The effects of antigenic competition on the efficacy of multivalent footrot vaccines

Risk factors associated with the infection of sheep with Dichelobacter nodosus

Ovine footrot is a highly contagious foot disease caused by the gram-negative bacterium Dichelobacter nodosus (D. nodosus). In a recent report, we showed a prevalence of 42.9% D. nodosus positive swabs across Germany. In this follow-up study, we used real-time PCR results for D. nodosus and footrot scores of 9297 sheep from 208 flocks and collated these data with survey data on herd and animal characteristics and herd management. The aims of the present study were to investigate herd and animal factors associated with D. nodosus infection and footrot scores in individual sheep. Multivariable analyses with generalized mixed models showed that month of recording, breed, herdbook membership, use of antibiotics, and footbaths in the past 3–10 years, signs of footrot in the past 12 months and flock environment of the sheep, modelled as a random farm effect within region, were significant risk factors. Among the 21 different breeds, Romney had the lowest risk of D. nodosus infection, while Swifter had the highest risk and German Merino and German White Heath were the next breeds at highest risk of D. nodosus infection. The variance between farms in the prevalence of D. nodosus was large and accounted for 84% of the total variance in the mixed model analysis. We conclude that specific and as yet unknown effects influencing D. nodosus infections in flocks, as well as breed and weather, are the most important effects on D. nodosus infection in sheep, pointing towards the need to establish adequate infection control at farm level.

Serological Diversity of Dichelobacter nodosus in German Sheep Flocks

Simple Summary

Footrot is an infectious hoof disease in sheep, caused by the bacterium Dichelobacter nodosus. The antigentic variation of the fimbrial proteins resulted in the description of up to ten serogroups (A–I and M). Vaccines against footrot target these fimbrial variants. Commercial vaccines are covering nine serogroups but have low efficacy compared to vaccines based on two serogroups. Therefore, our study investigated the prevalence and distribution of the nine serogroups A–I in German sheep flocks with the aim to detect the predominant serogroups guiding optimized vaccines based on two serogroups. Serogroup A was most common in our study, followed by serogroups B, H and C. More than one-third of the animals showed more than one serogroup. In flocks, we found, on average, 3.10 serogroups in a range of one to six. The nine serogroups were widely distributed across the flocks, with 50 different combinations across the 83 flocks investigated. The lack of two predominant serogroups in Germany impairs the nationwide protection against footrot by the usage of more efficient vaccines based on two serogroups and requires tailor-made flock-specific vaccines.

Abstract

Footrot is one of the major causes of lameness in sheep and leads to decreased animal welfare and high economic losses. The causative agent is the Gram-negative anaerobic bacterium Dichelobacter nodosus. The prevalence of D. nodosus in 207 sheep flocks across Germany was 42.9%. Based on the sequence variation in the type IV fimbrial gene fimA, D. nodosus can be subdivided into ten serogroups (A–I and M). There are commercially available vaccines covering nine serogroups, but the efficacy is low compared to bivalent vaccines. The aim of this study was to investigate the diversity of serogroups in Germany at the flock and animal levels. In total, we detected at least one serogroup in 819 samples out of 969 D. nodosus-positive samples from 83 flocks using serogroup-specific singleplex PCR for the serogroups A–I. Serogroup A was most prevalent at the animal level, followed by serogroups B, H and C. At the flock level, serogroups A and B had the highest prevalence, each with 64%, but only 40% of flocks had both. The average number of serogroups per animal was 1.42 (range one to five) and, per flock, 3.10 (range one to six). The serogrouping showed within-flock specific clusters but were widely distributed, with 50 different combinations across the flocks. The factors associated with the number of serogroups per animal and single serogroups were the load of D. nodosus, footrot score, sheep breed and flock. Our results indicate that efficient vaccination programs would benefit from tailor-made flock-specific vaccines and regular monitoring of circulating serotypes in the flock to be able to adjust vaccine formulations for nationwide progressive control of footrot in Germany.

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.

Protective efficacy of the peptide Subolesin antigen against the cattle tick Rhipicephalus microplus under natural infestation

The cattle tick Rhipicephalus microplus affect animal health, welfare, and cattle production in tropical and subtropical zones of the world. Anti-tick vaccines have been an effective alternative for cattle tick control instead of traditional chemical products. To date, Subolesin antigen has shown efficacy for the control of tick infestation in cattle, and previous studies showed that one peptide derived from this protein has demonstrated to elicit a strong and specific humoral immune response. Based on these findings, herein we characterized the efficacy of the peptide Subolesin for the control of cattle tick, R. microplus infestation under field conditions. Twenty-four female calves were assigned to four experimental groups and immunized with three subcutaneous doses of the peptide Subolesin, Bm86, both antigens (dual vaccine) and adjuvant/saline alone, respectively. Serum antibody levels (IgG) were assessed by ELISA and confirmed by Western blot; also, reproductive performance of naturally infested R. microplus was determined. The results showed that immunizations with the experimental antigens reduced tick infestations with vaccine's efficacy of 67 % (peptide Subolesin), 56 % (Bm86), and 49 % (dual vaccine) based on adult tick numbers, oviposition, and egg fertility between vaccinated and control animals. Peptide Subolesin-immunized calves developed a strong humoral immune response expressed by high anti-pSubolesin IgG levels, and the Western blot analysis confirmed that it is immunogenic. Cattle receiving Bm86 and dual vaccine showed less protection, although Bm86 was within the range reported previously. The negative correlation between antibody levels and reduction of naturally infested R. microplus strongly suggested that the effect of the vaccine was the result of the antibody response in immunized cattle. In conclusion, it was demonstrated that the peptide Subolesin induced a specific immune response in cattle under field conditions, resulting in reduced R. microplus populations in subsequent generations. Finally, integrated tick control must consider anti-tick vaccines as a cost-effective, sustainable, and successful tool for controlling cattle tick infestations.

