasma protein leaking onto the skin surface.
Autoradiographs of histological sections prepared from these fleece-rot sites showed that leakage of plasma protein had occurred via the follicular canals. Affected wool follicles frequently contained narrowed or broken regions of pre-keraterized fibre similar in appearance to the follicle 'plugs' observed by Nay and Watts (1977).
Histological examination of skin sections from fleece-rot sites also revealed, in addition to the moisture-induced dermatitis (maceration and hyperkeratosis of the epidermis and dermal oedema), described by Hayman (1953) that microabscessation of the stratum corneum may occur. It would appear, therefore, that the proteinaceous material of skin origin found in fleece-rot lesions may derive from two different sources (i.e. follicular and epidermal transfers).
(iii) Whilst enhanced activity of Ps. aeruginosa bacteria and the accumulation of plasma protein in discreet areas of fleece-rot lesions had been shown to occur (Merritt and Watts 1978b), it was still not clear how these factors influence susceptibility to fleece-rot or indeed whether the associations observed were of a causal or secondary nature. To ascertain their significance, a method of experimentally producing fleece in vitro was developed.
Fleece-rot was experimentally produced by wetting and incubating wool samples in serum agar. Gravid L. cuprina were readily attracted to these 'fleece-rot' lesions to oviposit. Using this technique, fleece-rot and oviposition were found to be influenced markedly by the availability of serum and by bacterial activity, particularly that of Ps. aeruginosa. These events varied according to the type of fleece (resistant or susceptible to fleece-rot) used, and could be prevented by the addition of a bactericide.
BODY STRIKE DEVELOPMENT
Previous studies
Various workers, notably Johnston (1923), Seddon (1931), Mackerras (1936) and Belschner (1937) have stressed bacterial activity in the fleece and on the skin as an antecedent to fly-strike. Similarly, there is clear experimental and field evidence that any inflammatory exudate (a protein-rich source) on the skin surface increases susceptibility to fly-strike (Mackerras 1936). The most common, and historically the most important is the dermatitis of wet, sodden breech skin folds described by Bull (1931).
However, an essential relationship between bacterial activity, soluble protein and body strike development had not been shown to exist. It was thought that bacterial activity on the serous exudate of fleece-rot lesions gave rise to odours that attracted the L. cuprina fly to lay its eggs in the sheep's fleece (Mackerras 1936). Experimentally, the volatiles from indole and ammonium carbonate solutions have been shown to be ovipositional attractants (Barton, Browne 1965). However, it is not known whether these compounds have any predisposing role to fly strike in the field.
Present studies
Specific details of individual experiments are reported by Merritt and Watts (1978a) and Watts et al.. (1981). The main conclusions of these studies are:
(i) The primary green blowfly, Lucilia cuprina, is stimulated to oviposit in response to the volatile odours emanating from fleece-rot lesions.
(ii) Fractionation of the fleece-rot wool headspace, was carried out by bubbling the volatiles through acid and/or alkali wash solutions contained in gas scrubbing tubes. Volatiles contained in the acid fraction of the fleece-rot odours and perfused into wet wool stimulated L. cuprina to oviposit. Preliminary examination of the total headspace volatiles and odour fractions of fleece-rot wool by gas chromatography have shown that at least two-thirds of the activity peaks present in the total odour, may now be considered relatively unimportant in the ovipositional response of the fly to fleece-rot lesions.
Further analysis of the acid fraction of the fleece-rot odours is in progress, using improved bioassay techniques, in an attempt to identify the volatile compounds responsible.
