Fleece Rot Research Articles

Cellular inflammatory responses in skin of sheep selected for resistance or susceptibility to fleece rot and fly strike

Leucocyte populations were examined in normal and inflamed skin of sheep bred for resistance (R) or susceptibility (S) to bacterial fleece rot and the common sequela, body strike caused by the dipteran parasite Lucilia cuprina. No differences between R and S lines were found in numbers of neutrophils accumulating in acute inflammatory lesions induced by activated complement, leukotriene B4, interleukin (IL)-1 beta, tumour necrosis factor-alpha, IL-8 or endotoxin from Pseudomonas aeruginosa. T19+ (alpha gamma delta T cell subset) lymphocytes and eosinophils were more prevalent in skin of sheep from the S line whereas IgE+ cells were more prevalent in skin of sheep from the R line. In an unrelated population of sheep, animals with low fleece rot scores had more intense neutrophil migration into inflammatory lesions induced by all the mediators examined than did animals with high fleece rot scores. IgE+ cells were more prevalent in animals with low fleece rot scores, although in contrast to R and S lines, T19+ cells tended to be elevated in this group of animals. The results suggest that defence mechanisms associated with IgE+ cells in skin may play an important role in resistance to fleece rot and fly strike.

Fleece rot and body strike in Merino sheep. VI. Experimental evaluation of some physical fleece and body characteristics as indirect selection criteria for fleece rot

Indicators of staple thickness, staple weight, staple density, wool weight cm -2, crimp frequency, dust penetration, fibre diameter (mean, variability), birthcoat score, and neck and body wrinkle were evaluated for their suitability as indirect selection criteria against fleece rot and body strike. The fleece traits were assessed in 949 Merino ewe hoggets representing the progeny from 134 sires in 15 contemporarily managed flocks. Fleece rot and body strike were assessed following experimental induction. In decreasing order of importance, variability (s.d.) in fibre diameter, average staple thickness, crimp frequency, variability (c.v.) in fibre diameter, mean fibre diameter, birthcoat score, staple density, variability in staple thickness, staple weight, and neck wrinkle score, accounted for a significant component of the variation in liability of sheep to fleece rot. Body wrinkle, variability (c.v.) in staple thickness, staple length, dust penetration, and wool weight cm-2 were not important. Based on combined estimates of the heritability of the indicator trait and its genetic correlation with fleece rot, variability in fibre diameter (s.d.) and birthcoat score showed the highest potential as indirect selection criteria. Variability in fibre diameter (s.d.) also showed a similar scope for selection against body strike. Indicators of mean staple thickness, crimp frequency, dust penetration and wrinkle score showed little or no scope for indirect selection against fleece rot or body strike. The role of variability in fibre diameter (s.d.) as an indirect selection criterion for fleece rot and body strike in designed breeding programmes which include other important objectives, will need to be further evaluated, given an undesirable genetic correlation with clean fleece weight. High between-flock correlations were observed for a number of fleece traits and susceptibility to fleece rot. Susceptible flocks were characterized by fleeces with longer, heavier, thicker staples with a lower crimp frequency, and higher fibre diameter with a higher variability. Predicting the susceptibility of flocks to fleece rot and body strike, through a between flock index based on the measurement of simple fleece traits, is suggested.

Plasma leakage in skin of sheep selected for resistance or susceptibility to fleece rot and fly strike

The leakage of plasma into skin following injection of histamine, bradykinin, activated complement, platelet-activating factor and serotonin was measured in sheep bred for resistance or susceptibility to fleece rot and fly strike. Genetically susceptible sheep had significantly greater plasma leakage to activated complement than genetically resistant sheep, and for all mediators there was a trend for plasma leakage to be greater in susceptible sheep. Within each genotype there was also a tendency for plasma leakage to be positively correlated with fleece rot score. In a flock of sheep of different genetic background not selected for resistance or susceptibility to fleece rot and fly strike, positive phenotypic correlations were also noted between fleece rot and plasma leakage. Plasma leakage provides nutrition for the first instar larvae of Lucilia cuprina, the major cause of primary blowfly strike in Australia. Diminished leakage of plasma following release of endogenous permeability mediators may be one component of the mechanism that confers resistance in animals bred for resistance to fleece rot and fly strike.

