Grading the lesions of ovine footrot

Gamithromycin in sheep: Pharmacokinetics and clinical evaluation against ovine footrot

In the context of a development program to obtain the market authorization of injectable gamithromycin 15% w/v solution (Zactran®, Boehringer Ingelheim) for use in sheep against footrot, the pharmacokinetic profile of gamithromycin was established and the safety and efficacy of the treatment were confirmed in a multicenter field study in Europe. The basic pharmacokinetic parameters established in healthy young Merino sheep administered gamithromycin at 6 mg/kg body weight based on the analysis of plasma samples which were collected in intervals up to 12 days after subcutaneous injection were: area under the curve until last quantifiable concentration, 8.88 ± 2.33 μg*h/mL; maximum plasma concentration, 448 ± 180 ng/mL; terminal half-life, 42.5 ± 5.25 h. The safety and clinical efficacy against footrot of gamithromycin 15% w/v solution were evaluated in comparison to tilmicosin 30% w/v solution (Micotil®, Elanco) treatment in 364 sheep of various breeds, sex and age from commercial farms in the United Kingdom (2 sites), Germany (3 sites) and France (1 site). Animals were enrolled based on lesions characteristic of footrot and lameness associated with the presence of footrot-related bacterial pathogens and were randomly allocated and treated in a 1:1 ratio with a single subcutaneous dose of gamithromycin or tilmicosin at label dosage (6 or 10 mg/kg body weight, respectively). Lameness and footrot lesions were evaluated at five and 21 days after treatment; the injection site in all animals was examined the day after treatment and followed up daily in the animals with injection site reaction until complete injection site reaction resolution. Samples of 310 and 120 animals tested positive for Dichelobacter nodosus and Fusobacterium necrophorum, respectively, at inclusion, and data of 359 animals were included into the combined analyses (5 animals excluded for unintentional overdosing [1], lack of follow-up [1], concurrent antibiotic medication for non-footrot conditions [3]). Lameness scores at 21 days after treatment demonstrated a significantly (p = 0.0396) better success for the gamithromycin treatment compared to the tilmicosin treatment (97.8% vs. 93.3%). Post-dosing footrot lesion scores followed similar trends of rapid and marked decrease (improvement) for both treatments with similar (p = 0.127) treatment success for the gamithromycin and tilmicosin treatments (97.8% and 96.0%, respectively). Both treatments were safe; injection site reactions noted in 19 gamithromycin- and 25 tilmicosin-treated animals resolved within five days or six days of treatment, respectively. Gamithromycin 15% w/v solution administered once to sheep by subcutaneous injection at 6 mg/kg body weight demonstrated a pharmacokinetic profile similar to that reported previously in sheep and cattle and was confirmed to be a safe and efficacious treatment for naturally occurring ovine footrot in a multicenter clinical field study conducted in Europe.

The severity of footrot lesions induced by aprV2-positive strains of Dichelobacter nodosus varies between strains

OBJECTIVES

The primary objective of this study was to evaluate the clinical virulence of aprV2-positive lesser virulent field isolates of footrot bacteria Dichelobacter nodosus in comparison with an aprV2-positive clinically virulent reference strain. Correlations between the clinical expression of the disease and the presence of aprV2 (detected using PCR tests) have been inconsistent. A second objective was to evaluate the elimination of D. nodosus following treatment of sheep as some strains of D. nodosus have been reported to be difficult to eliminate.

METHODS

The virulence of three aprV2-positive field isolates of D. nodosus which had lesser virulent phenotypes, and an aprV2-positive virulent reference strain was evaluated in a sheep trial using a pasture-based experimental infection model. In the second phase of the study, treatments including footbathing and a long-acting antibiotic were administered and their efficacy in elimination of these strains was evaluated.

RESULTS

Severe underrun (score 4) lesions developed in sheep infected with the aprV2-positive virulent reference strain but not in sheep infected with the field isolates; they had mild lesions (score 2 or 3). The three field isolates and the virulent reference strain of D. nodosus were eliminated by intensive foot bathing and antibiotic therapy in combination with housing the animals in dry conditions post-treatment.