Impact of Strain Variation of Dichelobacter nodosus on Disease Severity and Presence in Sheep Flocks in England

AprV2 and aprB2 are variants of the apr gene of Dichelobacter nodosus, the cause of footrot in sheep. They are putative markers for severe and mild disease expression. The aim of our study was to investigate the distribution of aprV2 and aprB2 in flocks with and without footrot. Our hypotheses were that both strains are present in endemically affected flocks, with aprB2 and aprV2 associated with mild and virulent phenotypes respectively but that D. nodosus is not present in flocks without footrot. Alternatively, aprB2 persists in flocks without footrot. Despite extensive searching over 3 years only three flocks of sheep without footrot were identified. D. nodosus was not detected in these three flocks. In one further flock, only mild interdigital dermatitis was observed, and only aprB2 was detected. Twenty-four flocks with endemic footrot of all severities were sampled on three occasions and all were positive for D. nodosus and the aprV2 variant; aprB2 was detected in only 11 of these flocks. AprB2 was detected as a co-infection with aprV2 in the 22% of samples positive for aprB2 and was more likely in mild footrot phenotypes than severe. Dichelobacter nodosus serogroups were not associated with footrot phenotype. We conclude that D. nodosus, even aprB2 strains, do not persist in flocks in the absence of footrot. Our results support the hypothesis that aprB2 is associated with mild footrot phenotypes. Finally, we conclude that given the small number of flocks without footrot that were identified, footrot is highly endemic in English sheep flocks.

Measurement over 1 Year of Neutralizing Antibodies in Cattle Immunized with Trivalent Vaccines Recombinant Alpha, Beta and Epsilon of Clostridium perfringens

The alpha (CPA), beta (CPB) and epsilon (ETX) toxins of Clostridium perfringens are responsible for causing diseases that are difficult to eradicate and have lethal potential in production animals. Vaccination of herds is still the best control strategy. Recombinant clostridial vaccines have shown good success at inducing neutralizing antibody titers and appear to be a viable alternative to the conventional production of commercial clostridial toxoids. Research is still needed on the longevity of the humoral immune response induced by recombinant proteins in immunized animals, preferably in target species. The objective of this study was to measure the humoral immune response of cattle immunized with trivalent vaccines containing the recombinant proteins alpha (rCPA), beta (rCPB) and epsilon (rETX) of C. perfringens produced in Escherichia coli at three different concentrations (100, 200, and 400 µg) of each protein for 12 months. The recombinant vaccines containing 200 (RV2) and 400 µg (RV3) yielded statistically similar results at 56 days. They performed better throughout the study period because they induced higher neutralizing antibody titers and were detectable for up to 150 and 180 days, respectively. Regarding industrial-scale production, RV2 would be the most economical and viable formulation as it achieved results similar to RV3 at half the concentration of recombinant proteins in its formulation. However, none of the vaccines tested induced the production of detectable antibody titers on day 365 of the experiment, the time of revaccination typically recommended in vaccination protocols. Thus, reiterating the need for research in the field of vaccinology to achieve greater longevity of the humoral immune response against these clostridial toxins in animals, in addition to the need to discuss the vaccine schedules and protocols adopted in cattle production.

The Evidence Base for Prevention of Infectious Bovine Keratoconjunctivitis Through Vaccination

Pili and cytotoxins are important virulence factors and antigens for Moraxella spp. Local and systemic immunity may play a role in the body's response to infectious bovine keratoconjunctivitis (IBK). No evidence exists that eliminating the carrier state for IBK is possible or beneficial. Evidence for efficacious transfer of passive immunity from dams to calves is conflicting. Autogenous vaccines and commercial vaccines for putative pathogens for IBK have not yet shown efficacy in blinded randomized field trials. Study design features, such as randomization, blinding, diagnostic criteria, and use of a placebo, reduce the risk of bias in vaccine studies for IBK.

Ovine footrot: A review of current knowledge

Footrot is a contagious foot disease mainly affecting sheep. It is caused by the Gram-negative anaerobic bacterium Dichelobacter nodosus. Warm, wet environmental conditions favour development of footrot, and under perfect conditions, it takes just 2-3 weeks from infection to manifestation of clinical signs. Affected sheep show lameness of various degrees and often graze while resting on their carpi. Local clinical signs vary in severity and extent from interdigital inflammation (benign footrot) to underrunning of the complete horn shoe in advanced stages of virulent footrot. Laboratory diagnosis ideally involves collection of four-foot interdigital swab samples followed by competitive real time PCR, allowing for detection of the presence of D. nodosus and differentiation between benign and virulent strains. Laboratory-based diagnostics at the flock level based on risk-based sampling and pooling of interdigital swab samples are recommended. The list of treatment options of individual sheep includes careful removal of the loose undermined horn, local or systemic administration of antimicrobials, systemic administration of non-steroidal anti-inflammatories (NSAIDs) and disinfectant footbathing. Strategies for control at the flock level are manifold and depend on the environmental conditions and the procedures traditionally implemented by the respective country. Generally, measures consist of treatment/culling of infected sheep, vaccination and prevention of reinfection of disease-free flocks. Gaining deeper insight into the beneficial effects of NSAIDs, screening for eco-friendly footbath solutions, developing better vaccines, including the development of a robust, reproducible infection model and elucidation of protective immune responses, as well as the elaboration of effective awareness training programs for sheep farmers, are relevant research gaps.

Anti-SU Antibody Responses in Client-Owned Cats Following Vaccination against Feline Leukaemia Virus with Two Inactivated Whole-Virus Vaccines (Fel-O-Vax® Lv-K and Fel-O-Vax® 5)

A field study undertaken in Australia compared the antibody responses induced in client-owned cats that had been vaccinated using two inactivated whole feline leukaemia virus (FeLV) vaccines, the monovalent vaccine Fel-O-Vax^® Lv-K and the polyvalent vaccine Fel-O-Vax^® 5. Serum samples from 428 FeLV-uninfected cats (118 FeLV-vaccinated and 310 FeLV-unvaccinated) were tested for anti-FeLV neutralising antibodies (NAb) using a live virus neutralisation assay to identify 378 FeLV-unexposed (NAb-negative) and 50 FeLV-exposed (NAb-positive; abortive infections) cats, following by anti-surface unit (SU) FeLV-A and FeLV-B antibody ELISA testing. An additional 42 FeLV-infected cats (28 presumptively regressively infected, 14 presumptively progressively infected) were also tested for anti-SU antibodies. NAb-positive cats displayed significantly higher anti-SU antibody ELISA responses compared to NAb-negative cats (p < 0.001). FeLV-unexposed cats (NAb-negative) that had been vaccinated less than 18 months after a previous FeLV vaccination using the monovalent vaccine (Fel-O-Vax^® Lv-K) displayed higher anti-SU antibody ELISA responses than a comparable group vaccinated with the polyvalent vaccine (Fel-O-Vax^® 5) (p < 0.001 for both anti-FeLV-A and FeLV-B SU antibody responses). This difference in anti-SU antibody responses between cats vaccinated with the monovalent or polyvalent vaccine, however, was not observed in cats that had been naturally exposed to FeLV (NAb-positive) (p = 0.33). It was postulated that vaccination with Fel-O-Vax^® 5 primed the humoral response prior to FeLV exposure, such that antibody production increased when the animal was challenged, while vaccination with Fel-O-Vax^® Lv-K induced an immediate preparatory antibody response that did not quantitatively increase after FeLV exposure. These results raise questions about the comparable vaccine efficacy of the different FeLV vaccine formulations and correlates of protection.