The susceptibility to body strike under high rainfall conditions of flocks selected for and against fleece rot

The susceptibility of Merino sheep to fleece rot and body strike was examined in a high risk environment. The ewes were a representative sample from 2 selection lines selected solely for increased (susceptible) and decreased (resistant) expression of hogget fleece rot in a low risk environment with the aid of induced fleece rot. A total of 244 fleece rot and body strike observations were made over 3 years. Ewes from the resistant flock had significantly fewer fleece rot lesions (P<0.05) of less severity (P<0.01) than ewes from the susceptible flock. The prevalence of body strike was high in all 3 years, with 11.3% affected in the resistant flock and 20.0% in the susceptible flock. The prevalence of fleece rot lesions was strongly correlated (P<0.001) with the prevalence of body strike. Results confirm the close phenotypic and genetic relationship between resistance to fleece rot and body strike, and show the potential of selection for reduced fleece rot in a dry environment to improve resistance to body strike in a wet environment.

Fleece rot and body strike in Merino sheep. V. Heritability of liability to body strike in weaner sheep under flywave conditions

The prevalence of body strike was monitored in 740 mixed sex weaner progeny from 43 sires, representing 15 different Merino genotypes. A flywave, characterized by a sudden onset in the prevalence of flystrike, occurred over a 2 month period which coincided with unseasonally high rainfall. A total of 190 strikes were treated in 176 sheep representing a strike rate of 25.7 strikes per 100 sheep and a prevalence of 23.8%. The majority of strikes (92.6%) were body strikes with 19.7% of all sheep affected. The heritability of susceptibility to body strike expressed as prevalence was 0.26�0.12 for both least squares (LS) and restricted maximum likelihood (REML) procedures. The heritability of liability to body strike using quasi-maximum likelihood procedures and assuming a continuous underlying scale was 0.53 which agreed well with estimates from the LS and REML analysis after transformation (0.54�0. 25). Predicted genetic progress in the reduction of susceptibility to body strike was calculated for mass selection where the best 5% of rams and 60% of ewes were selected, with a starting prevalence of 20% body strike, assuming (a) an environment exposed to continuous flywaves, and (b) an environment with infrequent flywaves. After 20 years of direct selection the prevalence of body strike would be reduced to 7.2% and 16.1% for the two environments respectively. Indirect selection based on (c) a variable prevalence of fleece rot with a long term average of 30% and (d) fleece rot expressed at 80% would reduce the prevalence of body strike to 6% and less than 2% respectively. Indirect selection based on (e) a hypothetical indicator trait with a continous distribution, a high heritability (0.4) and a moderate genetic correlation with liability to body strike (0.4), would also reduce the prevalence of body strike to less than 2% after 20 years of selection.

Prospects for the control of sheep blowfly strike by vaccination

Research into vaccination against flystrike is aimed at either controlling the predisposing condition, fleece rot, or direct control of the fly maggots. A vaccine against the major bacterial species found in fleece rot lesions, Pseudomonas aeruginosa, is undergoing field trials and results suggest that this vaccine may reduce fleece rot incidence. Problems to be investigated include the existence of variants of P. aeruginosa in the field and the involvement of other species of bacteria in fleece rot. Strategies for direct vaccination include immunization with larval products involved in wound formation and larval nutrition and immunization against novel antigens usually from the gut of first instar larvae. Both methods have resulted in significant inhibition of larval growth. Analysis of larval products has revealed a number of active proteases which degrade skin proteins such as collagen. Inhibition of these enzymes with plasma enzyme inhibitors also affects larval growth in vitro. Antibodies raised against these enzymes are being tested for inhibitory effects against larvae and used to isolate cDNA clones from Lucilia cuprina libraries. Antigens from the gut are able to induce antibodies inhibitory to larval growth both in vitro and in vivo. Isolation of these antigens is proceeding in a number of laboratories. Problems still to be analysed include whether growth inhibition produces effective protection in the field and whether sufficient antibody will have early access to the larvae to significantly affect them.