CONCLUSION

The results suggest that the presence of aprV2 gene in isolates of D. nodosus may not be a reliable indicator of virulence and that further investigation of the factors that determine clinical virulence is required. While the treatment regime was successful, based on a range of considerations, the use of such an intensive treatment involving antibiotics should be limited to small groups of high-value animals, such as rams.

A comparison of multivalent and bivalent vaccination strategies for the control of virulent ovine footrot

Virulent footrot is a significant economic and animal welfare concern. The disease can be treated, controlled, and eliminated with vaccine, but selecting the appropriate vaccination strategy can be challenging. There are two main strategies: outbreak (serogroup)-specific univalent or bivalent vaccination, or use of a multivalent vaccine containing up to nine of the most common serogroups. The objective of this study was to compare these approaches in sheep flocks infected with multiple Dichelobacter nodosus serogroups. In the first phase, we undertook an immunogenicity trial in which we compared four pre-commercial multivalent recombinant fimbrial vaccines containing six (A, B, C, G, H, I) or nine (A, B, C, D, E, F, G, H, I) serogroups, and compared them to commercial bivalent vaccines. Two multivalent vaccines stimulated significantly higher antibody responses than two other multivalent vaccines but the number of serogroups included in the multivalent vaccine formulations did not have a significant effect. In the first phase, we also compared inter-vaccination intervals of two- and three-months between sequential bivalent vaccines, and found that a two-month interval was sufficient to avoid antigenic competition. In the second phase, the most immunogenic multivalent vaccine (nine serogroups) was compared to sequential bivalent vaccines and monthly foot-bathing in a field trial in four commercial Merino flocks. The duration of protection afforded by the multivalent vaccine was likely to be less than that of the bivalent vaccines, as the antibody titres stimulated were lower and less persistent.

First cases of contagious ovine digital dermatitis in Germany

Contagious ovine digital dermatitis (CODD) is a significant disease of the ovine foot characterised by severe lameness and progressive separation of the hoof horn capsule from the underlying tissue. Similar to bovine digital dermatitis (BDD), pathogenic members of the genus Treponema including the Treponema medium phylogroup, Treponema phagedenis phylogroup and Treponema pedis are frequently found together in CODD lesions. To date, CODD was only described in Ireland and the United Kingdom. In northern Germany, cases of an unusually severe lameness presented in a sheep flock that had been affected by footrot for several years. These cases were non-responsive to conventional footrot therapies, with some sheep exhibiting substantial lesions of the claw horn that resulted in horn detachment. Lesion swab samples were collected from both clinically affected and asymptomatic animals. In all clinically affected sheep, CODD-associated Treponema phylogroups were detected by polymerase chain reaction. This is the first report of CODD in Germany and mainland Europe, indicating a wider geographic spread than previously considered. In cases of severe lameness attributed to claw lesions in sheep that fail to respond to footrot treatment, CODD should be considered irrespective of geographic location.

Amplification of acidic protease virulence gene (aprV2) in samples from footrot lesions did not help in diagnosis of clinical virulent footrot in affected sheep flocks in New South Wales

OBJECTIVE

Ovine footrot is a contagious bacterial disease that reduces meat and wool production and can trigger on-farm quarantine in New South Wales. Field diagnosis is based on the prevalence and severity of foot lesions, environmental conditions and flock history. The study evaluated whether a PCR assay or gelatin gel test for virulence in Dichelobacter nodosus isolated from hoof material could aid in the clinical diagnosis of virulent footrot.

METHODS

A quantitative polymerase chain reaction (qPCR) used for diagnosis of virulent footrot in some Australian states was evaluated on 218 hoof swabs taken from 44 sheep flocks from 36 NSW properties, quantifying both the aprV2 positive and aprB2 positive acidic protease genotypes of D. nodosus.

DESIGN

The same flocks/swabs were used to evaluate test agreement between the aprV2/B2 qPCR and the gelatin gel test, and a multiple logistic regression was used to identify factors critical for field diagnosis of virulent footrot.

