Comparison of an enzyme-linked immunospecific assay (ELISA) with the cold complement fixation test for the serodiagnosis of Brucella ovis infection

Diagnosis of canine brucellosis: comparison of various serologic tests and PCR

Canine brucellosis is an infectious and contagious disease associated with reproductive losses in breeding kennels. As a zoonotic disease, it poses a risk to human health, especially for veterinarians and breeders who handle materials potentially contaminated with Brucella canis. However, canine brucellosis is a neglected and underestimated disease given the difficulties in establishing a definitive diagnosis. We evaluated the frequency of detection of B. canis in 5 breeding kennels by using various serologic methods and PCR. Circulation of B. canis in these kennels was confirmed by bacterial isolation. The frequency of positive serologic results varied from 6.3% by AGID to 16.5% by dot-ELISA. There was no positive serology for smooth Brucella. PCR testing was positive in 13.9% of samples. The only detection tests with reasonable agreement were PCR and 2ME-MAT. The diagnosis of canine brucellosis remains challenging. The use of a single laboratory method, or even the use of different laboratory methods, may not be sufficient to reach a definitive diagnosis.

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.

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.

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.

Minor nucleotide substitutions in the omp31 gene of Brucella ovis result in antigenic differences in the major outer membrane protein that it encodes compared to those of the other Brucella species

The gene coding for the major outer membrane protein Omp31 was sequenced in five Brucella species and their biovars. Although the omp31 genes appeared to be highly conserved in the genus Brucella, nine nucleotide substitutions were detected in the gene of Brucella ovis compared to that of Brucella melitensis. As shown by differential binding properties of monoclonal antibodies (MAbs) to the two Brucella species, these nucleotide substitutions result in different antigenic properties of Omp31. The antigenic differences were also evidenced when sera from B. ovis-infected rams were tested by Western blotting with the recombinant B. melitensis or B. ovis Omp31 proteins. Twelve available sera reacted with recombinant B. ovis Omp31, but only four of them reacted with recombinant B. melitensis Omp31. These results validate previous evidence for the potential of Omp31 as a diagnostic antigen for B. ovis infection in rams and demonstrate that B. ovis Omp31, instead of B. melitensis Omp31, should be used to evaluate this point. The antigenic differences between the B. melitensis and B. ovis Omp31 proteins should also be taken into account when Omp31 is evaluated as a candidate for the development of subcellular vaccines against B. ovis infection. No reactivity against recombinant B. melitensis Omp31 was detected, by Western blotting, with sera from B. melitensis-infected sheep. Accordingly, Omp31 does not seem to be a good diagnostic antigen for B. melitensis infections in sheep. Two immunodominant regions were identified on the B. ovis Omp31 protein by using recombinant DNA techniques and specific MAbs. Sera from B. ovis-infected rams that reacted with the recombinant protein were tested by Western blotting against one of these immunodominant regions shown to be exposed at the bacterial surface. Only 4 of the 12 sera reacted, but with strong intensity.

Characterization of an immuno-dominant antigen in Brucella ovis and evaluation of its use in an enzyme-linked immunosorbent assay

A panel of 45 Brucella ovis serologically positive sera were tested in immunoblots against B. ovis outer membrane proteins Omp31 and Omp25, purified by preparative SDS-gel electrophoresis. Forty-three sera reacted with Omp31, while only 11 reacted with Omp25, suggesting that Omp31 is identical to the previously reported immuno-dominant 29-kDa protein. Attempts to purify Omp31 on a larger scale by using procedures such as ion exchange-, reversed phase-, affinity- and gel filtration chromatography suggested that the outer membrane proteins were aggregated with rough lipopolysaccharide. Only denaturing SDS-gel filtration chromatography was able to separate proteins of about 29 kDa from rough lipopolysaccharide but did not separate Omp31 from Omp25 in B. ovis preparations. When used in an enzyme-linked immunosorbent assay, this 29-kDa protein preparation was less sensitive and less specific than the routinely used heat-extracted B. ovis antigen. A readily available recombinant E. coli, expressing the gene for Omp31 from Brucella melitensis 16 M, was used to extract and enrich recombinant Omp31 by a temperature-dependent Triton X-114-based technique. When this material was used in immunoblots with the 45 sera from B. ovis-infected sheep and with 10 monoclonal antibodies, raised against B. ovis Omp31, major differences in the antibody reactivity between the recombinant B. melitensis Omp31 and the B. ovis Omp31 were found. Such differences were unexpected because of the known structural and immunological relatedness of outer membrane proteins from various Brucella species. These results indicated that the antibody-response in B. ovis naturally-infected sheep against the immuno-dominant Omp31 was directed against epitopes which were only accessible when the protein was aggregated with rough lipopolysaccharides, or which were formed after aggregation but were not present in the recombinant protein.

