Serum antibody responses in sheep after natural infection with Bacteroides nodosus

Investigation of immunity in sheep following footrot infection and vaccination

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

Antigens for serological diagnosis of ovine footrot

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

Immunoblot and ultrastructural analysis of antigens extracted from Dichelobacter nodosus with potassium thiocyanate

The antigens extracted from Dichelobacter nodosus with potassium thiocyanate were analyzed by western blotting with sera from footrot-free sheep and from sheep infected with D. nodosus to identify components specific for infection. Several components with molecular mass < 33 kD were associated with infection, particularly bands of 32.5 kD, 30.5 kD, and 28 kD. Components with molecular mass > 56 kD may be responsible for false-positive reactions observed when sera of footrot-free sheep react with the potassium thiocyanate extract in solid phase enzyme-linked immunosorbent assays (ELISAs). Bands of 62.5 kD, 56 kD, 40 kD, and 19 kD were recognized for most sheep regardless of their disease status or ELISA reactivity and therefore do not appear to be detected by the ELISA. No components of the potassium thiocyanate extract were completely specific for infection. Antigens in the extract were identified primarily on the cell envelope by immunogold electron microscopy. Labeling was concentrated in the periplasm, with minor labeling of the cell surface. The extract consisted of tangled strands of particles with electron-dense cores, and few vesicular structures were observed.

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

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

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

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

Grading the lesions of ovine footrot

Sixteen methods of grading the lesions of ovine footrot were assessed on the basis of the effect of the lesions on the humoral immune response of the host to a causative bacterium, Dichelobacter nodosus. Methods that allowed for qualitative and quantitative differences in lesion scores between sheep were the best predictors of host response, and methods that assessed the lesions in each of the eight digits were more efficient than methods that did not grade the digits within feet. Weighting the scores for lesions that involved underrunning of the keratin of the hoof provided the most powerful means of predicting host response. The correlations between host response and the more elaborate weighted scores were close to the highest possible among additive linear estimators. Total weighted footscore, which is the sum of the footscores of the four feet weighted for underrun lesions, is proposed as a simple and effective grading system for sheep with lesions of footrot. There was a significant association within sheep between the number of underrun feet and the severity of lesions in individual feet.

Application of ELISA to the serological diagnosis of virulent ovine footrot

The sensitivity and specificity of an ELISA that detects antibody in the serum of sheep against Dichelobacter nodosus was investigated. The sensitivity of the assay was determined in two trials in which sheep free of virulent footrot were exposed to a virulent strain of D. nodosus. In the first trial up to 96.6% of sheep that developed severe foot lesions were seropositive 2 weeks later. The proportion of seropositive sheep depended on the interval between exposure to D. nodosus and collection of blood samples and reached a maximum 6 weeks after exposure in the first trial and plateaued from 8 weeks after exposure in the second trial. Of sheep with severe lesions present for only 1 week, 40.0% became seropositive, while 98.3% of sheep with severe lesions present for 3 or more weeks became seropositive in Trial 1. Of the unexposed control sheep from the same flock, 93.3% remained seronegative. At least 75% of sheep that were vaccinated with commercial footrot vaccines remained seropositive for at least 24 months after last vaccination and 58.5% remained seropositive for at least 36 months. The specificity of the assay in flocks of sheep from different geographical regions was determined also. Overall, 95.2% of 2978 sheep free of virulent footrot were seronegative. However the rate of false positive serological reactions ranged from 1.6% to 11.3% between flocks and was higher in older sheep (7.0%) compared with sheep less than 1.5 years old (2.7%). Passively acquired colostral antibodies against D. nodosus were measured in 22 lambs and did not persist beyond 55 days mean age. Non-specific reactions with cross reactive antigens were proposed to explain age related changes in serological results. Applications of this ELISA in footrot control programmes are discussed.

Early diagnosis of typhoid fever by an enzyme immunoassay using Salmonella typhi outer membrane protein preparations

An enzyme immunoassay (EIA) for detection of serum antibodies in patients with typhoid fever was developed using Salmonella typhi outer membrane protein (OMP) preparations as antigen. Acute phase (first week) sera from adult typhoid fever patients were tested as well as sera from the following control groups: adult travellers with diarrhea caused by enterotoxigenic Escherichia coli, children infected with Campylobacter jejuni, healthy Mexican adult blood donors, and adults with septicemia caused by other organisms. At a 1:3,125 serum dilution, the mean absorbance values were 1.41 in the typhoid fever patients, and 0.57, 0.55, 0.51 and 0.52 in the respective control groups. Inhibition EIA studies using OMP preparations or lipopolysaccharide (LPS) as free antigen indicated that proteins can play an important role in the detection of antibodies in early typhoid fever. This EIA may be useful for the diagnosis of typhoid fever since results were obtained within about five hours and in an endemic area antibodies against Salmonella typhi OMP preparations appear early in the course of the disease.

Humoral responses to a multivalent vaccine in age-matched lambs of different bodyweight and nutrition

K-agglutination, pilus-enzyme-linked immunosorbent assay (ELISA) and outer membrane protein-ELISAs were used to assess humoral responses after vaccination with a commercial, multivalent, ovine foot rot vaccine (Dichelobacter nodosus whole cells) in three groups of nine-month-old lambs of markedly different bodyweight, nutritional history and dietary protein supply. Mean bodyweights of lambs in low (L), medium (M) and high (H) bodyweight/nutrition groups were 22, 32 and 48 kg, respectively, at the time of vaccination. Few significant differences in humoral responses to vaccine antigens were found between groups. However, lambs in group H tended to have lower levels of antibody to a greater number of component antigens than did lambs in the other groups. These results suggest that low bodyweight due to poor nutrition is unlikely to affect the response of sheep to multivalent foot rot vaccines.

Evaluation of a simple method for improving the precision of an ELISA detecting antibody in serum

A method of correcting OD values to improve between-assay precision (PCF4 method) was evaluated using an ELISA detecting anti-Dichelobacter nodosus antibody in ovine serum. Four control sera with pre-defined target OD were included on each plate and used to calculate a correction factor. The mean between-plate CV for eight reference sera tested over a period of 6 months was reduced from 15.2% to 9.4% in a polyclonal conjugate assay (114 plates) and from 13.3% to 9.8% in a monoclonal conjugate assay (128 plates) using this method. The PCF4 method was compared with an existing method that uses only one control serum and was found to be superior. Time course plots of PCF4 values were useful for laboratory quality control. The mean within-plate coefficients of variation for four reference sera tested in paired wells were 8.2% and 5.1% in the polyclonal and monoclonal conjugate assays respectively. Marked changes in OD due to experimental manipulation of antigen dilution, serum dilution and incubation time, conjugate dilution and chromogen-substrate incubation time were corrected by the method.

An anamnestic serological test for ovine footrot

Following recovery from ovine footrot, a proportion of sheep in a flock may carry the causative organism and spread it to other sheep if environmental conditions are favourable. Footrot affected sheep have elevated levels of serum antibody against Bacteroides nodosus, but these levels decline rapidly after clinical recovery. When challenged by subcutaneous injection with 470 micrograms of protein extracted from the cell membrane of B. nodosus, without adjuvant, sheep that had recovered clinically from virulent footrot produced a marked increase in specific serum antibody within 7 d, while antibody levels in footrot-free sheep injected with the same antigen, and in saline injected controls, did not increase over a period of 25 d. Artificial stimulation and serological detection of immune memory may be useful in footrot eradication programs by identifying sheep that have had clinical footrot infection. This procedure may be applicable to other diseases where antibody responses are inconsistent or transient.