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.

Efficacy of bivalent fimbrial vaccines to control and eliminate intermediate forms of footrot in sheep.

OBJECTIVE: The aim of this study was to determine the efficacy of serogroup-specific bivalent fimbrial vaccines in the control and elimination of relatively mild (intermediate) forms of footrot in sheep flocks in NSW, there being some evidence that such forms are difficult to control. METHODS: Four flocks of sheep with history of footrot of intermediate virulence were selected based on clinical and bacteriological diagnoses. Dichelobacter nodosus serogroups included in bivalent vaccines at each farm were based on on-farm serogroup-prevalence data. Two doses of bivalent vaccine were administered with a 4-week interval between doses. Repeated post-vaccination inspections of all feet of between 100 and 119 animals per mob were conducted and foot swabs were collected for bacteriological testing. Blood samples were collected from 10 to 24 individually identified animals per flock at each inspection to check for agglutinating antibody responses. RESULTS: In the majority of animals, antibody levels for serogroups included in each vaccine were above the level believed to be required for protective immunity. Footrot disappeared on farm 1 prior to vaccination, but did not reappear postvaccination. Footrot was controlled but not eliminated on farms 2, 3, and 4, where the prevalence and severity of the disease and number of serogroups present were reduced. CONCLUSION: Serogroup-specific bivalent vaccines can be effective at controlling footrot caused by intermediate strains of D. nodosus.

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.

The immunogenicity and tissue reactivity of Mycobacterium avium subsp paratuberculosis inactivated whole cell vaccine is dependent on the adjuvant used.

Johne's disease (JD) is a chronic enteritis caused by Mycobacterium avium subspecies paratuberculosis (MAP). Current commercial vaccines are effective in reducing the occurrence of clinical disease although vaccinated animals can still become infected and transmit MAP. Many vaccinated sheep develop severe injection site lesions. In this study a range of adjuvants (Montanide TM ISA 50V, ISA 50V2, ISA 61VG, ISA 70 M VG, ISA 71 VG, ISA 201 VG and Gel 01 PR) formulated with heat-killed MAP were tested to determine the incidence of injection site lesions and the types of immune profiles generated in sheep. All the novel formulations produced fewer injection site lesions than a commercial vaccine (Gudair®). The immune profiles of the sheep differed between treatment groups, with the strength of the antibody and cell mediated immune responses being dependant on the adjuvant used. One of the novel vaccines resulted in a reduced IFN-γ immune response when a second "booster" dose was administered. These findings have significance for JD vaccine development because it may be possible to uncouple protective immunity from excessive tissue reactivity, and apparently poorly immunogenic antigens may be re-examined to determine if an appropriate immune profile can be established using different adjuvants. It may also be possible to formulate vaccines that produce targeted immunological profiles suited to protection against other pathogens, i.e. those for which a bias towards cellular or humoral immunity would be advantageous based on understanding of pathogenesis.

Antibiotic resistant Escherichia coli in southeastern Australian pig herds and implications for surveillance.

To investigate public health implications of antibiotics to control post-weaning scours, we surveyed 22 commercial pig herds in southeastern Australia. Fifty faecal samples per herd were collected from pre- and post-weaned piglets. Presumptive Escherichia coli isolates were confirmed by MALDI-TOF MS. Isolates (n = 325) were screened for susceptibility to 19 veterinary antibiotics using MIC broth microdilution. All 325 E. coli isolates underwent further testing against 27 antibiotics used in human medicine and were screened for ETEC adhesin and enterotoxin genes (F4 (K88), F5 (K99), F6 (987P), F18, F41, STa, STb, Stx2e and LT) by multiplex PCR. Isolates identified as phenotypically resistant to third-generation cephalosporin (3GC) and aminoglycoside antibiotics were screened by multiplex PCR/reverse line blot to detect common ß-lactam and aminoglycosides resistance genes, confirmed by sequencing. Twenty (6.1%) of the E. coli isolates were resistant to 3GC antibiotics and 24 (7.4%) to the aminoglycoside antibiotic gentamicin. Genetic analysis revealed six different extended spectrum ß-lactamase (ESBL) genes (blaCTX-M-1, -14, -15, -27, blaSHV-12 and blaCMY-2-like genes), four of which have not been previously reported in Australian pigs. Critically, the prevalence of 3GC resistance was higher in non-pathogenic (non-ETEC) isolates and those from clinically normal (non-diarrhoeal) samples. This highlights the importance of non-ETECE. coli as reservoirs of antimicrobial resistance genes in piglet pens. Antimicrobial resistance surveillance in pig production focused on diagnostic specimens from clinically-affected animals might be potentially misleading. We recommend that surveillance for emerging antimicrobial resistance such as to 3GC antibiotics should include clinically healthy pigs.

