Development and validation of an indirect enzyme immunoassay for detection of ovine antibody to Brucella ovis

Development, validation and field evaluation of an indirect ELISA for the detection of antibodies against Brucella abortus in bulk and individual milk samples in dairy cattle

Brucellosis is an abortigenic and zoonotic disease. In cattle, it is mainly caused by Brucella abortus. The disease is endemic in low- and middle-income countries, being considered a neglected zoonotic disease. In these countries, it is of high importance to develop and validate sensitive, specific and low-cost diagnostic assays for brucellosis. The aim of the present study was the development of an indirect enzyme-linked immune assay (iELISA) to detect anti-B. abortus antibodies in milk samples. We purified the lipopolysaccharide antigen from B. abortus and produced an anti-bovine IgG monoclonal antibody to develop an iELISA (iELISA_INTA). The iELISA_INTA was validated using 1730 bulk milk samples and 1734 individual milk samples. The sampled dairy herds had at least 3 years of consistency at their positive or negative official brucellosis status. Individual milk samples were taken in parallel with serum samples from the cows. The status of the cows was defined by the result of the complement fixation test (CFT) performed with their serum sample. The reproducibility of the assay was evaluated in two laboratories. In addition, we evaluated the performance of the assay in the field, using 4385 bulk milk samples and 968 individual milk samples. The results of the iELISA_INTA were compared with those obtained using the officially accepted brucellosis techniques: iELISA from Canada (iELISA_CFIA) in milk samples, and the buffered plate antigen (BPA) and the CFT in serum samples. At validation, the sensitivity (Se) of the iELISA_INTA in bulk milk samples was 98.61 %, and the specificity (Sp) 98.79 % with a ≥ 10 % of positivity (PP) cutoff. In individual milk samples, the Se was 98.04 %, and the Sp 98.56 % with a ≥ 16 PP cutoff. The chance-corrected agreement kappa value (κ) between the results obtained in the different laboratories was κ = 0.87. In the field evaluation, in bulk milk samples the κ value between the iELISA_INTA and the iELISA_CFIA was κ = 0.86. On individual milk samples, the κ values were: between the iELISA_INTA and the iELISA_CFIA κ = 0.79, between the iELISA_INTA and BPA was κ = 0.85, and between the iELISA_INTA and CFT κ = 0.82. The developed iELISA_INTA showed a very good performance and it could be used as a screening assay for anti-B. abortus antibodies detection in individual milk samples and for epidemiologic surveillance in bulk milk samples.

Comparison of Serological Tests Based on Outer Membrane or Internal Antigens for Detecting Antibodies to Brucella Ovis in Infected Flocks

The aim of this work was to compare the performance of 6 serological tests using outer or internal antigens from Brucella for the diagnosis of Brucella ovis infection in sheep in an endemic area. Outer membrane antigens included a hot saline extract (HS) and the rough lipopolysaccharide (R-LPS) from B. ovis. Internal antigens were LPS-free total cytosolic proteins (CP) and an 18-kDa cytosolic protein (p18) from Brucella spp. Sera from 200 sheep from naturally infected flocks were assayed by agar gel immunodiffusion test (AGID) and by complement fixation test (CFT), both using HS, and by 4 ELISA using HS, R-LPS, CP, and p18, respectively. The percentage of positive results was 45.5% for ELISA with HS, 42.0% for ELISA with p18, 39.5% for CFT, 33.5% for ELISA with R-LPS, 29.0% for ELISA with CP, and 18.0% for AGID. Taking CFT as the reference test for calculating relative test parameters, the ELISA with HS had the best sensitivity (96.2%), while AGID and the ELISA with R-LPS had the best specificity (96.6%). The ELISA with CP was not more sensitive than the ELISA with p18 (67.1% vs. 79.7%) in spite of the higher number of antigens in CP. The lower relative sensitivity of tests using internal antigens might reflect a lack of antibodies to cytosolic proteins in some infected animals or a shorter persistence of these antibodies relative to antibodies to outer membrane components after recovery from infection.

Validation according to OIE criteria of a monoclonal, recombinant p26–based, serologic competitive enzyme-linked immunosorbent assay as screening method in surveillance programs for the detection of Equine infectious anemia virus antibodies

The Italian National Reference Center for equine infectious anemia (CRAIE; Rome, Italy) developed and validated a monoclonal, recombinant p26–based competitive enzyme-linked immunosorbent assay (cELISA) for the detection of EIA virus antibodies employing the 2010 criteria of the World Organization for Animal Health (OIE). The following parameters were evaluated: cutoff values, repeatability, reproducibility, concordance, analytical sensitivity (Se), absolute analytical specificity (Sp), and diagnostic Se and Sp. Positive and negative predictive values were also defined in relation to the estimated prevalence. When the cELISA was used as a screening test for 96,468 samples in the Italian EIA surveillance program, 17% more EIA cases were detected than by the agar gel immunodiffusion test, and the apparent diagnostic Sp estimated from these samples was 99.8%, which was more than the diagnostic Sp (80.2%) estimated from validation. The high Se and Sp of the cELISA confirm its fit for purpose as a screening test.