Fleece rot and body strike in Merino sheep. IV. Experimental evaluation of traits related to greasy wool colour for indirect selection against fleece rot

The potential for using indicators of greasy wool colour as indirect criteria for selection of sheep genetically resistant to fleece rot and body strike was examined. Nine traits based on the subjective grading of wool samples (1 trait), objective reflectance (6 traits) and colour development during incubation (2 traits) were evaluated for their genetic potential, ease and reliability of measurement, and relationship with other major production characteristics. The colour of wool was assessed in 949 Merino ewe hoggets representing the progeny of 134 sires before the induction of fleece rot. All colour traits were moderately heritable, ranging from 0.25 � 0.13 for subjective colour score to 0.40 � 0.14 (h2�.s.e.) for an objective reflectance index. Based on the combined estimate of the heritability of the indicator trait and its genetic correlation with fleece rot, indices based on the objective measurement of colour reflectance and colour development following incubation showed the highest genetic potential for indirect selection. Objective measurement of colour reflectance also showed a high genetic potential for indirect selection against body strike. Subjective colour assessment prior to the induction of fleece rot showed the lowest potential for indirect selection against both fleece rot and body strike. The cost of objective measurement of colour was less than that of colour development during incubation, and the former was found to be the most reliable indirect selection criterion in this experiment. A consequence of selecting solely for decreased greasy wool colour on other production characters is likely to result in decreased fibre diameter, increased wool yield, with a potentially small decrease in greasy fleece weight. Further evaluation of colour indicators as additional selection criteria should first focus on the likely role of fleece rot and body strike resistance as part of (national) breeding objectives. Only then can the full impact of using colour variables on the overall breeding aggregate be established.

Fleece rot and body strike in Merino sheep. II. Phenotypic and genetic variation in liability to fleece rot following experimental induction

Phenotypic and genetic variation in the susceptibility of Merino sheep to fleece rot and body strike were examined following experimental induction of fleece rot by wetting. A total of 949 Merino ewe hoggets, from 15 Merino bloodlines comprising 2 Fine-wool, 2 Medium Non-Peppin, 10 Medium Peppin and I Strong-wool flock were measured over a three-year period. Following wetting, 82.8'Yo of the hoggets developed fleece rot and 7.9% were affected with body strike. The incidence and severity of fleece rot was highest on the back, whereas body strike occurred mainly on the neck. Body strike was largely confined to the period of a flywave in the second year of the study. The low incidence of body strike reduced the reliability with which genetic variation in this trait could be estimated. Between-flock genetic variation in liability to induced fleece rot was largely strain variation (with increasing liability from Fine-wool to Medium-wool to Strong-wool strains). Variation between bloodlines within strains was not as large as those reported for the same bloodlines under natural conditions. The heritability ( � s.e.) of liability to fleece rot was 0.86 using quasi-maximum likelihood procedures, which agrees well with the heritability of fleece rot incidence (0.40 � 0.13) after transformation on to an underlying scale (0.87 � 0.29) obtained through restricted maximum likelihood (KEML). Estimates of genetic parameters obtained for these data through REML procedures were in good agreement with estimates obtained through ordinary least squares analyses. Fleece rot induction allowed screening of a large number of sheep in a relatively short period for their susceptibility to fleece rot. Further evaluation is needed to determine the role of fleece rot induction in breeding programmes aimed at reducing the susceptibility of Merino sheep to natural fleece rot and body strike.