Serogroups of Dichelobacter nodosus, the cause of footrot in sheep, are randomly distributed across England

We present the largest and most representative study of the serological diversity of Dichelobacter nodosus in England. D. nodosus causes footrot and is one of the top five globally important diseases of sheep. The commercial vaccine, containing nine serogroups, has low efficacy compared with bivalent vaccines. Our aim was to investigate the prevalence and distribution of serogroups of D. nodosus in England to elucidate whether a bivalent vaccine could protect the national flock. Farmers from 164 flocks submitted eight interdigital swabs from eight, preferably diseased, sheep. All serogroups, A-I, were detected by PCR in 687/1150 D. nodosus positive swabs, with a prevalence of 2.6-69.3% of positive swabs per serogroup. There was a median of two serogroups per flock (range 0-6). Serogroups were randomly distributed between, but clustered within, flocks, with 50 combinations of serogroups across flocks. H and B were the most prevalent serogroups, present in > 60% of flocks separately but in only 27% flocks together. Consequently, a bivalent vaccine targeting these two serogroups would protect 27% of flocks fully (if only H and B present) and partially, if more serogroups were present in the flock. We conclude that one bivalent vaccine would not protect the national flock against footrot and, with 50 combinations of serogroups in flocks, flock-specific vaccines are necessary.

Characterization of two putative Dichelobacter nodosus footrot vaccine antigens identifies the first lysozyme inhibitor in the genus

The Gram-negative anaerobic bacterium Dichelobacter nodosus (Dn) causes footrot in ruminants, a debilitating and highly contagious disease that results in necrotic hooves and significant economic losses in agriculture. Vaccination with crude whole-cell vaccine mixed with multiple recombinant fimbrial proteins can provide protection during species-specific outbreaks, but subunit vaccines containing broadly cross-protective antigens are desirable. We have investigated two D. nodosus candidate vaccine antigens. Macrophage Infectivity Potentiator Dn-MIP (DNO_0012, DNO_RS00050) and Adhesin Complex Protein Dn-ACP (DNO_0725, DNO_RS06795) are highly conserved amongst ~170 D. nodosus isolates in the https://pubmlst.org/dnodosus/ database. We describe the presence of two homologous ACP domains in Dn-ACP with potent C-type lysozyme inhibitor function, and homology of Dn-MIP to other putative cell-surface and membrane-anchored MIP virulence factors. Immunization of mice with recombinant proteins with a variety of adjuvants induced antibodies that recognised both proteins in D. nodosus. Notably, immunization with fimbrial-whole-cell Footvax vaccine induced anti-Dn-ACP and anti-Dn-MIP antibodies. Although all adjuvants induced high titre antibody responses, only antisera to rDn-ACP-QuilA and rDn-ACP-Al(OH)₃ significantly prevented rDn-ACP protein from inhibiting lysozyme activity in vitro. Therefore, a vaccine incorporating rDn-ACP in particular could contribute to protection by enabling normal innate immune lysozyme function to aid bacterial clearance.

Constituting a glutathione S-transferase-cocktail vaccine against tick infestation

Cocktail vaccines are proposed as an attractive way to increase protection efficacy against specific tick species. Furthermore, such vaccines made with different tick antigens have the potential of cross-protecting against a broad range of tick species. However, there are still limitations to the selection of immunogen candidates. Acknowledging that glutathione S-transferases (GSTs) have been exploited as vaccines against ticks and other parasites, this study aimed to analyze a GST-cocktail vaccine as a potential broad-spectrum tick vaccine. To constitute the GST-cocktail vaccine, five tick species of economic importance for livestock industry were studied (Rhipicephalus appendiculatus, Rhipicephalus decoloratus, Rhipicephalus microplus, Amblyomma variegatum, and Haemaphysalis longicornis). Tick GST ORF sequences were cloned, and the recombinant GSTs were produced in Escherichia coli. rGSTs were purified and inoculated into rabbits, and the immunological response was characterized. The humoral response against rGST-Rd and rGST-Av showed a stronger cross-reactivity against heterologous rGSTs compared to rGST-Hl, rGST-Ra, and rGST-Rm. Therefore, rGST-Rd and rGST-Av were selected for constituting an experimental rGST-cocktail vaccine. Vaccination experiment in rabbits showed that rGST-cocktail caused 35% reduction in female numbers in a Rhipicephalus sanguineus infestation. This study brings forward an approach to selecting immunogens for cocktail vaccines, and the results highlight rGST-Rd and rGST-Av as potentially useful tools for the development of a broad-spectrum tick vaccine.

Detection and Serogrouping of Dichelobacter nodosus Infection by Use of Direct PCR from Lesion Swabs To Support Outbreak-Specific Vaccination for Virulent Footrot in Sheep

Virulent footrot is an economically significant disease in most sheep-rearing countries. The disease can be controlled with vaccine targeting the fimbriae of virulent strains of the essential causative agent, Dichelobacter nodosus However, the bacterium is immunologically heterogeneous, and 10 distinct fimbrial serogroups have been identified. Ideally, in each outbreak the infecting strains would be cultured and serogrouped so that the appropriate serogroup-specific mono- or bivalent vaccine could be administered, because multivalent vaccines lack efficacy due to antigenic competition. If clinical disease expression is suspected to be incomplete, culture-based virulence tests are required to confirm the diagnosis, because control of benign footrot is economically unjustifiable. Both diagnosis and vaccination are conducted at the flock level. The aims of this study were to develop a PCR-based procedure for detecting and serogrouping D. nodosus directly from foot swabs and to determine whether this could be done accurately from the same cultured swab. A total of 269 swabs from the active margins of foot lesions of 261 sheep in 12 Merino sheep flocks in southeastern Australia were evaluated. DNA extracts taken from putative pure cultures of D. nodosus and directly from the swabs were evaluated in PCR assays for the 16S rRNA and fimA genes of D. nodosus Pure cultures were tested also by the slide agglutination test. Direct PCR using extracts from swabs was more sensitive than culture for detecting and serogrouping D. nodosus strains. Using the most sensitive sample collection method of the use of swabs in lysis buffer, D. nodosus was more likely to be detected by PCR in active than in inactive lesions, and in lesions with low levels of fecal contamination, but lesion score was not a significant factor. PCR conducted on extracts from swabs in modified Stuart's transport medium that had already been used to inoculate culture plates had lower sensitivity. Therefore, if culture is required to enable virulence tests to be conducted, it is recommended that duplicate swabs be collected from each foot lesion, one in transport medium for culture and the other in lysis buffer for PCR.