Longitudinal study of Escherichia coli O157 shedding and super shedding in dairy heifers.

A longitudinal study was conducted to assess the methods available for detection of Escherichia coli O157 and to investigate the prevalence and occurrence of long-term shedding and super shedding in a cohort of Australian dairy heifers. Samples were obtained at approximately weekly intervals from heifers at pasture under normal management systems. Selective sampling techniques were used with the aim of identifying heifers with a higher probability of shedding or super shedding. Rectoanal mucosal swabs (RAMS) and fecal samples were obtained from each heifer. Direct culture of feces was used for detection and enumeration. Feces and RAMS were tested by enrichment culture. Selected samples were further tested retrospectively by immunomagnetic separation of enriched samples. Of 784 samples obtained, 154 (19.6%) were detected as positive using culture methods. Adjusting for selective sampling, the prevalence was 71 (15.6%) of 454. In total, 66 samples were detected as positive at >10(2) CFU/g of which 8 were >10(4) CFU/g and classed as super shedding. A significant difference was observed in detection by enriched culture of RAMS and feces. Dairy heifers within this cohort exhibited variable E. coli O157 shedding, consistent with previous estimates of shedding. Super shedding was detected at a low frequency and inconsistently from individual heifers. All detection methods identified some samples as positive that were not detected by any other method, indicating that the testing methods used will influence survey results.

Comparison of recto-anal mucosal swab and faecal culture for the detection of Escherichia coli O157 and identification of super-shedding in a mob of Merino sheep.

We compared the use of recto-anal mucosal swab (RAMS) culture and faecal culture for the detection of E. coli O157 in a mob of Merino sheep. Fifty Merino wethers and maiden ewes housed in indoor pens were sampled on five occasions. We detected E coli O157 in 32% (16/50) of sheep, with weekly prevalence ranging from 4% (2/50) to 16% (8/50). Overall, 12·5% (2/16) were detected by RAMS culture only, and 37·5% (6/16) were detected by faecal culture only. The level of agreement between the two sampling methods was moderate [kappa statistic = 0·583, 95% confidence interval (CI) 0·460-0·707]. The relative sensitivities of RAMS and faecal culture were 67% (95% CI 41-86) and 57% (95% CI 34-77), respectively. We identified four super-shedding sheep using direct faecal culture. Although the majority of culture-positive sheep were detected at one sampling point only, 3/4 super-shedding sheep were culture-positive at two sampling points, and 1/4 was culture-positive at four sampling points. Persistent culture positivity may indicate sheep that could be considered 'super-shedders' at some point. The use of immunomagnetic separation further improved the rate of detection of E. coli O157, which was isolated from 1/34 animals that were previously negative by enrichment culture alone. A significant difference between sampling weeks was detected for both faecal (P = 0·021) and RAMS (P = 0·006), with the prevalence at the mid-point of sampling (week 4) significantly (P < 0·05) higher than at the beginning or end of the study. Study conditions (penned sheep) might have been responsible for the high prevalence and the epidemic pattern of infection observed, and could serve as a future model for studies of E. coli O157 transmission, shedding and super-shedding in sheep.

Daily variations in Escherichia coli O157 shedding patterns in a cohort of dairy heifers at pasture.

Escherichia coli O157 is a human pathogen carried asymptomatically by cattle and shed in their faeces. Infection can occur from the consumption of contaminated beef or by direct contact. Large variations of E. coli O157 shedding in cattle exist and vary in the number of cattle positive for E. coli O157 and the amount of bacteria (c.f.u./g faeces) shed by positive animals. To investigate E. coli O157 shedding and super-shedding (>104 c.f.u./g) we used daily sampling over two 8-day periods; in January 2013 (n = 12) and February 2013 (n = 21). Samples were tested by direct faecal culture for enumeration and by immunomagnetic separation to detect lower levels of shedding. We identified three patterns of shedding, similar to previously observed descriptions: intermittent, transient and consistent. The most commonly observed pattern was intermittent shedding and variation in the level of shedding could be large. This extreme variation is demonstrated by a heifer from which E. coli O157 could be not detected one day, was super-shedding E. coli O157 the next and was detected as shedding >100 c.f.u./g the following day. Recto-anal mucosal swab testing did not predict super-shedding in this cohort of heifers. The variable individual patterns of shedding suggest that a common mechanism of infection may not operate within such a herd when considering previously described patterns and the inferred mechanisms. The sporadic and intermittent nature of shedding is a challenge to identifying risk factors and potential intervention strategies.