Effect of early life selection using indirect characters on the subsequent incidence of fleece rot in a flock of South Australian Merino ewes

Correlations were estimated between 24 fleece and body characters measured at 1, 2, 3 and 4 years of age, and fleece rot induced by artificial wetting at 4 years of age in South Australian Merino ewes. The character most strongly and consistently related to fleece rot was greasy wool colour score which was correlated with fleece rot score when measured at all 4 ages (r = 0.21, 0.22, 0.31 and 0.26 respectively) and with liability to fleece rot when measured in 3 and 4-year-olds (r = 0.25, 0.38). A number of discriminant functions were developed from characters measured at each age and used to predict fleece rot susceptibility at 4 years of age. Culling 45% of hoggets on the basis of a function comprised of all characters measured would have reduced fleece rot by 39.3%. Culling on the basis of functions comprised of more manageable groups of characters (i.e. selected as (i) contributing significantly to the function when all characters were included, or (ii) on the basis of previous recommendations) would have reduced fleece rot by between 7.4 and 15.7% when 45% of hoggets were culled. Lower culling rates gave less reduction in fleece rot, except where the reduction was very small regardless of the per cent culled. It is concluded that in most practical situations there will be limited benefit from selecting ewes on the basis of indirect characters to reduce fleece rot susceptibility in the current generation.

Fleece rot and body strike in Merino sheep. III. Significance of fleece moisture following experimental induction of fleece rot

The relationship between fleece wetting and drying characteristics and liability of sheep to fleece rot and body strike was examined following experimental wetting. The liability of sheep to fleece rot was reflected by both wetting and drying characteristics. Wetness of the fleece was estimated gravimetrically and was found to be a simple, fast and relatively robust technique. Moisture increased from 13% in the dry fleece up to 123% at skin level in sheep exposed from 0 to 10 days' wetting. Consistent differences were reported in proximal and full staple moisture levels between sheep affected and unaffected with fleece rot or body strike. Drying characteristics of individual sheep were recorded by means of an electronic hygrometer in sheep up to 5 days after wetting. A lag phase in the drying of sheep susceptible to fleece rot or bodystrike was recorded, compared with resistant sheep. The most resistant sheep showed a slightly concave drying curve (-12.4% R.H. day-1 + 0.06% R.H. day-2) after wetting, whereas the most susceptible sheep showed a slightly convex drying curve (- 11.4% R.H. day-1 0.05% R.H. day-2). The maximum difference in fleece relative humidity between the two groups of sheep occurred between 1 and 3 days after wetting. Measurement of fleece relative humidity before wetting in the dry fleece, did not predict the liability of sheep to fleece rot and body strike.Moisture levels in the fleece during and immediately following wetting accounted for differences in fleece rot due to experimental treatments and genetic flock differences. Based on a single full-staple moisture measurement of 945 sheep, representing the progeny of 134 sires, the heritability of fleece moisture was 0.28 � 0.12 and had a genetic correlation with fleece rot incidence and severity of 0.74 � 0.24 and 0.79 � 0.19 respectively. Further work should identify fleece components which affect the wetting and drying characteristics of the fleece during the development of fleece rot.

Relationship between wax, suint and fleece rot: effect of sample preparation, time of sampling and fleece rot induction

The major factors thought to affect the relationship between wax and suint and fleece rot susceptibility were examined. Time of sampling and fleece rot induction were the major factors which resulted in changes in wax and suint levels in the fleece and their relationship with fleece rot. Variation in staple portion used for extraction did alter the yields of wax and suint, but these yields were highly correlated to full staple values in the case of wax (r = 0.77-0.92) and lower in the case of suint (r = 0.23-0.77). Variation in length of staple used did not alter the relationship between wax and suint levels and fleece rot. We conclude that wax would be a more suitable indicator of fleece rot susceptibility than would suint because of its more stable relationship with fleece rot. Differences between sheep affected and unaffected with fleece rot in wax and suint values were small and consistent for the 15 Merino genotypes sampled. The evaluation of wax for indirect selection against fleece rot will depend on further evaluation of the required genetic parameters and a more cost-effective means of measurement.