(iii) A potentially important consideration in the epidemiology of body strike predisposition has arisen from the demonstrated ability in vitro, of Ps. aeruginosa to inhibit the growth of Dermatophilus congolensis, the causal agent of mycotic dermatitis ('Lumpy wool'). The isolation of Ps. aeruginosa in pure culture from 8 field cases of 'lumpy wool' in sheep during an outbreak of body strike confirms this view. It indicates that it is not necessarily always D. congolensis activity in 'lumpy wool' lesions that render sheep immediately susceptible to body strike. The serous and cellular exudation in the fleece which follows D. congolensis infection of the sheep's skin provides abundant amounts of soluble protein for bacterial growth and fly larvae establishment. However, it appears in the eight sheep examined that the activity of Ps aeruginosa bacteria on this 'lumpy wool' material was responsible for generating odours attractive to gravid L. cuprina flies (J.E. Watts and G.C. Merritt, unpublished data).
FLEECE AND SKIN PROPERTIES RELATED TO FLEECE-ROT SUSCEPTIBILITY
Previous studies
Many attempts have been made to associate various properties of the fleece with susceptibility to fleece-rot. Initially the fleece properties examined were estimated by visual appraisal and included such features as: colour, character (or style, handle and 'density' of the greasy wool (Holdaway and Mulhear 1934, Belschner 1937).
'Character' signifies that the fleece shows clear-cut staple information with even, pronounced crimp. Handle is the softness of the wool to the touch. Fleece 'density', as used by sheep classers, denotes the compactness of staples in the fleece. Resistant sheep usually have bright or white wool which is soft-handling, 'dense' and possesses good character. On the other hand, susceptible sheep tend to have yellowish or dingy-coloured wool which is harsher handling, sometimes 'open' or 'slack' and lacks character (Belschner 1937). Although these subjective terms are difficult to define scientifically, and can be related only to some degree to certain fleece measurements, their meaning and significance are readily appreciated by experienced woolgrowers.
With the development of fleece metrology techniques (for example Freney 1940, Daly and Carter 1954, Lynch and Michie 1976), measurements such as wax content, suint content, wax to suint ratio, insoluble nitrogen content and fibre diameter variation have been incorporated into fleece-rot studies (Hayman 1953, Paynter 1961, Lipson 1978, Lipson et al. 1981, Watts et al. 1981).
Differing conclusions have been reached by various workers concerning the value of wax and suint estimations for identifying resistant sheep and sheep susceptible to fleece-rot. Hayman (1953) reported a highly significant difference for wax content (higher values for resistant animals) but not for suint content, whilst Paynter (1961) considered that neither of these fleece properties were important. Lipson (1978) showed that low suint content and high wax to suint ratio favour resistance to fleece-rot whilst high wax content was not as significant.
Several reasons have been suggested to explain this quandary. It is generally considered that this situation simply reflects the need to analyse these fleece properties over a wide range of sheep flocks before drawing any general conclusions. The conflict of results in the past may have been due to the different types of solvents used for wax extraction. Chlorinated hydrocarbons, such as chloroform and carbon tetrachloride, which were used in the studies of Hayman (1953) and Paynter (1961) will remove substantial amounts of suint in suspension with the wax and therefore give erroneous, results (Lipson 1978). Finally, experimental error may also arise where the full length of staple is analysed due to leaching of suint from the fleece tip. It is desirable therefore to analyse the base of the staple rather than the whole staple.
Fleece properties were measured in the present studies in a manner which avoided the sources of experimental error mentioned above. In addition, physical attributes of the fleece were considered together with skin properties, the latter for the first time in fleece-rot studies.
Present studies
Specific details of individual experiments are reported by Watts et al. (1980, 1981) and Lipson et al. (1981). The main conclusions of these studies are:
(i) Differences in skin histoanatomy between resistant and susceptible sheep to fleece-rot and body strike were identified in 2 merino flocks depastured together. Susceptible sheep were characterised by smaller follicle groups and more sudoriferous glands (which secrete the hygroscopic suint fraction of the fleece) than in resistant sheep. Fibres of coarser and more variable diameter were measured histometrically in these susceptible animals.
Of the skin properties measured, the number of sudoriferous glands and the mean fibre diameter together provided the best practical discrimination between the susceptible and resistant sheep assessed over both flocks, classifying 153 of the 180 sheep correctly. However, there are statistically differences between the discriminant lines for the two flocks, in part these reflect differences in the distributions of measurements of the individual skin properties. In extending these studies to other flocks of sheep, therefore, it is likely that the relative importance of the various skin properties associated with fleece-rot resistance will differ between flocks.