RESULTS

Only fair to moderate agreement (kappa test) and significant disagreement (McNemar's) was shown between the gelatin gel test and the ratio of aprV2 positive to total D. nodosus. The proportion of aprV2 positive D. nodosus was not significantly different between foot lesions scores of increasing severity. Field diagnosis of virulent footrot was best explained by the prevalence of score 4 and 5 lesions, wet and warm environmental conditions, and recent footrot diagnosis.

CONCLUSION

Although the apr2 gene could differentiate between benign and virulent strains of D. nodosus, the apr2 qPCR was of minimal use for field diagnosis of virulent footrot, where disease expression relies on host genetics, immunity and environmental conditions.

Diagnostic pathology in microbial diseases of sheep or goats

Post-mortem examination is a key step in the diagnostic process of infectious diseases in sheep and goats. Diagnostic pathology deals with identification and study of lesions, at the same time providing also significant clues regarding pathogenesis of the diseases. This article reviews the salient pathological findings associated with the most significant infectious diseases of sheep and goats present in countries where small ruminants are a relevant agricultural industry. Lesions are reviewed according to the different organ systems where they occur. Emphasis has been given in the description of the salient lesional patterns than can be identified in each organ and which can be of help in the differential diagnosis of the lesions caused by bacteria, viruses, fungi or prions. Finally, a review of the usefulness of ancillary tests that may be used on various tissue samples for performing an aetiological diagnosis, is included; the application of various techniques, from immunohistochemistry to molecular biology-based tests, is described.

Investigation of immunity in sheep following footrot infection and vaccination

Ovine footrot is a major disease affecting sheep welfare and production. The anaerobic Gram-negative bacterium Dichelobacter nodosus is the essential transmitting agent. Monovalent or bivalent vaccines induce high levels of D. nodosus antibodies and are the basis of several successful footrot treatment, control and eradication programs. Due to the rapid rate of disease transmission within a flock, the presence of therapeutic vaccination non-responders has major implications for a control program. The aim of this study was to assess the immunological basis of a therapeutic vaccination non-response. Sheep (n=120) were infected with D. nodosus in an artificial pen challenge. Once disease had established, animals were vaccinated with a serogroup specific D. nodosus fimbrial vaccine. Based on the response to therapeutic vaccination, animals were allocated into one of three groups: (i) TVNR where disease persisted despite vaccination (ii) non-diseased, where disease never established and (iii) TVR, where disease was established but resolved with vaccination. Factors related to both the innate and adaptive immune pathways were assessed. These included antigen-specific serum antibodies, interferon-γ, interleukin-10, proliferation of lymphocyte subsets and phagocytic activity of leukocytes. There was no significant difference between the three groups of sheep for any of these parameters. All three groups of sheep produced antibody in excess of a previously published minimum antibody titre required for protection. Opsonising activity in sera from the three groups of sheep was also not significantly different and phagocytic cells from sheep from all three groups were able to destroy D. nodosus intracellularly. These findings show that the measured systemic adaptive and innate immune responses were unlikely to be the cause of a therapeutic vaccination non-response. They also show that the accepted minimum protective titre may be incorrect and may need further examination.

Differential expression of Toll-like receptors and inflammatory cytokines in ovine interdigital dermatitis and footrot

Footrot is a common inflammatory bacterial disease affecting the health and welfare of sheep worldwide. The pathogenesis of footrot is complex and multifactorial. The primary causal pathogen is the anaerobic bacterium Dichelobacter nodosus, with Fusobacterium necrophorum also shown to play a key role in disease. Since immune-mediated pathology is implicated, the aim of this research was to investigate the role of the host response in interdigital dermatitis (ID) and footrot. We compared the expression of Toll-like receptors (TLRs) and pro-inflammatory cytokines and the histological appearance of clinically normal in comparison to ID and footrot affected tissues. Severe ID and footrot were characterised by significantly increased transcript levels of pro-inflammatory cytokines TNFα and IL1β and the pattern recognition receptors TLR2 and TLR4 in the interdigital skin. This was reflected in the histopathological appearance, with ID and footrot presenting progressive chronic-active pododermatitis with a mixed lymphocytic and neutrophilic infiltration, gradually increasing from a mild form in clinically normal feet, to moderate in ID and to a focally severe form with frequent areas of purulence in footrot. Stimulation with F. necrophorum and/or D. nodosus extracts demonstrated that dermal fibroblasts, the resident cell type of the dermis, also contribute to the inflammatory response to footrot bacteria by increased expression of TNFα, IL1β and TLR2. Overall, ID and footrot lead to a local inflammatory response given that expression levels of TLRs and IL1β were dependent on the disease state of the foot not the animal.

