The comparative susceptibility of five breeds of sheep to foot-rot

Five breeds of sheep, Romney Marsh, Dorset Horn, Border Leicester, Peppin Merinos and Saxon Merinos were examined for their susceptibility to foot-rot by exposure to natural transmission of infection on irrigated pasture or by the application of pure cultures of Bacteroides nodosus to each foot in a pen experiment. On pasture, the sheep encountered a moderate challenge and the British breeds were more resistant than Merinos to the development of severe foot-rot. Resistance was manifested by a rapid resolution of relatively benign lesions in the interdigital skin, rather than a reduction in the number of feet affected. However, under more severe challenges with foot-rot in pens, all breeds were equally susceptible. There was little difference between resistant and susceptible sheep in the kinetics and magnitude of their antibacterial immune responses indicating that resistance did not depend on pre-existing antibody or a more rapid induction of antibody production. In each experiment, humoral immune responses against B. nodosus were not greatly elevated until under-running lesions of the hoof developed.     

Motility in relation to virulence of Bacteroides nodosus

Fourteen Bacteroides nodosus isolates from footrot lesions of sheep were examined microscopically and all were found to have twitching motility. The mean percentage of cells showing motility was 40% and 9% for virulent and benign strains, respectively. This corresponded with mean agar colony diameters of 17 mm and 7 mm, respectively, for these strains. Two strains of intermediate virulence had values of motility and colony diameter similar to the benign strains. However, the intermediate and the virulent strains produced relatively stable protease compared to the benign strains. All virulent, benign and intermediate strains produced abundant pili. Included for comparison in this study was an avirulent variant strain which was highly motile, formed large colonies and produced stable protease, but showed no pili on electron microscopy. It was concluded that the properties of motility and protease stability may be used to distinguish, in the laboratory, wild-type virulent, benign and intermediate strains of B. nodosus.     

A comparison of the efficacy of footbaths containing formalin or zinc sulphate in treating ovine foot-rot under field conditions

A comparison was made of the efficacy of zinc sulphate and formalin footbaths in treating foot-rot of a wide range of severity in a British lowland sheep flock. Solutions of 10% zinc sulphate and 10% formalin in a walk-through trough were used at weekly intervals on three occasions following initial foot trimming. Paring allowed accurate scoring of foot lesions. Ninety affected sheep were randomly divided between three groups: two groups having footbaths and a control group. Both agents were effective in reducing the total lesion score and the number of feet affected.     

The virulence of Dichelobacter nodosus,measured by the elastase test,is an important predictor for virulent footrot diagnosis in New South Wales sheep flocks

Ovine footrot is a contagious bacterial disease that causes foot lesions, and depending on the virulence of the causative strains, may lead to severe underrunning of the hoof and lameness. Virulent footrot can be identified, treated and controlled more effectively than less virulent benign forms. The in vitro elastase test for virulence of the causative bacteria, Dichelobacter nodosus, has been used to support clinical diagnosis. However, not all laboratory-designated virulent D. nodosus strains cause clinical signs of virulent footrot. This study evaluated retrospectively how well the elastase test supported clinical footrot diagnosis in 150 sheep flocks examined for suspect footrot in New South Wales between August 2020 and December 2021. Flocks were included if measures of clinical disease, environmental conditions and the virulence of D. nodosus isolates were available. Variation in the elastase activity result between D. nodosus isolated from the same flock made bacterial virulence hard to interpret, but calculating the mean elastase rate for all isolates from the same flock made correlations between bacterial virulence and flock footrot diagnosis possible. Simplifying bacterial virulence into whether there were any elastase-positive D. nodosus isolates before 12 days increased the predictive value of elastase results for virulent diagnosis, compared with using the first day that any isolate was elastase positive or the percentage of elastase-positive isolates by 12 days, but not all clinically virulent flocks had isolates with elastase activity before 12 days. Logistic regression models were fitted to identify the minimum number of predictors for virulent footrot diagnosis, with models suggesting that virulent footrot diagnosis was best predicted by adding the elastase test result and environmental conditions to the prevalence of severe foot lesions (score 4 and 5). However, performing the same analysis with different breeds, ages of sheep and seasons might highlight other factors important in the diagnosis of virulent footrot.     