Microbial diseases of the genital system of rams or bucks

Objective of the present paper is to review microbial diseases of the genital system of male small ruminants. The paper reviews the infections and the diseases by taking an organ approach within the genital system, whilst relevant health management actions are also discussed. Diseases of the genital organs of male small ruminants include orchitis, of bacterial or viral aetiology, epididymitis, primarily caused by Brucella ovis, by other bacteria as well (e.g., Actinobacillus seminis, Haemophilus somni), infections of the accessory glands, orf, other infections of the penis or prepuce and infections of the scrotum. The health management of rams/bucks include the appropriate diagnostic investigations, the relevant therapeutic approaches and, finally, the preventive measures.

Contagious epididymitis due to Brucella ovis: relationship between sexual function, serology and bacterial shedding in semen

BACKGROUND

Contagious Epididymitis (CE) due to Brucella ovis (B. ovis) is a contagious disease that impairs rams' fertility due to epididymis, testicle and accessory sexual gland alterations. An increased incidence of CE has been observed in South Eastern France ("PACA" region) since the Rev.1 vaccination against B. melitensis has been stopped in 2008. The objective of this study was to evaluate the relationship between the infection by B. ovis and the sexual function of rams. Two-hundred eighteen sexually-mature rams, from 11 seropositive flocks, were submitted to a clinical examination of the genital tract, a semen collection by electro-ejaculation for spermogram and culture, and a serological examination for anti-B. ovis antibodies by complement fixation test (CFT) and indirect ELISA (I-ELISA). The relationships between clinical, seminal, bacteriological and serological parameters were studied using the Fisher exact test and a logistic regression model (binomial logit).

RESULTS

B. ovis shedding in semen was significantly associated with seropositivity (CFT and I-ELISA; p < 0.001 and 0.01 respectively), genital tract alterations (p < 0.05) and poor semen quality (p < 0.001). Seropositive rams presented significantly more genital tract alterations (p < 0.001) and a poor seminal score (p < 0.001) than seronegative rams.

CONCLUSIONS

Since semen culture is not routinely feasible in field conditions, a control plan of CE should be based, where Rev.1 vaccination is not possible, on both systematic clinical and serological examination of rams, followed by the culling of seropositive and/or genital tract alterations carrier rams.

Assessment of the diagnostic sensitivity and specificity of an indirect ELISA kit for the diagnosis of Brucella ovis infection in rams

Background

Brucella ovis causes an infectious disease responsible for infertility and subsequent economic losses in sheep production. The standard serological test to detect B. ovis infection in rams is the complement fixation test (CFT), which has imperfect sensitivity and specificity in addition to technical drawbacks. Other available tests include the indirect enzyme-linked immunosorbent assays (I-ELISA) but no I-ELISA kit has been fully evaluated.

The study aimed to compare an I-ELISA kit and the standard CFT. Our study was carried out on serum samples from 4599 rams from the South of France where the disease is enzootic. A Bayesian approach was used to estimate tests characteristics (diagnostic sensitivity, Se and diagnostic specificity, Sp). The tests were then studied together in order to optimise testing strategies to detect B. ovis.

Results

After optimising the cut-off values in order to avoid doubtful results without deteriorating the concordance between the results of the two tests, the I-ELISA appeared to be slightly more sensitive than CFT (Se _I-ELISA = 0.917 [0.822; 0.992], 95% Credibility Interval (CrI) compared to Se _CFT = 0.860 [0.740; 0.967], 95% CrI). However, CFT was slightly more specific than I-ELISA (Sp _CFT = 0.988 [0.947; 1.0], 95% CrI) compared to Sp _I-ELISA =0.952 [0.901; 1.0], 95% CrI).

The tests were then associated with two different interpretation schemes. The series association increased the specificity of screening and could be used for pre-movement testing in rams from uninfected flocks. The parallel association increased sequence sensitivity, thus appearing more suitable for eradicating the disease in infected flocks.

Conclusions

The high sensitivity and acceptable specificity of this I-ELISA kit support its potential interest to avoid the limitations of CFT. The two tests could also be used together or combined with other diagnostic methods such as semen culture to improve the testing strategy. The choice of test sequence and interpretation criteria depends on the epidemiological context, screening objectives and the financial and practical constraints.