A distinct bacterial dysbiosis associated skin inflammation in ovine footrot

Ovine footrot is a highly prevalent bacterial disease caused by Dichelobacter nodosus and characterised by the separation of the hoof horn from the underlying skin. The role of innate immune molecules and other bacterial communities in the development of footrot lesions remains unclear. This study shows a significant association between the high expression of IL1β and high D. nodosus load in footrot samples. Investigation of the microbial population identified distinct bacterial populations in the different disease stages and also depending on the level of inflammation. Treponema (34%), Mycoplasma (29%) and Porphyromonas (15%) were the most abundant genera associated with high levels of inflammation in footrot. In contrast, Acinetobacter (25%), Corynebacteria (17%) and Flavobacterium (17%) were the most abundant genera associated with high levels of inflammation in healthy feet. This demonstrates for the first time there is a distinct microbial community associated with footrot and high cytokine expression.

What can we learn from the microbial ecological interactions associated with polymicrobial diseases?

Periodontal diseases in humans and animals are model polymicrobial diseases which are associated with a shift in the microbial community structure and function; there is therefore a need to investigate these diseases from a microbial ecological perspective. This review highlights three important areas of microbial ecological investigation of polymicrobial diseases and the lessons that could be learnt: (1) identification of disease-associated microbes and the implications for choice of anti-infective treatment; (2) the implications associated with vaccine design and development and (3) application of the dynamics of microbial interaction in the discovery of novel anti-infective agents. This review emphasises the need to invigorate microbial ecological approaches to the study of periodontal diseases and other polymicrobial diseases for greater understanding of the ecological interactions between and within the biotic and abiotic factors of the environment.

Selenium supplementation restores innate and humoral immune responses in footrot-affected sheep

Dietary selenium (Se) alters whole-blood Se concentrations in sheep, dependent upon Se source and dosage administered, but little is known about effects on immune function. We used footrot (FR) as a disease model to test the effects of supranutritional Se supplementation on immune function. To determine the effect of Se-source (organic Se-yeast, inorganic Na-selenite or Na-selenate) and Se-dosage (1, 3, 5 times FDA-permitted level) on FR severity, 120 ewes with and 120 ewes without FR were drenched weekly for 62 weeks with different Se sources and dosages (30 ewes/treatment group). Innate immunity was evaluated after 62 weeks of supplementation by measuring neutrophil bacterial killing ability. Adaptive immune function was evaluated by immunizing sheep with keyhole limpet hemocyanin (KLH). The antibody titer and delayed-type hypersensitivity skin test to KLH were used to assess humoral immunity and cell-mediated immunity, respectively. At baseline, FR-affected ewes had lower whole-blood and serum-Se concentrations; this difference was not observed after Se supplementation. Se supplementation increased neutrophil bacterial killing percentages in FR-affected sheep to percentages observed in supplemented and non-supplemented healthy sheep. Similarly, Se supplementation increased KLH antibody titers in FR-affected sheep to titers observed in healthy sheep. FR-affected sheep demonstrated suppressed cell-mediated immunity at 24 hours after intradermal KLH challenge, although there was no improvement with Se supplementation. We did not consistently prevent nor improve recovery from FR over the 62 week Se-treatment period. In conclusion, Se supplementation does not prevent FR, but does restore innate and humoral immune functions negatively affected by FR.

Outbreak-specific monovalent/bivalent vaccination to control and eradicate virulent ovine footrot

Footrot is a contagious disease of small ruminants which is caused by the bacterium Dichelobacter nodosus. In its virulent form there are severe economic losses and a very significant animal welfare issue. Sheep and goats can be vaccinated for treatment and prevention of the disease. There are 10 different serogroups of D. nodosus (A-I and M) and immunity is serogroup-specific. When all 10 serogroups are presented together in a vaccine, protection persists for only a few months due to "antigenic competition". Consequently we evaluated the use of sequential monovalent or bivalent vaccines to control/eliminate/eradicate virulent footrot in a longitudinal intervention study on 12 commercial farms in southeast Australia with flock sizes of approximately 1200-4200 sheep. Overall, virulent footrot was eradicated from 4 of the flocks, 2 of which had 2 serogroups, and the others 4 or 5 serogroups. Where there were only 1 or 2 serogroups (3 farms) the clinical response was rapid and dramatic; prevalence was reduced from 45 to 50% before vaccination to 0% (2 farms) or 0.4% (1 farm) after one round of vaccination. In the remaining 9 flocks there were more than 2 serogroups and successive bivalent vaccines were administered leading to eradication of virulent footrot on 2 farms over 4 years and control of the disease on all but 3 of the others. Of the latter farms, 1 discontinued, and 2 initially had poor response to vaccine due to misdiagnosis of serogroup 'M', which was previously unknown in Australia. Control was achieved after administration of a serogroup M vaccine. These results provide clear evidence for control, elimination and eradication of virulent footrot by outbreak-specific vaccination in Australia.

Ultrahigh resolution and full-length pilin structures with insights for filament assembly, pathogenic functions, and vaccine potential

Pilin proteins assemble into Type IV pili (T4P), surface-displayed bacterial filaments with virulence functions including motility, attachment, transformation, immune escape, and colony formation. However, challenges in crystallizing full-length fiber-forming and membrane protein pilins leave unanswered questions regarding pilin structures, assembly, functions, and vaccine potential. Here we report pilin structures of full-length DnFimA from the sheep pathogen Dichelobacter nodosus and FtPilE from the human pathogen Francisella tularensis at 2.3 and 1 Å resolution, respectively. The DnFimA structure reveals an extended kinked N-terminal α-helix, an unusual centrally located disulfide, conserved subdomains, and assembled epitopes informing serogroup vaccines. An interaction between the conserved Glu-5 carboxyl oxygen and the N-terminal amine of an adjacent subunit in the crystallographic dimer is consistent with the hypothesis of a salt bridge between these groups driving T4P assembly. The FtPilE structure identifies an authentic Type IV pilin and provides a framework for understanding the role of T4P in F. tularensis virulence. Combined results define a unified pilin architecture, specialized subdomain roles in pilus assembly and function, and potential therapeutic targets.