The role of elastase in the differentiation of Bacteroides nodosus infections in sheep and cattle

Eighty-seven Bacteroides nodosus isolates were examined for elastase production by clearing of elastin particles in TAS agar medium. These included 54 ovine virulent isolates, 28 ovine benign isolates and five bovine isolates. In addition 22 ovine virulent, 16 ovine benign and two bovine isolates were examined for decline in proteolytic activity over a 13-day period in the degrading proteinase test using hide power-azure as substrate. There was a remarkable correlation between elastase production, relative stability of proteolytic activity in the hide powder-azure test and virulence of B nodosus. Ovine virulent isolates invariably produced elastase whereas ovine benign isolates and bovine isolates were elastase negative. Bovine isolates produced only mild lesions in the feet of challenged sheep.     

Characterisation of virulent and benign strains of Bacteroides nodosus

The extracellular proteases of 395 isolates of B. nodosus from ovine, bovine and caprine foot lesions were classified as either thermostable or thermolabile. Stable protease was associated with one and unstable protease with four distinctive isoenzyme patterns, each pattern differentiated by the relative mobility of paired isoenzymes. Pathogenicity tests on 64 isolates showed a correlation between the production of stable protease and the production of virulent ovine footrot lesions. The mean values for total protease activity, twitching motility and colony diameter were significantly higher for virulent compared to benign isolates, but the range of values overlapped. SDS-PAGE whole-cell electrophoretic profiles of virulent isolates were similar to the profiles of some benign isolates.     

Cultural characteristics and virulence of strains of Fusobacterium necrophorum isolated from the feet of cattle and sheep

Sixty-one isolates of Fusobacterium necrophorum were recovered for study. Thirty-one were obtained from lesions of foot abscess in cattle (25) and sheep (6), 28 were from interdigital lesions in cattle and 2 were from the normal interdigital skin of cattle. The majority of isolates from lesions of foot abscess were virulent, belonged to biotype AB (Fievez 1963), produced flat, irregular shaped, greyish colonies and haemolysis on blood agar, and grew as turbid filamentous suspensions in liquid media. They produced a soluble exotoxin, a leucocidin, and were pathogenic for cattle and mice. Virulent isolates also produced a haemolysin which most readily lysed bovine, equine and chicken erythrocytes; those from sheep were less susceptible while those of rabbit and pig were the most resistant. Isolates recovered from lesions of the feet not classified as foot abscess and from clinically normal feet were predominantly of the B biotype and caused few experimental lesions, produced convex, round, yellow colonies, flocculated and sedimented while growing in liquid medium and produced little or no haemolysin or leucocidin. Routine differentiation between virulent and non-virulent bovine isolates of F. necrophorum could be achieved by assessing the colour, morphology, and degree of haemolytic activity of colonies grown on blood agar.     

The pathogenicity and cultural characteristics of virulent, intermediate and benign strains of Bacteroides nodosus causing ovine foot-rot

The relationship between the cultural and biochemical characteristics of 22 strains of Bacteroides nodosus and their virulence for sheep was examined. Virulent, intermediate and benign strains were recognised. Although there was some relationship between virulence and colony morphology on hoof medium with 4% agar, colonies of one virulent and 4 intermediate strains resembled those of benign strains. However, on hoof medium with 2% agar and on blood Euonagar, colonies of this virulent and one intermediate strain differed from each other and the other 3 intermediate strains, which in turn differed from the benign. The degree of piliation, as assessed by electron microscopy, was not a reliable indicator of virulence in strains not possessing a beaded colony type. Together, the results of colony morphology and proteolytic tests such as zymogram, degrading proteinase and elastin-agar tests allowed better discrimination of virulent and benign strains. Intermediate strains generally possessed virulent protease activity. In strains with benign zymogram patterns, activity bands 2 and 3 were more labile than in strains with virulent patterns. The addition of CaCl2 to the culture medium resulted in greater stability of proteolytic activity, particularly with benign strains, and prevented the disappearance of protease activity in the band 5 position in virulent, intermediate and benign strains during prolonged incubation. There were slight differences in the sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) patterns of outer membrane proteins of some benign strains but those of intermediate category resembled virulent strains. There was some relationship between the apparent Mr of the pilin monomer on SDS-PAGE gels and serogroup specificity.     