Inquérito soro-epidemiológico e fatores de risco associados à infecção por Brucella ovis em carneiros deslanados do semi-árido da Paraíba

Foi realizado um levantamento soro-epidemiológico da brucelose por Brucella ovis em reprodutores ovinos deslanados na Paraíba com os objetivos de verificar a prevalência e distribuição da infecção por B. ovis em propriedades rurais e analisar os possíveis fatores de risco associados à infecção. Foram investigadas 283 propriedades criadoras de ovinos, das quais foram colhidas 498 amostras de soro sanguíneo de carneiros, a partir de oito meses de idade, nas mesorregiões do Sertão Paraibano e Borborema. Todos os soros foram examinados pela IDGA (teste de triagem) e RFC (teste confirmatório). De acordo com as análises, 8,59% (I.C.95% = 5,83%-12,48%) das propriedades apresentaram evidência sorológica de infecção por B. ovis, com uma prevalência de 5,57% (I.C.95% = 3,86%-7,97%) de reprodutores sororreagentes. Nas propriedades que higienizavam suas instalações com maior freqüência, a soropositividade foi estatisticamente inferior (p < 0,05).

Detection of antibodies to Brucella ovis in sheep milk using B. ovis and B. canis antigen

The diagnostic techniques most widely used for detecting brucellosis caused by Brucella ovis are serological tests such as complement fixation (CFT), agar gel immunodiffusion (AGID), and ELISAs. However, to our knowledge, milk tests, with the advantage that samples may be taken in a non invasive manner, have not been investigated as diagnostic tools. We studied 144 samples of milk and sera from lactating ewes, comparing bacteriological studies, serological and milk tests using Brucella canis and B. ovis antigens. A group of 75 ewes in an uninfected flock were serologically negative to rapid slide agglutination test (RSAT), indirect ELISA (IELISA)-B. canis, AGID and IELISA-B. ovis. The milk of these ewes had an IELISA-B. canis mean (%P) value of 16.18 (S.D. 5.63), while the IELISA-B. ovis had a mean (%P) value of 12.52 (S.D. 4.94). A cut-off value of (%P) 27.44 (+2 S.D.) or (%P) 33 (+3 S.D.) was determined by milk-ELISA-B. canis and (%P) 22.4 (+2 S.D.) and (%P) 27.34 (+3 S.D.) by milk-IELISA-B. ovis. These cut-off values were adjusted by receiver-operator characteristics (ROC) analysis using 23 positive samples from infected ewes, which indicated a milk-IELISA-B. canis cut-off value of (%P) 33 and milk-IELISA-B. ovis of (%P) 26 with 100% sensitivity and specificity. Based on our results, we propose that, following a study of a larger number of samples, the milk-IELISA-B. canis could be considered a suitable test for the diagnosis of B. ovis brucellosis in lactating ewes.

Use of Brucella canis antigen for detection of ovine serum antibodies against Brucella ovis

Brucella ovis causes a genital disease of sheep manifested by epididymitis in rams and placentitis in ewes producing reduced fertility in the flock. Clinical diagnosis is not sensitive enough and bacteriological testing is not feasible for detection of the disease in large numbers of animals. Indirect methods of serological testing are preferred for routine diagnosis, of which agar gel immunodiffusion (AGID), complement fixation (CF) and ELISA tests are recommended as the most efficient. Since B. ovis shares antigenic components with Brucella canis, it would seem that either strain could be used as antigen with the same results; however, the advantage of the B. canis (M-) strain variant is that it can be used to develop a satisfactory antigen for agglutination tests. We present data on AGID and IELISA tests using B. ovis antigen and rapid screening agglutination test (RSAT), 2-mercapto-ethanol RSAT (2ME-RSAT) and IELISA using B. canis antigen. We tested 225 animals. The cut-off values were adjusted by ROC analysis using 51 negative and 32 positive sera; the IELISA-B. canis cut-off value was 39 (%P) and IELISA-B. ovis, 51 (%P), with 100% sensitivity and specificity. Of the 32 positive sera from the infected flock RSAT detected 32 (100%), 2ME-RSAT 29 (91%) and AGID 31 (97%). Of the 142 sera from suspicious flocks, 46 were negative and 56 positive in all the tests; 16 were positive by RSAT, IELISA-B. canis and IELISA-B. ovis, 20 positive only with RSAT and 2 positive only by both IELISAs. RSAT is a very sensitive screening test that, because of its simplicity and easy interpretation, following a study in larger sample, could replace AGID as a screening test for diagnosis of ovine brucellosis caused by B. ovis. The IELISA-B. canis or IELISA-B. ovis could be used as confirmatory tests, since they show equal specificity and sensitivity.