Fleece rot and body strike in Merino sheep. I. Evaluation of liability to fleece rot and body strike under experimental conditions

The conceptual model that variation in resistance of Merino sheep to fleece rot and body strike could be attributed to an underlying variable with a continuous normal distribution, termed 'liability', was examined in four experiments. A total of 640 young Merino sheep were exposed from 0 to 13 days to a simulated rainfall treatment to allow the development of fleece rot. The incidence of fleece rot ranged from 0% of sheep affected in unwetted control groups and increased up to 100% affected with increased exposure to wetting. Assuming binomial variation, the response in fleece rot and flystrike incidence was fitted on an underlying scale using generalized linear models. In the case of fleece rot severity, grades of exudative fleece rot were shown to fit an ordered categorical response model. The assessment, induction and liability of sheep to fleece rot were found to be highly repeatable in all three cases, indicating that liability to fleece rot could be accurately screened for using experimental induction. Fleece rot severity was found to be an accurate indicator of liability to body strike but not of other forms of flystrike. The possible mechanisms and factors contributing to the liability of sheep to fleece rot and body strike have been discussed, and moisture status of the fleece during the development of the two diseases was indicated as an essential common denominator.

A scanning electron microscope study of L. cuprina larvae and the development of blowfly strike in sheep

Scanning electron microscopy was used to determine whether first instar larvae were capable of physically damaging sheep skin and thus initiating blowfly strike without predisposing conditions. In addition, damage at the lesion site was monitored throughout the infection period. It was found that first instar larvae do not possess the mouth hooks seen in second and third instars. They do however have sets of spines within the oral cavity which, with the spines on the creeping welts, may be able to abrade the outermost skin cell layers. Damage to the epidermis was observed within the first 8 h after larval implantation on sheep. The dermis was exposed within the first 24 h and after this time the normal skin architecture was rapidly destroyed. At 72 h areas of wool were missing and the collagen matrix of the dermis was clearly visible. Predisposing conditions such as fleece rot or wounding were not necessary for strike initiation. It is suggested that a combination of physical irritation and abrasion together with enzymes released by larvae enable them to initiate formation of the strike lesion. Thus, the only mandatory predisposing condition for fly strike may be wetted wool and skin.

An evaluation of classing Merino sheep for fleece rot susceptibility

Two experiments were conducted to evaluate the ability of sheep classers to predict the susceptibility of Merino sheep to fleece rot. A total of 626 hogget ewes, from 15 genotypes known to vary in fleece rot susceptibility, were assessed by 3 classers in each experiment before fleece rot was induced. In all cases the classers were able to identify flocks resistant and susceptible to fleece rot (between-flock correlations between predicted and observed fleece rot susceptibilities ranged from 0.37 to 0.81). Some classers were also able to distinguish between resistant and susceptible sheep within a flock (within-flock phenotypic correlation between predicted and observed fleece rot susceptibilities ranged from 0.05 to 0.39). The potential role of sheep classers in relation to selective breeding against fleece rot is discussed.

Effect of wither malformation on the pre-disposition of sheep to fleece rot and body strike

The presence of the conformational fault 'pinch' (narrow depression over the 4th-6th ribs) was examined in relation to fleece rot susceptibility. A field study involving 189 Merino ewes showed that the identification of the fault (incidence was 11%) by 4 independent observers was highly repeatable. Fleeces on the pinch site showed a disorderly staple arrangement with a higher incidence of tangled and narrow staples compared with unaffected sheep. The pinch fault appeared to be associated with excessively sloping front pasterns ( lt; 45� from the ground) compared with unaffected sheep (about 70�). In 2 experiments, it was shown that the 'pinch' fault was associated with a higher incidence of fleece rot and a higher level of moisture penetration and retention in the fleece during fleece rot induction. Staple crimp, staple density and fibre diameter (mean and variability) were associated with the wetting and drying characteristics of the fleece but not specifically with the pinch fault. It is suggested that the mechanical movement of the fleece at the pinch site was responsible for the increased moisture penetration which, in turn, increased the chance of development of fleece rot and fly strike.