Higher whole-blood selenium is associated with improved immune responses in footrot-affected sheep

We reported previously that sheep affected with footrot (FR) have lower whole-blood selenium (WB-Se) concentrations and that parenteral Se-supplementation in conjunction with routine control practices accelerates recovery from FR. The purpose of this follow-up study was to investigate the mechanisms by which Se facilitates recovery from FR. Sheep affected with FR (n = 38) were injected monthly for 15 months with either 5 mg Se (FR-Se) or saline (FR-Sal), whereas 19 healthy sheep received no treatment. Adaptive immune function was evaluated after 3 months of Se supplementation by immunizing all sheep with a novel protein, keyhole limpet hemocyanin (KLH). The antibody titer and delayed-type hypersensitivity (DTH) skin test to KLH were used to assess humoral immunity and cell-mediated immunity, respectively. Innate immunity was evaluated after 3 months of Se supplementation by measuring intradermal responses to histamine 30 min after injection compared to KLH and saline, and after 15 months of Se supplementation by isolating neutrophils and measuring their bacterial killing ability and relative abundance of mRNA for genes associated with neutrophil migration. Compared to healthy sheep, immune responses to a novel protein were suppressed in FR-affected sheep with smaller decreases in FR-affected sheep that received Se or had WB-Se concentrations above 250 ng/mL at the time of the immune assays. Neutrophil function was suppressed in FR-affected sheep, but was not changed by Se supplementation or WB-Se status. Sheep FR is associated with depressed immune responses to a novel protein, which may be partly restored by improving WB-Se status (> 250 ng/mL).

The pathogenesis of ovine footrot

Ovine footrot is a contagious and debilitating disease that is of major economic significance to the sheep meat and wool industries. The causative bacterium is the gram negative anaerobe Dichelobacter nodosus. Research that has used a classical molecular genetics approach has led to major advances in our understanding of the role of the key virulence factors of D. nodosus in the disease process. D. nodosus strains produce polar type IV fimbriae and extracellular serine proteases. Mutagenesis of the fimbrial subunit gene fimA and the pilT gene, which is required for fimbrial retraction, and subsequent testing of these mutants in sheep virulence trials has shown that type IV fimbriae-mediated twitching motility is essential for virulence. The extracellular protease genes aprV2, aprV5 and bprV have also been mutated. Analysis of these mutants has shown that ArpV5 is the major extracellular protease and that AprV2 is the thermostable protease that is responsible for the extracellular elastase activity. Structural analysis of AprV2 has revealed that it contains several novel loops, one of which appears to act as an exosite that may modulate substrate accessibility. Finally, virulence experiments in sheep have shown that the AprV2 protease is required for virulence.

A within farm clinical trial to compare two treatments (parenteral antibacterials and hoof trimming) for sheep lame with footrot

From observational studies, farmers who use parenteral antibacterials to promptly treat all sheep with footrot (FR) or interdigital dermatitis (ID) have a prevalence of lameness of < 2% compared with a prevalence of 9% lameness reported by farmers who treat lame sheep by trimming affected feet. We tested the hypothesis that prompt treatment of sheep lame with naturally developing FR or ID with parenteral and topical antibacterials reduces the prevalence and incidence of lameness with these conditions compared with less frequent treatment with trimming of hoof horn and applying topical antibacterials.A further hypothesis was that reduction of ID and FR would improve productivity. A lowland sheep flock with 700 ewes was used to test these hypotheses in an 18-month within farm clinical trial with four groups of ewes: two intervention and two control. The duration and severity of lameness was used to categorise sheep into three weighted scores of lameness (WLS): never lame (WLS0), mildly lame/lame for < 6 days (WLS1) and severely or chronically lame (WLS2). The intervention reduced the prevalence of lameness due to FR and ID in ewes and lambs and the incidence of lameness in ewes. The WLS was also significantly lower in sheep in the intervention groups. Ewes with a higher WLS were subsequently significantly more likely to have a body condition score < 2.5 and to have lame lambs. Significantly more ewes lambed and successfully reared more lambs that were ready for slaughter at a younger age in the intervention versus control groups. There was an increase in the gross margin of Pound630/100 ewes mated in the intervention group, including the cost of treatment of Pound150/100 ewes mated. We conclude that prompt parenteral and topical antibacterial treatment of sheep lame with ID and FR reduced the prevalence and incidence of these infectious conditions and led to improved health, welfare and productivity.

Ovine footrot: new approaches to an old disease

Footrot is a bacterial disease that has substantial economic and welfare impacts in sheep and can be difficult to manage. Research is focussed on reducing the impact that footrot has on farmers and their flocks and better understanding the aetiology of the disease. Key areas of current research include, developing better vaccines, deploying tailored vaccines in a specific and targeted fashion on individual farms, analysing and developing better farm management practices to suit specific sheep farming environments, elucidating the virulence genes and bacterial population dynamics that drive footrot and using genetic testing in combination with selective breeding to produce stock that are more resilient to disease.