A longitudinal study of the risks for introduction of severe footrot into sheep flocks in the south west of Norway

In 2008, ovine footrot was detected in Norway for the first time since 1948. By December 2012 it had spread to 99 flocks, all in the county of Rogaland in the south west of Norway, and 42% of which were located in the municipality of Rennesøy in Rogaland. The aim of this study was to investigate risk factors for contracting severe footrot in flocks of sheep. A flock was considered positive for severe footrot based on positive virulence test or by clinical signs in addition to a positive PCR test. A retrospective longitudinal study was performed with a questionnaire as the main data source. All sheep farmers (107) in the municipality of Rennesøy were selected for inclusion in the study. The questions focused on direct and indirect contacts between sheep in different sheep flocks and general information about the farm. The questions covered the years 2007-2011. Data were analysed using discrete time survival modelling. A total of 81 (76%) farmers responded to the questionnaire including 29 of 41 (71%) farmers with flocks positive for severe footrot. Factors that increased the risk of a flock becoming positive for severe footrot in the final multivariable survival model were sheep that trespassed boundary fences and came into contact with a flock positive for severe footrot (odds ratio 11.5, 95% confidence interval 4.1-32.2) and at least one flock with severe footrot within 0-1km radius of a farm (odds ratio 8.6, 95% confidence interval 2.3-32.6). This study highlights the importance of upgrading and maintaining boundary fences and encouraging farmers to avoid direct and indirect contact between nearby flocks.

Producer-initiated field research leads to a new diagnostic test for footrot

The Cicerone Project was formed in 1998 to address problems faced by wool producers. In the New England area, the issue of suspected false positive diagnoses of virulent footrot, which can be a significant cause of economic loss to individual producers, was investigated. In New South Wales, footrot diagnosis is primarily a field diagnosis supported by the gelatin gel laboratory test. The principal causative agent of footrot is Dichelobacter nodosus. If the gelatin gel test finds strains of D. nodosus to be thermostable (gel stable), a finding of virulent footrot is likely and quarantine of the affected property follows. However, livestock producers and inspectors reported that there were a considerable number of cases where laboratory tests found strains to be stable but these strains did not cause virulent footrot in the field. Preliminary results using DNA markers associated with virulent footrot showed that one of these markers, intA, was absent in gel stable, field benign strains but present in all strains tested which caused field virulent footrot. A trial conducted at Uralla, New South Wales, demonstrated conclusively that there were strains of D. nodosus which were stable in the gelatin gel test but did not cause virulent footrot in the field. All of these strains were negative in the intA DNA test. These results were confirmed in a second field trial at Molong, New South Wales. These trials were instrumental in establishing that the gelatin gel test at times gave results inconsistent with the clinical expression of footrot, potentially leading to a false positive diagnosis of virulent footrot. Subsequent research led to confirmation of the intA test, which is now available as an additional tool for footrot diagnosis.

Why are sheep lame? Temporal associations between severity of foot lesions and severity of lameness in 60 sheep