Eradication of ovine footrot by repeated daily footbathing in a solution of zinc sulphate with surfactant

OBJECTIVE: To investigate the effect on ovine footrot of repeated daily footbathing in a solution of zinc sulphate with surfactant. DESIGN: Merino sheep were allocated to control and treatment groups of 119 sheep each at week 0. The sheep had a history of S1, U1, T and/or U6 types of Dichelobacter nodosus in interdigital and underrunning footrot lesions. Feet were not pared prior to treatment. PROCEDURE: Treatment sheep were footbathed in a 15 to 18% (w/v) solution of zinc sulphate with surfactant for 10 min on five consecutive days during week 1. At week 2, and fortnightly to week 52, all feet were inspected, lesion scores were recorded and samples were taken for laboratory tests. At week 53, all feet with no lesions at week 52, but with underrunning lesions prior to week 1, were pared and samples were taken. RESULTS: After footbathing, there were no lesions in any treatment sheep at any inspection to week 52. The percentage of feet of control sheep with lesions increased from 9% (391 of 4,284) between weeks 20 and 36, to 14% (593 of 4,284) between weeks 36 and 52. Ninety-five of 96 control sheep with no lesions at week 20 were still asymptomatic at week 52. D nodosus was not isolated from samples taken from 99 and 87 pared feet of treatment and control sheep, respectively. CONCLUSION: Repeated daily footbathing combined with prolonged exposure to a dry environment eradicated footrot in sheep with both interdigital and underrunning lesions in feet that were not pared prior to treatment.     

Cross-protection from Bacteroides nodosus vaccines and the interaction of pili and adjuvants

The effects of vaccination of Merino sheep with the purified pili or the whole cells of Bacteroides nodosus strain 198, either in oil or alum-oil adjuvant, on the severity of foot-rot induced with the homologous strain (198) and a heterologous strain (217) were determined in a field experiment, on flood irrigated pasture. The efficacy of the whole cell vaccines was comparable to that of purified pili vaccines, against homologous challenge, when both had a similar content of pilus antigen although the purified pili vaccines induced significantly greater homologous pilus agglutinating antibody titres than the whole cell vaccines. However, against heterologous challenge, the whole cell vaccines in oil (CO) or alum-oil (CAO) provided significantly greater protection than a purified pili-in-oil (PPO) vaccine, the number of severely affected feet in sheep vaccinated with PPO being similar to that of the unvaccinated group. The group vaccinated with purified pili in alum-oil (PPAO) was intermediate between these two extremes. The superior performance of the PPAO in comparison to the PPO vaccine, against heterologous challenge, was associated with significantly higher mean ELISA titres to the outer membrane complex. Western blot analyses implicated a role in cross-protection for outer membrane proteins, in particular a protein Mr 78,000. The PPO vaccine produced fewer, smaller and less persistent vaccination reactions at the inoculation sites than did the other vaccines. Bodyweight gains in the period prior to challenge were much lower for the groups vaccinated with CO and CAO than for the controls and those vaccinated with purified pili, due presumably to the larger vaccination reactions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparative antigenic analysis of extracellular proteins of Bacteroides nodosus isolated from virulent and benign ovine footrot

Antigens in the extracellular protein (ECP) complexes of Bacteroides nodosus, isolated from sheep with either benign or virulent footrot, were studied by immunoelectrophoresis (IEP). Rabbit antisera against ECP from virulent and benign strains, were used in homologous and heterologous crossed IEP. Four precipitin peaks unique to the virulent strain, and five peaks unique to the benign strain were identified. In an attempt to characterize the different antigens in ECP, rabbit antisera were raised against an outer membrane protein (OMP, mol. wt. 35 000 daltons), pili and various proteases of virulent and benign strains of B. nodosus. No precipitin band was observed when ECP from both B. nodosus strains were reacted against anti-OMP and anti-pilus antisera. However, single precipitin bands unique to one protease from the benign strain and one protease from the virulent strain were identified. The results suggest that specific antigens other than proteases or pili are important in determining whether a B. nodosus isolate is virulent or benign.     

Improved diagnosis of virulent ovine footrot using the intA gene

Footrot is a mixed bacterial infection of the hooves of sheep. The gram-negative anaerobic bacterium Dichelobacter nodosus is the principal causative agent, with different strains causing diseases of different severity, ranging from benign to virulent. In Australia, in the state of New South Wales (NSW), only virulent footrot is subject to regulatory action, including quarantine. However, it is often difficult to distinguish benign footrot from virulent footrot in the initial stages of infection, or under adverse climatic conditions. The gelatin gel test, which measures the thermostability of secreted bacterial proteases, is the laboratory test most widely used in Australia to aid in the differential diagnosis of footrot. The proteases of virulent strains are, in general, more thermostable than the proteases of benign strains. However, there are some false positives in the gelatin gel test, which may lead to unnecessary quarantine procedures. We used Southern blot analysis on 595 isolates of D. nodosus from 124 farms on which sheep had benign or virulent footrot to test for the presence of the intA gene. We found that for D. nodosus strains which are stable in the gelatin gel test, there is a high correlation between the presence of the intA gene and the ability of the strain to cause virulent footrot. We also developed a PCR-based assay for the rapid detection of intA, which can be used to test DNA extracted from colonies grown on plates, or DNA extracted from cotton swabs of culture plates.     