Risk factors for Escherichia coli O157 shedding and super-shedding by dairy heifers at pasture.

We undertook a longitudinal study within a cohort of 52 dairy heifers maintained under constant management systems and sampled weekly to investigate a comprehensive range of risk factors which may influence shedding or super-shedding of E. coli O157 (detected by direct faecal culture and immunomagnetic separation). E. coli O157 was detected from 416/933 (44.6%) samples (faeces and recto-anal mucosal swabs) and 32 (3.4%) samples enumerated at >10000 c.f.u./g. Weekly point prevalence ranged from 9.4% to 94.3%. Higher temperature (P < 0.001), rainfall (P = 0.02), relative humidity (P < 0.001), pasture growth (P = 0.013) and body score (P = 0.029) were positively associated with increased shedding. Higher rainfall (P < 0.001), hide contamination (P = 0.002) and increased faecal consistency (P = 0.023) were positively associated with super-shedding. Increased solar exposure had a negative effect on both shedding and super-shedding within bivariate analyses but in the final multivariate model for shedding demonstrated a positive effect (P = 0.017). Results suggest that environmental factors are important in E. coli O157 shedding in cattle.

A longitudinal study of the prevalence and super-shedding of Escherichia coli O157 in dairy heifers.

The fecal shedding and super-shedding of the human pathogen Escherichia coli O157 by cattle has been the focus of many previous studies with varied results observed. The heterogeneity of shedding is becoming more accepted, both in the numbers of animals shedding and the levels at which animals shed. To clarify patterns in shedding and super-shedding we undertook a longitudinal study to investigate shedding within a cohort of replacement dairy heifers. The cohort of 52 heifers was sampled 18 times at approximately weekly intervals with no significant changes in management during the sampling period. An overall prevalence of 44.3% (412/930 samples) was detected with prevalence ranging from 9.6 to 94.3% at individual sampling points. Each of the 52 heifers yielded at least one sample which was detected positive for E. coli O157. Super-shedding was detected at a sample level of 3.6% (32/893) and ranged between 0 and 9.6% at each sampling point. Of the 52 heifers, 24 (46.2%) were detected to be super-shedding at some point during the study, 19 of which were detected as super-shedding at only one point. From our findings we conclude that super-shedding is not associated with a small subset of animals that shed at high levels continually as had been proposed by earlier studies. We propose that the term 'super-shedding event' as opposed to 'super-shedding animal' better describes the nature of shedding.

Relative sensitivity of Escherichia coli O157 detection from bovine feces and rectoanal mucosal swabs.

The need to quantify the potential human health risk posed by the bovine reservoir of Escherichia coli O157 has led to a wealth of prevalence studies and improvements in detection methods over the last two decades. Rectoanal mucosal swabs have been used for the detection of E. coli O157 fecal shedding, colonized animals, and those predisposed to super shedding. We conducted a longitudinal study to compare the detection of E. coli O157 from feces and rectoanal mucosal swabs (RAMS) from a cohort of dairy heifers. We collected 820 samples that were tested by immunomagnetic separation of both feces and RAMS. Of these, 132 were detected as positive for E. coli O157 from both samples, 66 were detected as positive from RAMS only, and 117 were detected as positive from feces only. The difference in results between the two sample types was statistically significant (P < 0.001). The relative sensitivities of detection by immunomagnetic separation were 53% (confidence interval, 46.6 to 59.3) from RAMS and 67% (confidence interval, 59.6 to 73.1) from fecal samples. No association between long-term shedding (P = 0.685) or super shedding (P = 0.526) and detection by RAMS only was observed.

Pilot trials in Australia on eradication of footrot by flock specific vaccination.