We investigated the temporal associations between the severity of foot lesions caused by footrot (FR) and the severity of lameness in sheep. Sixty sheep from one farm were monitored for five weeks. The locomotion of each sheep was scored once each week using a validated numerical rating scale of 0-6. All feet were then examined, FR was the only foot lesion observed; the severity of FR lesions was recorded on a scale from 0 to 4. Sheep had a locomotion score > 0 on 144/298 observations. FR lesions were present on at least one foot on 83% of observations of lame sheep but also present on 27% of observations where sheep were not lame; 95% of these sheep with a lesion but not lame had FR score 1. The results from a linear mixed model with locomotion score as the outcome were that the mean (95% CI) locomotion score of 0.28 (0.02, 0.53) in sheep with no lesions increased by 0.35 (0.05, 0.65) in sheep with FR score 1 or 2 and by 1.55 (1.13, 1.96) in sheep with FR score > 2 at the time of the observation; indicating that as the severity of the lesion increased, the severity of lameness increased. One week before an FR score > 2 was clinically apparent, sheep had a locomotion score 0.81 (0.37, 1.24) higher than sheep that did not have an FR score > 2 in the subsequent week. One week after treatment with intramuscular antibacterials the locomotion score of lame sheep reduced by 1.00 (0.50, 1.49). Our results indicate a positive association between severity of FR lesions and locomotion score and indicate that some non-lame and mildly lame sheep have footrot lesions. Treatment of even those mildly lame will facilitate healing and probably reduce the spread of infection to other sheep in the same group.

The subtilisin-like protease AprV2 is required for virulence and uses a novel disulphide-tethered exosite to bind substrates

Many bacterial pathogens produce extracellular proteases that degrade the extracellular matrix of the host and therefore are involved in disease pathogenesis. Dichelobacter nodosus is the causative agent of ovine footrot, a highly contagious disease that is characterized by the separation of the hoof from the underlying tissue. D. nodosus secretes three subtilisin-like proteases whose analysis forms the basis of diagnostic tests that differentiate between virulent and benign strains and have been postulated to play a role in virulence. We have constructed protease mutants of D. nodosus; their analysis in a sheep virulence model revealed that one of these enzymes, AprV2, was required for virulence. These studies challenge the previous hypothesis that the elastase activity of AprV2 is important for disease progression, since aprV2 mutants were virulent when complemented with aprB2, which encodes a variant that has impaired elastase activity. We have determined the crystal structures of both AprV2 and AprB2 and characterized the biological activity of these enzymes. These data reveal that an unusual extended disulphide-tethered loop functions as an exosite, mediating effective enzyme-substrate interactions. The disulphide bond and Tyr92, which was located at the exposed end of the loop, were functionally important. Bioinformatic analyses suggested that other pathogenic bacteria may have proteases that utilize a similar mechanism. In conclusion, we have used an integrated multidisciplinary combination of bacterial genetics, whole animal virulence trials in the original host, biochemical studies, and comprehensive analysis of crystal structures to provide the first definitive evidence that the extracellular secreted proteases produced by D. nodosus are required for virulence and to elucidate the molecular mechanism by which these proteases bind to their natural substrates. We postulate that this exosite mechanism may be used by proteases produced by other bacterial pathogens of both humans and animals.

Evolution of oxidative/nitrosative stress biomarkers during an open-field vaccination procedure in sheep: effect of melatonin

Melatonin has been shown to exert immunomodularory properties with broad application in veterinary medicine. In previous work we have described that subcutaneous coadministration of melatonin to seeps vaccinated against two stumps of A1 and C strains of Dichelobacter nodosus enhanced both the antibody titer and serum IgG levels to A1 and C strains of D. nodosus compared to vaccinated animals not treated with melatonin. Following a similar protocol here we have investigated the effect of a higher dose of melatonin (36mg/animal) in the improvement of the immune response and in the possible oxidative/nitrosative stress produced during the immunization protocol. Our results show that footrot vaccine application induced nitrosative but not oxidative stress at 42 days post-vaccination, which was neutralized by melatonin administration. On the other hand, melatonin improved the immune response with respect to our previous data increasing the time of permanence of antibodies in serum, opening new perspectives for melatonin as prophylactic drug.