The occurrence, prevalence and transmission of Bacteroides nodosus infection in cattle

Following reports of findings of ovine foot-rot flora in the feet of cattle, a study was undertaken to determine the prevalence of Bacteroides nodosus infection in the apparently normal cattle population. We found that 34.5 to 74.2 per cent of the animals examined on different farms had B nodosus present in one or more feet. B nodosus was not the most prevalent bacterium observed in smears from cattle. Other Gram negative species including Fusiformis necrophorus and many Gram positive cocci and coccobacilli were also present. Macroscopic lesions in the interdigital skin characterised by erosion and hyperkeratosis were usually associated with the occurrence of B nodosus. B nodosus isolated from cattle induced mild interdigital dermatitis in experimental cattle and sheep and the infection was transmitted to recipient cattle and sheep under field conditions. Virulent foot-rot of sheep was not transmitted to recipient cattle in conditions where the disease spread to susceptible sheep.     

Virulence of recent isolates of Mycoplasma synoviae in turkeys

Systemic Mycoplasma synoviae (MS) infection was induced experimentally in commercial turkeys with recent MS isolates (K4822D and K4774J) from turkey breeder flocks that exhibited no clinical signs typical of MS infection except for a low incidence of swollen footpads. The virulence of each strain was compared by evaluating gross and microscopic lesions, serologic responses, and MS isolation rates at 10 and 21 days postchallenge and by comparing these results with those obtained from a known virulent isolate (K1968), another previously characterized field isolate (K4463B), and unchallenged controls. All strains induced lesions typical of infectious synovitis but showed distinct differences in the extent of the gross and microscopic lesions and in the isolation rates from the tissues in turkeys. K1968 induced the most extensive lesions in hock and stifle joints and footpads, but strains K4822D, K4774J, and K4463B all induced synovitis and were similar in virulence for synovial tissues. Very mild respiratory lesions were induced by all of the strains studied. All strains yielded strong positive serologic responses. We concluded that these recent field isolates, although able to induce synovitis, are less virulent for turkeys than a known virulent strain. Nevertheless, under severe experimental challenge, these strains have the capability of causing lesions that may be incompatible with economical turkey production.     

A field trial with a commercial vaccine against foot-rot in sheep

A commercial, polyvalent, alum-oil adjuvanted vaccine against foot-rot was tested under conditions where climatic and management circumstances have traditionally made foot-rot difficult to control or eradicate. The vaccine had a protection effectiveness of 84.4% 120 days after the completion of vaccination. Although 93.5% of ewes developed lesions at inoculation sites, they caused no management problems. A negative correlation between foot-rot status and the development of lesions at inoculation sites was considered to show that a sheep factor exists which is related to susceptibility to both foot-rot and reactions at the site of inoculation. The nature of this factor was not determined. Vaccination was considered the easiest and most effective way of controlling the spread of foot-rot among ewes with lambs at foot in the test district.     

腐蹄病节瘤拟杆菌的分子致病机制的研究进展

腐蹄病是反刍动物绵羊、山羊、鹿和奶牛等常见的一种高度接触性传染病。节瘤拟杆菌是致病作用的主要病菌之一,它是通过IV型纤毛和细胞外蛋白酶而产生作用。但从腐蹄病发病症状看,节瘤拟杆菌所致疾病的严重程度不是相同的,据此节瘤拟杆菌被分为毒性、弱毒性和良性菌,这种分类通过对该菌基因组毒性关联蛋白Vap(Virulence-associated protein)区域和毒性相关位点vrl(Virulence Related Locus)区域的研究,发现其致病特点与基因的顺序有很大联系。     

Histopathology of antitrochanteric degeneration in adult female turkeys of four strains of different mature size