Footrot is a contagious disease of ruminants requiring strains of Dichelobacter nodosus that possess virulence factors including proteases and fimbriae. Sheep can be immunised against footrot using vaccine-containing fimbriae, either native or recombinant. The fimbriae are responsible for the serological K-agglutination reaction, which has been used to classify field isolates into nine major serogroups. The range of protection conferred by vaccination is largely restricted to the serogroup involved, but antigenic competition precludes effective vaccination with multivalent vaccines that contain all serogroups. However, vaccination with specific bivalent recombinant fimbrial vaccine led to eradication of virulent footrot from small ruminants in Nepal and the same result was obtained in Bhutan using a specific whole cell vaccine. In the study reported here two pilot trials have been conducted in Australian sheep flocks, one with a virulent form of footrot caused by a single serogroup F, and the other with an intermediate form also caused by a single serogroup C. In trial 1 pre-vaccination prevalence of clinical footrot in a group of randomly selected animals was 44%. This reduced to 2% at 3 months and 0.5% at 4 months, and there were no clinical cases at 5 months or at 16 months post-vaccination in the whole flock. Similarly in trial 2 pre-vaccination whole flock prevalence was 8.5%, while it was 2% at 3 months, 0.3% at 6 months and zero at 18 months post-vaccination. Use of flock specific monovalent whole cell vaccines over whole flocks for only one season and culling of the few non-responders has been a successful approach in eradication of the disease from both these flocks. This is the first study to report the successful use of specific vaccine for the intermediate form of footrot.

Distribution and prevalence of footrot in Bhutan.

The first cases of footrot in Bhutan were reported in sheep in 1990 at the National Sheep Breeding Centre (NSBC), which supplies breeding animals to village sheep flocks throughout Bhutan. Despite the presence of footrot at the Centre the distribution of apparently disease-free sheep continued. Cases of footrot were reported in village flocks soon after the disease was diagnosed at NSBC. A national survey was designed to establish the distribution and prevalence of footrot in Bhutan. This detected footrot in 19/94 village sheep flocks surveyed. The 19 affected flocks were distributed among nine different administrative districts whereas the villages selected were in 13 of a total of 16 sheep growing districts. The highest within-flock prevalences were among the seven flocks sampled in Bumthang district (mean 20.4%). The prevalence of the disease within flocks was generally much lower in other affected districts and in three districts a single affected animal was identified in the sample of 14 sheep examined in each village. Nationally, footrot prevalence was estimated to be 3.1% (95% CI 2.16-4.04%). There was a positive association between the receipt of animals from NSBC and the presence of footrot. The prevalence of the disease was higher in flocks with a migratory system of management than in those using a sedentary system. The relative risk of there being footrot in a migratory flock was nine-times higher than in a non-migratory flock. Only one strain of Dichelobacter nodosus (serogroup B) was identified among the 234 isolates obtained from the 19 affected flocks. Sheep with footrot healed quickly when treated with a vaccine made from this strain.

The use of an autogenous Dichelobacter nodosus vaccine to eliminate clinical signs of virulent footrot in a sheep flock in Bhutan.

An outbreak of virulent footrot was investigated in a flock of 605 Merino cross-bred sheep in Bhutan. Conventional control methods in the preceding eight years had reduced its prevalence from 36-79% in different components of the flock to about 15% overall. Only one serogroup (B) of Dichelobacter nodosus was identified among 40 isolates cultured from affected sheep. A vaccine prepared from this strain was used in a pilot trial to compare the response of 14 treated and 14 untreated sheep. All affected, vaccinated animals in this trial healed quickly and were protected against re-infection while additional cases developed among untreated sheep during a period favourable for the spread of footrot. The serogroup B vaccine was administered to the whole flock for two successive years. No other footrot treatment was given during these or subsequent years. The whole flock was examined three times, foot by foot, for two years and twice yearly for another two years. When vaccination began there were 88 affected sheep in the flock, an affected sheep being defined as an animal with a foot-score of 2 or greater in one or more feet. There were neither affected sheep in the flock 30 days after the first dose of vaccine nor were any identified in later inspections. Virulent footrot, originating from the farm under investigation, persisted in neighbouring village flocks during this period. It was concluded that whole flock specific D. nodosus vaccination made a major contribution to the elimination of all clinical signs of footrot from the flock of 605 sheep where the condition had previously persisted for 10 years.

Eradication of virulent footrot from sheep and goats in an endemic area of Nepal and an evaluation of specific vaccination.