Footrot and interdigital dermatitis in sheep: farmer satisfaction with current management, their ideal management and sources used to adopt new strategies

The aims of this research were to identify management practices that sheep farmers currently use to treat and prevent footrot in sheep and whether they consider that these are successful management tools and to find out how sheep farmers would ideally like to manage footrot in their flock. Over 90% of lameness in sheep in the UK is caused by Dichelobacter nodosus, which presents clinically as interdigital dermatitis (ID) alone or with separation of hoof horn (FR). A questionnaire was sent to 265 farmers to investigate their current management and their satisfaction with current management of the spectrum of clinical presentations of footrot. Farmers were also asked their ideal management of footrot and their interest in, and sources of information for, change. Approximately 160 farmers responded. Farmers satisfied with current management reported a prevalence of lameness < or = 5%. These farmers caught and treated lame sheep within 3 days of first seeing them lame, and treated sheep with FR and ID with parenteral antibacterials. Farmers dissatisfied with their management reported a prevalence of lameness >5%. These farmers practised routine foot trimming, footbathing and vaccination against footrot. Whilst 89% of farmers said they were satisfied with their management of FR over 34% were interested in changing management. Farmers identified veterinarians as the most influential source for new information. Farmers reported that ideally they would control FR by culling/isolating lame sheep, sourcing replacements from non-lame parents, trimming feet less, using antibacterial treatments less and using vaccination more. Footbathing was a commonly used management that was linked with dissatisfaction and that also was listed highly as an ideal management. Consequently, some of the ideal managements are in agreement with our understanding of disease control (culling and isolation, sourcing healthy replacements) but others are in contrast with our current knowledge of management and farmers self-reporting of satisfaction of management of footrot (less use of antibacterial treatment, more footbathing and vaccination). One explanation for this is the theory of cognitive dissonance where belief follows behaviour, i.e. farmers report that they believe an ideal which is what they are currently doing, even if the management is sub-optimal.

Diagnostic sampling strategies for virulent ovine footrot: simulating detection of Dichelobacter nodosus serogroups for bivalent vaccine formulation

Dichelobacter nodosus is a slow-growing anaerobic bacterium that is the causative agent of virulent ovine footrot. Vaccination targeted at up to two specific serogroups can eliminate those serogroups from infected flocks, but requires identification of serogroups present in infected flocks. Serogroups can be identified using slide agglutination or polymerase chain reaction (PCR) methods. The objectives of this project were to use stochastic simulation modeling to estimate the efficacy of sampling strategies encompassing 5-40 sheep per flock and 2-4 colonies per sheep, and to compare efficacies based on slide agglutination or multiplex PCR test results. Foot swabs collected from sheep in 12 flocks were used as the basis for a sampling strategy simulation model. None of the evaluated sampling strategies identified the two most common serogroups in the flock, or all serogroups present in the flock, in 95% of iterations. However, a simulated sample of 22 sheep/flock and 2 colonies/sheep resulted in a simulated vaccine that protected 95% of the sheep that could be protected by a single bivalent vaccine, while a sample of 24 sheep/flock and 2 colonies/sheep resulted in a series of simulated bivalent vaccines that protected 95% of diseased infected sheep. The difference in outcome was due to the distribution and frequency of serogroups within certain flocks where some serogroups were uncommon and others dominant. A sampling strategy (>40 sheep/flock, 4 colonies/sheep) that will identify the two most common serogroups in a flock 95% of the time may not be cost effective. Evaluating efficacy based on the expected effect on the flock may be more useful than one which seeks to determine the most common serogroups. These findings are broadly applicable to diseases where more than one strain or type of pathogen may be present and must be represented in a vaccine.

Modulation of inter-vaccination interval to avoid antigenic competition in multivalent footrot (Dichelobacter nodosus) vaccines in sheep

Virulent footrot is a significant disease of sheep in most sheep farming countries; a strain/serogroup of the anaerobic bacterium Dichelobacter nodosus is the essential transmitting agent. Commercial multivalent footrot vaccines containing nine fimbrial serogroups (A through I) of D. nodosus produce relatively low and short term antibody responses due to antigenic competition, in contrast to higher and longer responses provided by monovalent or bivalent vaccines. The latter were important components of successful eradication programs for endemic footrot caused by either one or two serogroups of D. nodosus in Nepal, Bhutan, and several flocks in Australia. However, the presence of up to six serogroups in some Australian flocks and the use of an annual bivalent vaccination regime to progressively eradicate serogroups would require a long term program. In this study we report the results of a sequential vaccination trial testing different time intervals between different bivalent vaccinations. Intervals of 12, 9, 6, 3 and 0 months were tested. The 1st vaccination was with recombinant fimbrial antigens for serogroups A and B while the 2nd vaccination was with D and E. There were no significant differences between the antibody responses for time intervals of 3, 6, 9 and 12 months whereas there was a reduced response when sheep were vaccinated with two bivalent vaccines (four antigens) concurrently, indicating antigenic competition. Therefore an inter-vaccination interval of 3 months can be applied between two different bivalent vaccines without detrimental impact on the humoral immune responses to the various fimbrial antigens of D. nodosus. These results could have wider applications in vaccination against diseases caused by multivalent or multistrain microbes.

Genome sequence and identification of candidate vaccine antigens from the animal pathogen Dichelobacter nodosus

Dichelobacter nodosus causes ovine footrot, a disease that leads to severe economic losses in the wool and meat industries. We sequenced its 1.4-Mb genome, the smallest known genome of an anaerobe. It differs markedly from small genomes of intracellular bacteria, retaining greater biosynthetic capabilities and lacking any evidence of extensive ongoing genome reduction. Comparative genomic microarray studies and bioinformatic analysis suggested that, despite its small size, almost 20% of the genome is derived from lateral gene transfer. Most of these regions seem to be associated with virulence. Metabolic reconstruction indicated unsuspected capabilities, including carbohydrate utilization, electron transfer and several aerobic pathways. Global transcriptional profiling and bioinformatic analysis enabled the prediction of virulence factors and cell surface proteins. Screening of these proteins against ovine antisera identified eight immunogenic proteins that are candidate antigens for a cross-protective vaccine.

Assessment of current knowledge of footrot in sheep with particular reference to Dichelobacter nodosus and implications for elimination or control strategies for sheep in Great Britain

A brief summary of the currently accepted aetiology and pathogenesis of footrot and interdigital dermatitis in sheep in Great Britain is presented together with a description of the transmission dynamics of Dichelobacter nodosus. Approaches that may be successful for control or elimination of the disease are proposed and areas of uncertainty in current knowledge highlighted. Research and practice in Great Britain is compared with that from other countries and the proposal made that the environment and/or carrier (non-diseased) sheep may assist persistence of footrot.

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.

Current approaches to the management of ovine footrot

Ovine footrot remains the most important cause of lameness in sheep flocks in the UK, despite the existence of proven methods for the control of the disease. Recent research suggests that sheep farmers may be unaware of these methods and may allocate greater resources to treatment of footrot rather than to its prevention. Foot paring, topical treatments, vaccination and parenteral antibiotic therapy all have a role in treating sheep with advanced footrot infections, but prevention of severe infections is best achieved by the timely implementation of control programmes. These are usually based on footbathing and vaccination. For control programmes to be effective it is essential that the pathogenesis and epidemiology of footrot is understood and that control methods are implemented at appropriate times in the season, depending on climatic and pasture conditions. This article reviews these strategies and makes recommendations for steps to reduce the spread of footrot between flocks and to reduce the incidence of footrot within UK flocks.