In further work, it may be useful to try to quantify sheep susceptibility with both measurement and prediction of the degree of susceptibility on a continuous scale. Such procedures appear possible, as susceptibility of sheep to fleece-rot is not in reality on all-or-none quality. This may provide an improvement to the present qualitative approach, especially in terms of predicting the susceptibility of flocks of sheep.
(ii) An automated method (CSIRO fibre fineness distribution analyser of Lynch and Michie 1976) of measuring the diameters of large numbers of individual wool fibres was used to provide accurate estimates of fibre diameter variation. In a study of resistant and susceptible sheep from another merino flock, the relationship between high fibre diameter variation and susceptibility to fleece-rot was confirmed. This relationship is thought to be causal in nature since other indicators of fleece-rot susceptibility (wax content, suint content, wax to suint ratio, suint pH, insoluble nitrogen content, wool colour and fleece wettability did not differ significantly between the resistant and susceptible groups of animals. It is suggested that sheep with irregular fibre size may retain free moisture in the fleece for longer, and thereby become more susceptible to fleece-rot than sheep with uniform fibre diameter, other predisposing factors being equal.
(iii) Fleece samples from susceptible and resistant sheep from each of 14 flocks in New South Wales, Victoria, South Australia and Tasmania were examined for wax and suint contents, suint pH, insoluble, nitrogen content, fleece wettability and fibre diameter variation. Contents of suint and insoluble nitrogen were significantly (P<0.05) higher in susceptible animals from 11 and 10 respectively of the 14 flocks. Fleece wettability and suint pH were significantly higher in 8 and 7 flocks respectively, whilst fibre diameter variation were each significantly higher in 6 flocks.
Wax to suint ratio could differentiate 9 flocks but this was due almost entirely to the effect of suint content.
(iv) It is suggested that fleece-rot resistance of sheep can be viewed in terms of 3 barriers, involving both the fleece and skin of sheep, to wetting and infection which have to be breached before the final and characteristic phase of serous fluid loss from the dermis with its associated bacterial proliferation, discolouration and odours can occur. These barriers are: the topography of the fleece surface and the arrangement and form of staples in the fleece; the water-absorbing suint and water-repellent wax fractions of the fleece; and the skin itself. They act to hinder the penetration of water into the dermis. The fleece and skin properties which are seen to differ between resistant sheep and susceptible sheep in a particular flock will, under this hypothesis, depend on which of the 3 barriers is mainly responsible for discriminating between these two types of sheep. This, in turn, will depend on the duration and frequency of rain to which the flock is subjected.
CONCLUSIONS
The development of fleece-rot and body strike involves a complex chain of reactions which depend on a range of prerequisites in the fleece and skin of sheep. Firstly, water must penetrate to the skin surface. This may occur by direct skin wetting in an open fleece,by diffusion where the fleece is dense and compact and, in some cases, by a combination of the two.
In dense and compact fleeces, the shape and arrangement of staples appear to have an important bearing on the wetting and drying behaviour of the sheep's fleece. In sheep which are susceptible to fleece-rot, long coarse wool fibres are often present in large numbers in the staple. These fibres tend to protrude above the staple ends and impart a 'fuzzy' appearance to the fleece surface. The 'fuzzy' tip forms a web which traps and holds rain droplets on the sheep's back. These droplets are channelled directly into poorly defined and unevenly crimped staples.