Differentiation of wool and spruce waxes affected by environmental influences using fingerprinting by soft ionization mass spectrometry

Wool wax from two sheep which showed different susceptibility towards fleece rot was analysed by field desorption mass spectrometry. In the mass range from m/z 360 to m/z 410 clear differences of ion abundances were found indicating lower proportions of oxidized cholesterone in the wax of the resastent sheep. This suggests that the weathering of the fleece wax could be a factor in relation to the cause of fleece rot.

Characterization of Pseudomonas maltophilia Isolates from Fleece Rot

A total of 286 isolates of Pseudomonas maltophilia was collected from sheep exhibiting brown or yellowish fleece rot and from fly-strike lesions. Enzyme activities for 10 of the isolates were examined by plate tests and with the API ZYM system and compared with the enzymatic profile of a human type strain of P. maltophilia. The fact that ovine isolates of P. maltophilia are biochemically similar to pathogenic human strains suggests there may be an association between this organism and the brown to yellow type of fleece rot.

Characterisation of isolates of Pseudomonas aeruginosa from sheep

Fleece rot isolates of Pseudomonas aeruginosa were characterised in terms of biochemical reactions, serological typing and production of keratinase, lipase and the potentially dermal necrotic enzymes exotoxin A, protease, elastase phospholipase and lecithinase. The similarities generally obtained between these characteristics of isolates from sheep and man suggests a role for P. aeruginosa and mechanisms of pathogenesis in the development of the dermatitis associated with fleece rot.

Characterization of pseudomonads isolated from diseased fleece.

A total of 59 Pseudomonas isolates was obtained from 11 samples of diseased fleece taken from live sheep. All but four of the isolates could be assigned to one of nine Pseudomonas species, of which P. aeruginosa, P. alcaligenes, P. mendocina , and P. putida were the most common. P. aeruginosa was found in four of the fleece samples and, when present, appeared to predominate. Although several of the isolates of P. aeruginosa lacked the ability to produce pyocyanin (and some produced neither pyocyanin nor fluorescein), nearly all produced several virulence factors. Of the other pseudomonads, many produced proteinase, esterase, and catalase, several were able to grow at 42 degrees C and reduce nitrate, and some also produced lipase and hemolysin and, like P. aeruginosa, might serve to initiate (or sustain) the dermatitis frequently associated with fleece rot in sheep.

Fleece rot in Merino sheep. 1. The heritability of fleece rot in unselected flocks of medium-wool Peppin Merinos

Fleece rot was observed on hogget ewes and rams in unselected medium-wool Peppin Merino flocks over a 15-year period at Trangie Agricultural Research Centre, N.S.W. Each sheep was given a score of from 0 (no fleece rot) to 5 (very severe fleece rot) following a subjective visual assessment along the backline prior to shearing. Data were analysed as two measures of susceptibility to fleece rot: a score as defined above; and incidence, where fleece rot was treated as an all-or-none trait. The average incidence of fleece rot over 23 flock x year combinations was 36%, but this varied widely, from 8 to 90%. Other environmental factors (sex, birth type and age of dam) generally had small and non-significant effects on fleece rot. After adjusting for significant environmental effects, the half-sib heritability estimates for sires:with at least three progeny were 0.36 (�0.07) for score and 0.23 (�0.07) for incidence. Separate anaiyses were conducted for flock x year data sets of low, intermediate and high incidences. The highest heritability estimate for incidence was, as expectedly theoretically, obtained in the data set of intermediate incidence. Offspring-dam heritability estimates ( � s.e.) for fleece rot score and incidence were respectively 0.21 (� 0 0 5 ) and 0.14(�0.04). Corresponding offspring-sire estimates for score and incidence were 0.20 (�0.06) and 0.17 (�0.05). It is concluded that a realistic estimate of the heritability of the underlying liability to fleece rat is of the order of 40%.