Eradication of footrot of lesser clinical severity (intermediate footrot)

OBJECTIVE

To determine if intermediate footrot (IFR) can be eradicated from a flock of sheep by inspection and culling of cases during a non-transmission period and if prior antibiotic treatment or vaccination increases the likelihood of eradication.

PROCEDURE

A replicated field experiment that compared the three eradication strategies was followed by an observational study of the best of these applied in a commercial flock of 3000 sheep.

RESULTS

In the replicated experiment, IFR was eradicated either by inspection and culling alone, or when combined with vaccination. Eradication failed when the sheep were treated with parenteral antibiotics before inspection and culling during the non-transmission period. In the whole-flock program, eradication by repeated inspection and culling of footrot cases during the non-transmission period was successful and the flock remained free of infection 3 years later.

CONCLUSIONS

IFR can be eradicated by inspection and culling but latent infections, which may persist undetected for at least 34 weeks, require surveillance inspections to be repeated during the non-transmission phase of the program. The use of parenteral antibiotics as an aid to the eradication of IFR is contraindicated.

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.

Extensive diversity in New Zealand Dichelobacter nodosus strains from infected sheep and goats

Footrot is a contagious bacterial disease of ruminants spread by the Gram-negative, anaerobic organism, Dichelobacter nodosus. It is endemic in New Zealand and throughout sheep and goat farming regions of the world. Using the polymerase chain reaction (PCR) to amplify fragments of the fimbrial gene (fimA), D. nodosus was detected in 14 hoof scrapings, sampled from six farming regions within New Zealand. DNA sequencing revealed 15 strains covering eight serogroups on the New Zealand farms. The predominant serogroup was B which contained six strains, followed by serogroups F, H and G. No strains from serogroups D and I were detected in this investigation. Eleven out of the 15 D. nodosus strains had fimbriae sequences different to those previously reported and the presence of multiple strains on a single hoof was common (86% samples). Individual sheep from the same farm, or the same paddock, were often infected by a different range of strains, which suggests a host role in mediating footrot infection.

Manipulation of the helper T cell response to influence antigenic competition occurring with a multivalent vaccine

The reduction in antibody observed following inoculation with multiple heterologous Dichelobacter nodosus pili antigens is thought to be due to competition between antigen-specific B cells for a limited amount of T cell help. We demonstrate here that this competition is not further influenced by the expansion of cross-reactive antibody secreting cells at the expense of serogroup specific antibody secreting cells. The T cell determinants of pili recognized by sheep and BALB/c mice have been defined using 15 residue peptides. These T cell determinants include cross-reactive determinants in the conserved amino terminal region of the antigen. Here we investigate the effect of expanding the pili-specific T cell population by priming with pili derived T cell determinants. It was not possible to increase the antibody elicited in response to the multivalent vaccine by priming mice with either a synthetic peptide spanning a T cell determinant or with reduced and alkylated or heterologous serogroups of pili 4 weeks before inoculation with the multivalent vaccine. A strategy designed to increase the T cell population by inoculating animals with pili covalently coupled to an extrinsic T cell determinant was pursued.

Immunological parameters associated with antigenic competition in a multivalent footrot vaccine

A murine model for antigenic competition with multivalent D. nodosus pili vaccine has been established that parallels the phenomenon observed in sheep where levels of antibody, specific for any particular serogroup of pili, are significantly lower following vaccination in the presence of multiple serogroups of pili than with that serogroup alone. This competition was observed in both high and low responder strains of mice and was not dependent on the multiplicity of the antigens in the multivalent vaccine but could be observed with a large excess of a single heterologous serogroup. Competition was manifest by a reduction in the number of serogroup-specific antibody secreting cells elicited in response to vaccination. The antibody response to a single serogroup of pili reached a plateau at high doses and it was at these doses that antigenic competition was most pronounced, under conditions where both B- and T-cell responses were limiting. The limit in T-cell responsiveness was not imposed at the level of presentation of antigen. Pili-specific T cells were largely cross-reactive for different serogroups, and under conditions of limiting T-cell stimulation within a lymph node the available T cells would have to be shared between B cells specific for each serogroup of pili, which may in turn result in the decrease of serogroup-specific antibody induced following inoculation with the multivalent vaccine.

The protective efficacy of cloned Moraxella bovis pili in monovalent and multivalent vaccine formulations against experimentally induced infectious bovine keratoconjunctivitis (IBK)

Calves were vaccinated with cloned Moraxella bovis pili of serogroup C (experiment 1) or B (experiment 2) either as a monovalent formulation or as part of a multivalent preparation with pili of six other serogroups. Within 4 weeks of the second vaccine dose vaccinated calves and non-vaccinated controls were challenged via the ocular route with either virulent M. bovis strain Dal2d (serogroup C) or M. bovis strain 3WO7 (serogroup B) in experiments 1 and 2, respectively. Calves vaccinated with multivalent vaccines had significantly lower antibody titres than those vaccinated with monovalent preparations. Nevertheless, the levels of protection against infectious bovine keratoconjunctivitis (IBK) achieved with multivalent vaccines were 72% and 83% for the groups challenged with M. bovis strains of serogroups B and C, respectively. The serogroup C monovalent vaccine gave 100% protection against experimentally induced IBK and M. bovis isolates cultured from the eyes 6 days post-challenge were identified as belonging solely to serogroup C. Unexpectedly, only 25% protection was achieved against homologous strain challenge of calves that received the monovalent serogroup B vaccine. Furthermore, the majority of M. bovis isolates recovered from calves in this group belonged to serogroup C, as did half of those isolates cultured from the multivalent vaccinates. The remaining bacterial isolates from the latter group, together with all isolates from the non-vaccinated controls, belonged to serogroup B. Results are consistent with the hypothesis that derivatives of the serogroup B challenge inoculum had expressed serogroup C pilus antigen within 6 days of the challenge, possibly as a result of pilus gene inversion occurring in response to the presence of specific antibody in eye tissues and tears.