Development and evaluation as vaccines in mice of Brucella melitensis Rev.1 single and double deletion mutants of the bp26 and omp31 genes coding for antigens of diagnostic significance in ovine brucellosis

The live attenuated Brucella melitensis Rev.1 strain is considered the best vaccine available for the prophylaxis of brucellosis in sheep caused by either B. melitensis or Brucella ovis. However, its application stimulates antibody responses in vaccinated animals indistinguishable by the current conventional serological tests from those observed in infected animals. The periplasmic protein BP26 and the outer membrane protein (OMP) Omp31 are immunodominant antigens in the serological responses of B. melitensis and B. ovis infected sheep, respectively. Accordingly, vaccine strain Rev.1 single and double deletion mutants of the bp26 and omp31 genes were developed, based on the principle that the use of such mutants as vaccines in association with diagnostic tests based on BP26 and Omp31 antigens would allow the serological differentiation between infected and vaccinated animals. The deletion mutants obtained were indistinguishable from the parental Rev.1 strain by conventional bacteriological and typing tests. The expression of their major surface antigens, as determined by reactivity with specific monoclonal antibodies (MAbs), remained unaffected, i.e. smooth-lipopolysaccharide (S-LPS) and OMPs besides in the expression of the antigens whose respective genes were deleted. The bp26 and omp31 deletions did not modify the kinetics of splenic infection nor the residual virulence of Rev.1 in the BALB/c mouse model. Vaccination of BALB/c mice with the deletion mutants conferred significant protective immunity against B. melitensis strain H38 or B. ovis strain PA challenges, to the same extent as that induced by parental Rev.1 strain. Thus, these Rev.1 bp26 or omp31 deletion mutants are promising vaccine candidates against B. melitensis and B. ovis infections and will be further evaluated in sheep.

Immunogenicity of recombinant Omp31 from Brucella melitensis in rams and serum bactericidal activity against B. ovis

Detergent-extracted recombinant Omp31 (rOmp31 extract) from Brucella melitensis produced in Escherichia coli was previously identified as a protective immunogen against B. ovis in mice. In this study, we evaluated the immunogenicity of rOmp31extract in rams. This immunogen was emulsified in an oil adjuvant and administered three times with 4 and 8 weeks intervals. Antibody response was measured in serum by whole B. ovis ELISA. Specific antibodies to purified rOmp31 (pET-Omp31) were detected by Western blotting and indirect ELISA. In addition, isotype specific antibodies were measured in tears. Serum bactericidal activity against B. ovis in the presence of complement was measured in vitro. Cellular immune response was explored by intradermal testing with purified rOmp31. Immunization with rOmp31 extract induced IgG specific antibodies in serum able to bind to whole B. ovis cells. Furthermore, strong inhibition in a competitive ELISA (with an Omp31-specific monoclonal antibody) suggested that a proportion of Omp31-specific antibodies were directed against a loop containing a protective epitope. Serum antibodies killed efficiently B. ovis in vitro in the presence of either guinea pig or ovine serum. Tears had both IgG and IgA antibodies to equivalent titers. Finally, immunized rams showed skin reactivity to Omp31. These data demonstrate that B. melitensis Omp31, a protective antigen identified in the mouse model, induces antibody and cellular immune mechanisms in sheep.

Rough Lipopolysaccharide ofBrucella abortusRB51 as a Common Antigen for Serological Detection ofB. ovis,B. canis, andB. abortusRB51 Exposure Using Indirect Enzyme Immunoassay and Fluorescence Polarization Assay

Rough lipopolysaccharide (RLPS) antigens were prepared from cultures of Brucella abortus RB51, B. ovis, and B. canis. The preparations were standardized by weight and tested with sera from cattle immunized with B. abortus RB51, sheep infected with B. ovis, and dogs infected with B. canis. Populations of unexposed animals of each species were also tested. The tests used were the indirect enzyme immunoassay (IELISA) using RLPS and the fluorescence polarization assay (FPA) using RLPS core fractions, labeled with fluorescein isothiocyanate. The IELISA using B. abortus RB51 RLPS antigen resulted in sensitivity and specificity values of 94.8% and 97.3%, respectively, when testing bovine sera, 98.5% and 97.8% when testing ovine sera, and 95.8% and 100% when testing dog sera. The IELISA using B. ovis RLPS antigen gave sensitivity and specificity values of 80.5% and 91.7%, respectively with bovine sera, 98.9% and 93.8% with sheep sera, and 70.8% and 79.8% with dog sera. The IELISA using B. canis RLPS antigen resulted in sensitivity and specificity values of 97.0% and 97.4%, respectively, with bovine sera, 96.2% and 96.3% with sheep sera, and 95.8% and 98.8% with dog sera. Labeling RLPS core from B. ovis and B. canis with fluorescein was not successful. B. abortus RB51 core labeled with fluorescein resulted in sensitivity and specificity values of 93.5% and 99.8%, respectively, with bovine sera and 78.1% and 99.0% with sheep sera. It was not possible to test the dog sera in the FPA.