Twitching motility is essential for virulence in Dichelobacter nodosus

Type IV fimbriae are essential virulence factors of Dichelobacter nodosus, the principal causative agent of ovine foot rot. The fimA fimbrial subunit gene is required for virulence, but fimA mutants exhibit several phenotypic changes and it is not certain if the effects on virulence result from the loss of type IV fimbria-mediated twitching motility, cell adherence, or reduced protease secretion. We showed that mutation of either the pilT or pilU gene eliminated the ability to carry out twitching motility. However, the pilT mutants displayed decreased adhesion to epithelial cells and reduced protease secretion, whereas the pilU mutants had wild-type levels of extracellular protease secretion and adherence. These data provided evidence that PilT is required for the type IV fimbria-dependent protease secretion pathway in D. nodosus. It was postulated that sufficient fimbrial retraction must occur in the pilU mutants to allow protease secretion, but not twitching motility, to take place. Although no cell movement was detected in a pilU mutant of D. nodosus, aberrant motion was detected in an equivalent mutant of Pseudomonas aeruginosa. These observations explain how in D. nodosus protease secretion can occur in a pilU mutant but not in a pilT mutant. In addition, virulence studies with sheep showed that both the pilT and pilU mutants were avirulent, providing evidence that mutation of the type IV fimbrial system affects virulence by eliminating twitching motility, not by altering cell adherence or protease secretion.

Current understanding of the aetiology and laboratory diagnosis of footrot

Footrot is a highly contagious disease of the feet of ruminants caused by the synergistic action of certain bacterial species of which Dichelobacter nodosus (D. nodosus) is the main transmitting agent. The infection is specific to sheep and goats, although it has also been reported in cattle, horses, pigs, deer and mouflon. The antigenic diversity of D. nodosus is due to variations in the DNA sequence of its fimbrial subunit gene (fimA) and provides the basis for classification of the organism into at least 10 major serogroups (A-I and M), the distribution of which varies with different geographical locations. Host immune response to vaccination is serogroup specific. There are three different clinical forms of disease caused by virulent, intermediate and benign strains of D. nodosus, respectively. In order to facilitate rapid and reliable clinical diagnosis, virulence determination, strain differentiation and serogroup identification for effective control measures, immunological tests, DNA probes and PCR based techniques have been introduced. This review summarises the current understanding of the mechanisms of antigenic diversity of D. nodosus as well as advances made in its strain differentiation and diagnosis.

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 use of melatonin as a vaccine agent

Molecules with immunomodulatory properties determine the magnitude and quality of immune responses specific for the coadministered antigen. Melatonin is considered a biological-response modifier of the immune system with broad application in veterinary medicine. In seasonally-breeding animals, the indolamine is able to improve reproductive performance. With the purpose of expanding new advantageous roles for melatonin, we investigated the effect of subcutaneous slow-release melatonin implants in the humoral response after a vaccination. We reported here a new feature of melatonin as an adjuvant-like system towards Dichelobacter nodosus (A1 and C serotypes)--the bacterium which cause ovine footrot--the most important cause of lameness in sheep. Antibody titres determined by both agglutination and ELISA techniques were substantially higher and were sustained for a longer duration than non-implanted animals. Remarkably, the effect of melatonin was completely dependent on the presence of aluminium hydroxide. The finding that melatonin enhances a defined immune response in vivo opens new perspectives for the improvement of Th2-biased immune responses by alum adjuvants.

Diagnosis of footrot in goats: application of ELISA tests for response to antigens of Dichelobacter nodosus

Goats are an important natural host for footrot and are infected with Dichelobacter nodosus that have virulence characteristics similar to those of sheep strains. However, the humoral response of goats to D. nodosus antigens and the possibility of a serological diagnosis of footrot in goats have not been studied. With the aim of evaluating a diagnostic ELISA test, we investigated the primary immune response of goats to experimental and natural infection, the memory response in recovered animals, and the transfer and persistence of colostral antibodies in kids. Footrot stimulated the goat's immune system and, as in sheep, under-running lesions were the primary stimulus for production of anti-D. nodosus antibodies. The immune response could be detected in ELISA using either fimbrial or outer membrane protein (KSCN) antigens of D. nodosus. Antibody titres resulting from infection declined quickly after recovery and reached pre-infection levels within 3-4 months. Previously affected animals, however, mounted a memory response when injected with purified D. nodosus antigens. Antibody levels attained after anamnestic challenge were correlated with the maximum levels attained during infection, and were therefore indicative of the infection status. Anti-D. nodosus antibodies were also transferred to kids via colostrum, but these antibodies did not persist and therefore were unlikely to interfere with the diagnostic ELISA after 3 months of age. Though these ELISA tests were highly specific, their sensitivity was rather low. Therefore, they are only suitable for a herd diagnosis of footrot in goats and are dependent on the development of advanced under-running infections in a proportion of affected goats.