A haemolytic cell-free preparation of Moraxella bovis confers protection against infectious bovine keratoconjunctivitis

Protection conferred by a cell-free preparation from a haemolytic Moraxella bovis isolate, UQV 148NF, was compared to an equivalent fraction from a non-haemolytic M. bovis isolate, Gordon 26L3, and to a recombinant DNA-derived pili vaccine. Three groups of ten calves were vaccinated twice with one of the three preparations and, together with ten non-vaccinated calves, challenged with virulent M. bovis isolate Dal 2d. Compared to the control group, significant protection was observed in the group receiving the pili vaccine and the group receiving the preparation from haemolytic isolate, UQV 148NF.

Antigenic competition in a multivalent foot rot vaccine

The antigenic competition that occurs when pilus antigens of different serogroups are combined in multivalent vaccines for foot rot has been investigated using recombinant pilus antigens. Our prototype vaccine contains pili from nine serogroups of Dichelobacter nodosus which are expressed in Pseudomonas aeruginosa. Sheep inoculated with this multivalent vaccine were not as well protected against foot rot as those given the monovalent vaccine. Levels of agglutinating and total antibody specific for any particular pili serogroup were found to be significantly reduced in sheep vaccinated with six or more closely related pili. This effect was more pronounced for agglutinating antibody, which is thought to mediate protection, but was also observed with total antibody levels measured by ELISA. The antigenic competition was not associated with the total antigen load as a tenfold higher dose of monovalent pili induced high titres of antibody. Furthermore, distributing the vaccine to four sites, each draining to a different lymph node, failed to overcome the competition. Experiments with mixtures of monospecific sera indicate that the phenomenon is unlikely to be due to blocking of serogroup-specific protective antibodies by an excess of cross-reactive non-protective antibody elicited by heterologous pili.

Protective antibody titres and antigenic competition in multivalent Dichelobacter nodosus fimbrial vaccines using characterised rDNA antigens

The relationship between K-agglutination antibody titres and protection against experimental challenge with Dichelobacter nodosus, the effect of increasing the number of D. nodosus fimbrial antigens, and the importance of the nature of additional antigens in multivalent vaccines on antibody response and protection against experimental challenge with D. nodosus were examined in Merino sheep. A total of 204 Merino sheep were allocated to one of 12 groups, and vaccinated with preparations containing a variable number of rDNA D. nodosus fimbrial antigens. The most complex vaccine contained ten fimbrial antigens from all major D. nodosus serogroups, while the least complex contained a single fimbrial antigen. In addition to D. nodosus fimbrial antigens, other bacterial rDNA fimbrial antigens (Moraxella bovis Da12d and Escherichia coli K99), and bovine serum albumin (BSA) were used in some vaccines. Antibody titres to fimbrial antigens and BSA were measured by agglutination and ELISA tests, respectively. Antibody titres were determined on five occasions (Weeks 0, 3, 6, 8, and 11 after primary vaccination). All sheep were exposed to an experimental challenge with virulent isolates of D. nodosus from either serogroup A or B, 8 weeks after primary vaccination. For D. nodosus K-agglutinating antibody titres, a strong negative correlation between antibody titre and footrot lesion score was observed. This relationship was influenced by the virulence of the challenge strain. Increasing the number of fimbrial antigens in experimental rDNA D. nodosus fimbrial vaccines resulted in a linear decrease in K-agglutinating antibody titres to individual D. nodosus serogroups. Similarly, a linear decrease in protection to challenge with homologous serogroups was observed as the number of D. nodosus fimbrial antigens represented in the vaccine increased. The reduction in antibody titres in multicomponent vaccines is thought to be due to antigenic competition. The level of competition between individual antigens is not constant and appears to be related to the immunodominance (nature) of the competing antigens. Both BSA ELISA, and M. bovis K-agglutinating antibody titres were adversely affected by the presence of two D. nodosus fimbrial preparations, whereas the antigenicity of E. coli K99 was unchanged by the presence of two additional D. nodosus antigens. Further studies are required to determine the step(s) in the immune response which are influenced by antigenic competition. Our results suggest that antigen presentation, particularly following primary vaccination, is the step most strongly influenced by antigenic competition.

High and low responsiveness to vaccines in farm animals

Low responsiveness in some farm animals is emerging as a problem in the application of newly developed vaccines which operate at skin surfaces and mucous membranes. Breeding for resistance to specific diseases seems to be associated with breeding for specific immune responsiveness in farm animals and very likely this involves selection for major histocompatibility complex (MHC) haplotype. However, other factors contribute to low responsiveness and these include poor nutrition, sire effects, antigenic competition and inadequate effector mechanisms. The future of newly developed vaccines will rest on the solution to the low responder problem, and once solved, the application of these vaccines will be fully utilized for disease control in farm animals.

Recombinant vaccines against ovine footrot

For the past 20 years footrot vaccines have evolved from simple bacterins to highly specific recombinant DNA (rDNA) fimbrial vaccines. The development of these vaccines has left a trail of discoveries, challenges and solutions; these processes continue as we move closer to understanding the requirements of a footrot vaccine. The initial whole cell vaccines were unsuccessful due to the short duration of immunity and incorporation of limited serotypes. A multistrain vaccine eliminated the problem of serotype inclusion, although the duration of immunity in many cases is still inadequate. The proteases of Dichelobacter nodosus appear to be cross protective; however, little is known of their ability to protect sheep against footrot. The major protective immunogen is the bacterial fimbriae, which also forms the basis for the K-agglutination serotyping system. K-agglutinin titre correlates directly with resistance to challenge. The protective fimbrial epitope is conformationally dependent, suggesting little advantage in the development of synthetic peptide vaccines. To enhance the efficiency of vaccine production D. nodosus fimbrial genes were eventually cloned and successfully expressed in Ps. aeruginosa. Monovalent vaccines based on recombinant fimbriae are omnipotent, inducing high levels of agglutinins and long lasting immunity. In multivalent vaccines, on the other hand, incorporation of each additional serogroup into the vaccine results in reduced efficacy both in terms of reduced K-agglutinin titres and reduced protection following challenge. The least effective are multivalent formulations representing all major serogroups. In addition, considerable genetic variation has been observed in the ability of sheep to respond optimally to each serogroup in a multivalent vaccine. Results show that the limitation of the sheep to mount an effective immune response, rather than the quality or quantity of the immunogen, limits the efficacy of current footrot vaccines. Studies are being undertaken to examine in detail the immune response of sheep to potentially highly effective footrot vaccines.