The left and right antitrochanters of 80 female turkeys of four strains were examined for histopathological changes at sexual maturity. Ten birds of each strain were fed ad libitum and 10 restricted to achieve 0.6 of the bodyweight of the ad libitum fed birds at 24 weeks old when the birds were photostimulated. Bodyweights at sexual maturity (28 to 31 weeks) for the four strains were 5.4, 6.5, 7.6 and 13.2 kg for restricted and 7.5, 9.0, 11.1 and 17.1 kg for ad libitum fed birds. The prevalence and severity of cartilage change increased with mature size and was lower in restricted turkeys then in turkeys fed ad libitum. The results showed that abnormal cartilage changes forming the basis of clinical disease occur in females. The prevalence and severity of lesions were directly related to bodyweight and were not limited to particular genotypes. Lesions were more severe in the centre of the antitrochanter, possibly because of forces exerted by the trochanter. Concentrations of basophilic cells in the hyaline cartilage were associated with the increased prevalence and severity of lesions in large turkeys and may be of significance in the development of osteochondrotic lesions in the antitrochanter.     

An investigation into the use of infrared thermography (IRT) as a rapid diagnostic tool for foot lesions in dairy cattle

Previous research has suggested that temperature at the coronary band increases in the presence of claw horn lesions in dairy cattle. However the reliability of using infrared thermography (IRT) as a method of distinguishing between lesions has not been investigated. The objective of this study was to examine the potential of IRT as a non-invasive tool for rapidly screening for the presence of digital dermatitis (DD). Eighty-two cows which either had no skin lesions on the hind feet (controls, n=41feet) or a DD lesion on one or both feet were selected during milking. Following selection, each cow was moved into the farm crush where thermal images were taken from the plantar aspect of each foot at the pastern when the foot was dirty, cleaned and raised for visual inspection. Following recording of thermal images each hind foot was trimmed and the presence of any lesion recorded. It was found that the temperature did not differ significantly between feet with DD lesions and other skin or claw horn lesions, regardless of whether the feet were dirty, clean or raised for inspection. As IRT was not sensitive enough for lesion specific detection, the reliability of setting a temperature threshold above which any type of lesion causing lameness could be detected was investigated. The optimal trade off between sensitivity and specificity could be reached without having to either clean or lift the feet. Setting the threshold for disease at 27°C for dirty feet identified 80% of feet with lesions and 73% of feet without lesions correctly. In conclusion, IRT was reliable in detecting elevated temperature associated with foot lesions. Future research investigating the development of lesions may identify a temperature threshold for early treatment intervention. This technique may also be useful to quantify the effectiveness of early treatment by tracking recovery and recurrence of cases.     

Pathogenicity of field isolates of Erysipelothrix rhusiopathiae in mice, rats and pigs

Eight field isolates of Erysipelothrix rhusiopathiae serotypes 1 and 2, from different sources, were examined for their pathogenicities for mice and pigs. Arthritogenicity for pigs correlated with virulence for mice at the highest and lowest levels, but not with strains of intermediate virulence. The most virulent strain was also arthritogenic in rats. In pigs, after repeated intravenous challenge the number of affected joints ranged from 0 to 11 of 12 examined. For the 8 strains, the mean number of affected joints ranged from 1 to 7.7 per pig. Clinical course and pathological findings were correlated, but the onset, severity and duration of lameness was variable both within and between groups. Clinical lameness, joint swelling and urticariae were of limited use as indicators of joint changes. The more virulent strains caused lameness as early as 2 days, whereas strains of low virulence took up to 8 weeks.     

The Salmonella excretor incidence of dogs in Hawke's Bay

Two groups of adult Merino sheep, initially grazed on pasture, were dosed daily with zinc sulphate (1 mg Zn/kg LW) or zinc oxide (15 mg Zn/kg LW) for six and seven weeks, respectively. On the 18th day, both groups were transferred to indoor pens together with unmedicated control sheep, and five days later the feet of all animals were infected artificially with a virulent strain of Bacteroides nodosus. By the fourth week after challenge, 85% or more of the feet challenged had developed advanced footrot, and no significant differences in the incidence and severity of lesions between dosed and undosed groups were recorded. Plasma zinc concentrations, monitored throughout the experiment, remained at similar levels in sheep receiving the lower dose rate and in the controls. At the higher dose rate, plasma zinc levels increased till Day 23, but thereafter declined to values comparable to those of undosed animals. At post mortem, no evidence of zinc toxicosis was found, and only the kidneys of sheep receiving the higher dosage showed a significant accumulation of zinc compared with the controls.