The recombinant Omp31 from Brucella melitensis alone or associated with rough lipopolysaccharide induces protection against Brucella ovis infection in BALB/c mice

Immunogenicity and protective activity against Brucella ovis of detergent-extracted recombinant Omp31 (rOmp31 extract) from Brucella melitensis produced in Escherichia coli, purified rough lipopolysaccharide from B. ovis (R-LPS) and a mixture of rOmp31 extract and R-LPS (rOmp31 extract + R-LPS) were assessed in BALB/c mice. The experimental vaccines were compared with a hot saline extract (HS extract) from B. ovis mainly composed of outer membrane proteins (OMPs) and R-LPS, and known to be protective in mice against a B. ovis infection. Serum antibodies to Omp31 and R-LPS were detected in the corresponding mice using Western blotting with B. ovis whole-cell lysates and ELISA with purified antigens. Protection was evaluated by comparing the levels of infection in the spleens of vaccinated mice challenged with B. ovis. A significantly lower number of B. ovis colony-forming units in spleens relative to unimmunized (saline injected) controls were considered as protection. Mice immunized with rOmp31 extract or rOmp31 extract mixed with R-LPS developed antibodies that bound to the B. ovis surface with similar titers. Vaccination with rOmp31 extract plus R-LPS provided the best protection level, which was comparable with that given by HS extract. Similar protection was also obtained with rOmp31 extract alone and, to a lesser degree, with R-LPS. Comparisons between groups showed that an extract from E. coli-pUC19 (devoid of Omp31) provided no protection relative to either HS extract, rOmp31 extract or rOmp31 extract mixed with R-LPS. In conclusion, the recombinant Omp31 associated or not with B. ovis R-LPS, could be an interesting candidate for a subcellular vaccine against B. ovis infection.

Diagnosis of brucellosis by serology

Serological diagnosis of brucellosis began more than 100 years ago with a simple agglutination test. It was realized that this type of test was susceptible to false positive reactions resulting from, for instance, exposure to cross reacting microorganisms. It was also realized that this test format was inexpensive, simple and could be rapid, although results were subjectively scored. Therefore, a number of modifications were developed along with other types of tests. This served two purposes: one was to establish a rapid screening test with high sensitivity and perhaps less specificity and a confirmatory test, usually more complicated but also more specific, to be used on sera that reacted positively in screening tests. This led to another problem: if a panel of tests were performed and they did not all agree, which interpretation was correct? This problem was further compounded by the extensive use of a vaccine which gave rise to an antibody response similar to that resulting from field infection. This led to the development of an assay that could distinguish vaccinal antibody, starting with precipitin tests. These tests did not perform well, giving rise to the development of primary binding assays. These assays, including the competitive enzyme immunoassay and the fluorescence polarization assay are at the apex of current development, providing high sensitivity and specificity as well as speed and mobility in the case of the fluorescence polarization assay.

Single-step purification and evaluation of recombinant BP26 protein for serological diagnosis of Brucella ovis infection in rams

To investigate the value of the BP26 protein in the serological diagnosis of ovine brucellosis caused by Brucella ovis, recombinant BP26 protein was produced in Echerichia coli and purified for use in an indirect enzyme-linked immunosorbent assay (I-ELISA). The majority of the recombinant protein was recovered from the supernatant of sonicated recombinant E. coli cells in a soluble form. This facilitated the purification of the recombinant BP26 protein which was achieved by using ion-exchange chromatography. After one step of purification, the purity of the recombinant BP26 protein was analyzed by using SDS-PAGE, Coomassie blue staining, and Western blot with a monoclonal antibody (MAb) directed against the BP26 protein. The degree of purity appeared satisfactory so that it could be directly used in I-ELISA. Although the recombinant BP26-ELISA appeared less useful than I-ELISA using the B. ovis hot saline (HS) extract as antigen, the high number of sera from B. ovis infected rams found positive (90%) in the recombinant BP26-I-ELISA indicated that the BP26 protein may be an additional suitable antigen for serological diagnosis of B. ovis infection in rams.