Water in the fleeces of susceptible sheep appears to be more easily transferred to skin level and, more importantly take longer to evaporate since large quantities of hygroscopic suint are usually present. Rapid wetting and slow drying of the sheep's fleece encourage fleece-rot development as the continued presence of free moisture at skin level is an essential prerequisite. Resistant sheep, however, usually have better fleece structure than susceptible sheep. Rain tends to run off the sheep's back more easily as the closely packed staples with squared ('blocky') tips impede water penetration. Droplets of water which do enter the fleece tend to lodge mainly between rather than within the well-defined and evenly crimped staples. In this location, ventilation is better and the well-structured fleece tends to dry rapidly. Free moisture that enters the body of the staples would probably be slower in reaching the skin surface and evaporate more freely as smaller quantities of suint are usually present in the resistant type of fleece.
In susceptible sheep, the intense and prolonged wetting of the fleece and skin allows the opportunist pathogen, Ps. aeruginosa to multiply rapidly and to produce a bacteriostat which suppresses the growth of other resident micro-organisms. The wetting also allows the suint soaps to emulsify and disrupt its protective sebaceous film so that a mild dermatitis characterised by maceration of the stratum corneum develops. The unprotected skin also becomes more permeable to irritant substances.
The development of fleece-rot is characterised by a sudden and substantial increase in the concentration of soluble protein on the skin surface (Merritt and Watts 1978b). Within the fleece-rot area, discreet sites ('hotspots') often no bigger than 1 cm in diameter can be found in which serum has leaked onto the skin surface via damaged wool follicles. It is also possible that the purulent material that accumulates on the skin surface in response to Ps. aeruginosa and its soluble products may contribute substantially to the high levels of soluble protein present in these fleece-rot 'hotspots'. Such sites blend imperceptibly with surrounding areas of fleece-rot, but nevertheless remain highly attractive to the gravid L. cuprina fly. Odours emanating from these sites as the result of Ps. aeruginosa activity in the presence of serum, wax, suint and skin debris, are responsible, in part, for the fly laying its eggs in the wet fleeces of susceptible sheep. Newly hatched larvae feed on the rich supplies of protein available to them and develop their mouthparts sufficiently to abrade the sheep's skin. Copious quantities of blood are thereby released, and the body-struck area extended by the burrowing blowfly larvae.
PROSPECTS FOR FURTHER INVESTIGATIONS
Further studies on the role of Pseudomonas aeruginosa and the effect of immunisation.
Soluble antigenic products of Ps. aeruginosa exacerbate the leakage of vascular element from sheep's skin under wet conditions (D.H. Burrell, J.E. Watts and G.C. Merritt, unpublished data). In vitro assay techniques have been developed to study these pathogenic factors as well as a hitherto undescribed protein precipitating substance. Corresponding inhibition tests indicate that sheep can be immunised against these antigens. Experimental work will be carried out to determine whether sheep can be vaccinated against fleece-rot as a means of preventing body strike.
Indirect selection of sheep for reduced incidence of fleece-rot and body strike.
Genetic improvement of body strike resistance in sheep appears to be both a feasible and desirable breeding objective. Indirect selection methods have more appeal than direct selection against fleece-rot and body strike because of the irregular occurrence of these conditions in most sheep-raising areas. The way in which industry might use such predictor traits is left open at this stage. If any of the predictor traits showed a strong negative association with sheep productivity there would be a case for a high producing but fleece-rot susceptible type adapted to dry areas; plus a slightly lower producing but fleece-rot resistant type adapted to wet areas. This is approximately the situation as it exists today, with strong and fine wool types of merino sheep largely confined to semi-arid, areas and high rainfall areas respectively. If, on the other hand, there were no strong negative associations, it should be possible to breed one high-producing sheep suited to both dry and wet areas.
A breeding experiment to determine whether high wool-producing merino sheep can be selected for fleece-rot and body strike resistance without loss of productivity commenced in 1978. In it, 750 ewes have been divided into 3 closed lines, one selected to improve the skin and fleece barriers to wetting (that is, low values for insoluble nitrogen content, suint content and fibre diameter variation), a second selected only to improve the skin barrier (low insoluble nitrogen content) and a third left unselected to enable progress in the other 2 lines to be measured. In each selection line, equal emphasis is given to greasy fleece weight and the indirect selection criteria for fleece-rot resistance.
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