Programmes based on the identification and treatment of cases and the culling of animals refractory to treatment had failed to eradicate virulent footrot from two districts in the western region of Nepal. From 1993 to 1996 vaccination against two endemic virulent strains of Dichelobacter nodosus was tested for its potential to contribute to the eradication of footrot from the region. Only sheep and goats which had been free of signs of footrot at three inspections at monthly intervals before their annual migration to alpine pastures were eligible for inclusion. From November 1992, the treatment of cases identified during inspections included the injection of specific vaccine. Successfully treated cases migrated with their flocks but were excluded from the vaccine trial. Non-responding cases were culled. Forty combined flocks of sheep and goats (approximately 9500 animals) were used initially to compare three vaccination regimens. Eleven flocks (sheep and goats) were treated with two doses of specific vaccine (group A), nine (sheep and goats) were treated with commercial vaccine followed by specific vaccine (group B) and 10 (sheep and goats) were treated with two doses of commercial vaccine (group C) in March to April 1993 before the annual migration; 10 flocks (sheep and goats) remained unvaccinated (group D). Only sheep and goats free of signs of footrot were allowed to migrate. Nevertheless, virulent footrot recurred in many flocks three months later. However, its prevalence was significantly lower in group A than in the other three groups combined. Groups A, B and C then received the specific vaccine before their migrations in 1994 to 1996; group D remained unvaccinated. The annual programme of inspection and identification and treatment of cases continued for seven years, but the vaccinations ceased after four years. There was no recurrence of virulent footrot after November 1993. After the first season the virulent strains of D nodosus used in the specific vaccine could no longer be isolated, although antigenically distinct, benign strains of the organism persisted in cases of benign footrot.

Serogroup specific single and multiplex PCR with pre-enrichment culture and immuno-magnetic bead capture for identifying strains of D. nodosus in sheep with footrot prior to vaccination.

The identification of Dichelobacter nodosus present in a flock is a prerequisite to specific (autogenous) vaccination. Current methods of identification of the serogroup present in a population requires that the organisms be isolated, identified visually in mixed culture on streak plates, subcultured to purify and subjected to antigenic analysis. This process takes at least 3 to 4 weeks. This study describes the development of a simple and rapid serogroup specific PCR test for D. nodosus. A common forward primer was designed from the conserved amino-terminal region of the fimbrial gene (fimA) and 9 (A-I) serogroup specific reverse primers were designed from the carboxy-terminal regions of fimA of the different serogroups. To verify the specificity within D. nodosus, each specific primer pair was tested in PCR against 18 serogroups/serotypes (prototypes) and found to be specific for all the serotypes within the homologous serogroups. Eighty four other bacterial strains, either commonly occurring in sheep or found in the environment of sheep, and including organisms related taxonomically to D. nodosus, were used to check the specificity of these assays. They were found to be specific for D. nodosus as none of the 84 bacterial stains reacted. These primers detected 1 pg of purified chromosomal DNA, or 50-100 cells of D. nodosus in crude lysates. Sensitivity was markedly improved when an immuno-magnetic capture was employed. Single tube multiplex PCRs were tested with different combinations of common forward primer and groups of 3, 4 or 5 reverse primers chosen so that amplicon size for each reaction product was different. These were able to amplify DNA of isolates from all the relevant serogroups included in the reactions. These tests were evaluated with samples taken directly from lesions of footrot, either directly or preceded by DNA purification, immuno-magnetic capture, enrichment broth culture and culture on hoof agar media. Of these methods only PCR on mixed colonies from 4-day-old cultures on 4% hoof agar media yielded results of practical value.

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.

Transmission of virulent footrot between sheep and goats.

OBJECTIVE: To determine the infectivity of ovine and caprine strains of Dichelobacter nodosus for both sheep and goats. DESIGN: Pen experiments in which 20 sheep and 19 goats were challenged directly with the two strains, and transmission experiments on pasture, using donors infected by experimental challenge. RESULTS: Sheep and goat strains of D nodosus infected both animal species in experimental challenges. Animals so infected transmitted footrot to both sheep and goats on pasture plots. A significantly smaller proportion of goats than sheep was infected when challenged with either strain. The interval between exposure and development of footrot in goats was longer than in sheep when recipient animals were exposed to infected donors on pasture. The disease was less invasive in goats than in sheep. CONCLUSIONS: With the strains of D nodosus used there was no evidence of host specificity. Direct transmission of footrot can occur between sheep and goats in the same environment. There is a need to include goats in ovine footrot eradication programs and vice versa.