The type IV fimbrial subunit gene (fimA) of Dichelobacter nodosus is essential for virulence, protease secretion, and natural competence

Dichelobacter nodosus is the essential causative agent of footrot in sheep. The major D. nodosus-encoded virulence factors that have been implicated in the disease are type IV fimbriae and extracellular proteases. To examine the role of the fimbriae in virulence, allelic exchange was used to insertionally inactivate the fimA gene, which encodes the fimbrial subunit protein, from the virulent type G D. nodosus strain VCS1703A. Detailed analysis of two independently derived fimA mutants revealed that they no longer produced the fimbrial subunit protein or intact fimbriae and did not exhibit twitching motility. In addition, these mutants were no longer capable of undergoing natural transformation and did not secrete wild-type levels of extracellular proteases. These effects were not due to polar effects on the downstream fimB gene because insertionally inactivated fimB mutants were not defective in any of these phenotypic tests. Virulence testing of the mutants in a sheep pen trial conducted under controlled environmental conditions showed that the fimA mutants were avirulent, providing evidence that the fimA gene is an essential D. nodosus virulence gene. These studies represent the first time that molecular genetics has been used to determine the role of virulence genes in this slow growing anaerobic bacterium.

Pilus ELISA and an anamnestic test for the diagnosis of virulent ovine footrot and its application in a disease control program in Nepal

The immunological memory (anamnestic) responses in sheep recovered from virulent footrot (VFR) can be aroused by subcutaneous injection of outer membrane protein (OMP) antigens of Dichelobacter nodosus. The magnitude of this response is directly correlated to the highest antibody response attained during infection and memory lasts at least a year after recovery from VFR. However, some older animals show non-specific responses to OMP antigens. In this study an evaluation of D. nodosus pilus antigen for the anamnestic diagnosis of footrot in sheep was undertaken. The results indicated that the primary and anamnestic responses to pilus were similar in character to OMP antigen but were highly specific. The sensitivity of the procedure for detection of sheep with a history of VFR was approximately 80%. A low proportion of sheep with mild lesions due to virulent strains of D. nodosus reacted to anamnestic challenge. Anamnestic challenge with 10 microg pilus was used in a VFR surveillance program in migratory sheep flocks in Nepal. Conventional diagnostic methods could not be applied during the disease transmission periods in these flocks because of their migration to alpine pastures far away from human habitation. The results supported clinical and bacteriological findings suggesting that virulent strains of D. nodosus have apparently been eliminated from these flocks in Nepal.

Antigens for serological diagnosis of ovine footrot

An antigen extracted from Dichelobacter nodosus with potassium thiocyanate (KSCN) is currently used in enzyme-linked immunosorbent assay (ELISA) for serological diagnosis of ovine footrot, but the test lacks specificity in mature sheep. Other antigens were therefore evaluated for use in this test. Structural components of the cell envelope of D. nodosus including outer membrane, cytoplasmic membrane, lipopolysaccharide and pilus and extracellular proteases were purified from cultured D. nodosus while recombinant membrane proteins, protease and pilus antigens were also evaluated. Many antigenic components of D. nodosus participated in reactions in ELISA that were not specific for infection with D. nodosus and apart from pilus, none of the antigens resulted in improved specificity of the ELISA. Using a positive-negative cut-off to yield sensitivity of 70%, ELISA using pili from cultured D. nodosus serogroup A had a specificity of 98.3% compared with 89.7% for the ELISA with KSCN-extract as antigen (P < 0.001). Recombinant pili morphogenetically expressed in Pseudomonas aeruginosa were unsuitable for use in ELISA due to copurification of Pseudomonas antigens to which apparently healthy sheep directed antibodies. The application of ELISA with D. nodosus pilus as antigen in footrot control programs is discussed.

Improved laboratory diagnosis of ovine footrot: an update

Ovine footrot is a complex clinical disease syndrome primarily resulting from infection by the anaerobic bacterium Dichelobacter nodosus. In order to aid clinical diagnosis, various laboratory tests based on the detection and measurements of phenotypic properties of D. nodosus have been developed for genus-specific detection as well as virulence determination. However, these tests are generally time-consuming, and tend to be variable with external factors that affect the growth and metabolism of the bacterium. A new generation of diagnostic reagents, such as monoclonal antibodies, gene probes and polymerase chain reaction, has been developed recently. Preliminary assessment of these reagents has shown potential to vastly improve the laboratory identification and determination of the virulence of D. nodosus. It is important that these new reagents are vigorously assessed against existing laboratory tests, such as the elastase test and gelatin gel test, prior to their adoption for the routine diagnosis of footrot.

Factors associated with the effectiveness of antibiotic treatment for ovine virulent footrot

Factors associated with the proportion of sheep cured of virulent footrot after antibiotic treatment were studied in a field trial under dry environmental conditions. From 2 similar flocks, 1091 Merino sheep weighing about 50 kg and infected with virulent footrot received an intramuscular injection of either 12 mL of a mixture of penicillin (250 mg/mL) and streptomycin (250 mg/mL), 6 mL of long acting oxytetracycline (200 mg/mL) or 6 mL of a mixture of lincomycin (50 mg/mL) and spectinomycin (100 mg/mL). Variables that were significantly associated with the proportion of sheep cured were: the type of antibiotic used, the number of feet infected and the flock from which the sheep came. There was an interaction between antibiotic type and number of feet infected and between antibiotic type and flock in association with the proportion of sheep cured. The extent of paring and the occurrence of blowfly strike in footrot lesions treated with diazinon had no significant association with the proportion of sheep cured.

Further observations on the primary and anamnestic humoral responses to Dichelobacter nodosus in sheep in relation to the diagnosis of footrot

An anamnestic serological test for ovine footrot was evaluated. Footrot-free lambs were infected with Dichelobacter nodosus and treated four, six or eight weeks later. There were strong linear correlations between the severity of the lesions and both the primary response and the anamnestic response evoked by the subcutaneous injection of an antigen from D nodosus 16 weeks after the treatment of the lambs; the latter correlation was stronger than the correlations reported elsewhere in mature sheep. Similar anamnestic responses were elicited six and 12 months after the treatment of mature sheep which had had severe lesions. Natural anamnestic responses were demonstrable in sheep which had had recurrent clinical episodes of virulent footrot. The non-specific humoral responses after the anamnestic challenge of footrot-free sheep increased with age and did not depend on the dose of the antigen between 10 and 200 micrograms. Using the pooled data from sheep of all ages and a positive-negative cut-off which was selected to obtain a sensitivity of 75 per cent, the specificity of the anamnestic test was 90 per cent, similar to that reported for the primary response when it was used to diagnose footrot. The anamnestic test can be applied to determine the presence and severity of footrot in young sheep.

Effects of the severity and duration of lesions on the primary and anamnestic humoral responses of sheep to Dichelobacter nodosus and observations of natural resistance to footrot

In a flock of 137 sheep naturally infected with Dichelobacter nodosus the severity of the lesions was the principal factor associated with the humoral response early in the period of spread of D. nodosus, underrun lesions having the greatest effect. However, after five to six weeks, the duration of underrun lesions rather than their severity or number primarily influenced the response. Sheep first affected late in the period of spread had fewer affected feet, milder lesions and a lower humoral response than those affected earlier. An anamnestic humoral response was stimulated by injecting membrane-protein antigen of D nodosus subcutaneously 18 weeks after the sheep had been treated parenterally with antibiotics and antiseptic footbathing. The anamnestic response was related to the antibody level reached during the infection phase, and hence with the duration and severity of the lesions, and with the residual antibody level at the time of the anamnestic challenge, suggesting that the population of memory B cells specific for D nodosus was proportional to the size of the originally activated B cell population. Even after allowing for differences between the duration and severity of the lesions differential responses were detectable among the sheep. Primary and anamnestic responses of a non-specific nature occurred in control sheep.