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

An indirect enzyme-linked immunosorbent assay (ELISA) for the serodiagnosis of Brucella ovis infection was developed. The assay uses a mouse monoclonal antibody to bovine IgG1 horseradish peroxidase (HRPO) conjugate that cross-reacts with immunoglobulin from sheep and a purified antigen from Brucella ovis. The ELISA data were read and analyzed according to a targeting procedure. The ELISA results were compared with a cold complement fixation test (CFT). Sera from 675 rams from three uninfected flocks were used to determine the ELISA cut-off value (O.D. 405 nm: 0.095) and the diagnostic specificity of the ELISA (100%) and the CFT (99.69% +/- 0.42). The ELISA cut-off value was corroborated by receiver operating characteristic (ROC) analysis. Six hundred and forty semen and serum samples from 419 rams from two naturally infected flocks were collected before and after mating-time during two consecutive years. All semen samples were cultured and Brucella ovis was isolated from 28 samples. Sera from the 28 rams with positive semen were used to determine the diagnostic sensitivity of the ELISA (96.43% +/- 6.8) and of the CFT (including suspected positive samples with titers of 1:5; 88.89% +/- 11.85). Considering the CFT suspicious and the anti-complementary reactions as positive resulted in a diagnostic sensitivity value of 89.28% +/- 11.46. Six hundred and ten serum samples from the 640 sera were used to determine relative sensitivity (excluding sera with 1:5) at: ELISA/CFT 97.26% +/- 3.74 and CFT/ELISA was 71.72% +/- 8.87. The percent agreement, beyond chance measured by the Kappa index was 79.7. Relative sensitivity ELISA/CFT (including 1:5 titers in the CFT as positive) was 94.9% +/- 4.83 and CFT/ELISA was 72.84% +/- 8.59. The Kappa index was 79.4.

Identification and characterization of immunodominant antigens during the course of infection with Brucella ovis

The seroresponse against Brucella ovis of 8 intrapreputially and 6 intravenously infected rams and 9 ewes infected through mating was analyzed by electrophoretic immunoblotting. Additionally, 87 sera from chronically infected rams that were shedding B. ovis in their semen, 226 sera from rams belonging to infected flocks, and 324 sera from false-positive complement fixation test (CFT) reactors were examined. In all infected animals, antibody reactivity was predominantly found against 5 B. ovis components of 8-12, 17, 19, 29, and 63 kD, of which the 29-kD antigen was most dominant in the seroresponse. Antibodies to the 29-kD component were present in 93-100% of the infected sheep in each infected group, whereas the frequency of antibodies to the 4 other components varied considerably among and within the different groups. No reactivity against the 29-kD antigen was found in the false-positive CFT reactors. By using monoclonal antibodies against known bacterial macromolecules, the immunodominant antigens were identified as rough lipopolysaccharide (8-12 kD), outer membrane proteins (17, 19, 29 kD), and a heat-shock protein (63 kD).

Serological and necropsy findings for rams infected withBrucella oviswhich were not identified by the complement fixation test

The eradication of Brucella ovis from a commercial flock of 36 Romney rams was complicated by four infected rams remaining undetected despite four successive flock examinations using the complement fixation test. These four rams were subsequently tested using an enzyme-linked immunosorbent assay and a gel diffusion test and shown to be infected by semen culture. All four rams could have been identified as infected at the initial test if the enzyme-linked immunosorbent assay had been used in addition to the complement fixation test. Although gross evidence of epididymitis was found in only one ram at necropsy, three had histological lesions of epididymitis and all four had a seminal vesiculitis.

Antibody response to Brucella ovis outer membrane proteins in ovine brucellosis

Hot saline extracts of Brucella ovis were composed of vesicles with outer membrane proteins (OMPs), lipopolysaccharide, and phospholipid as constituents. Extraction with petroleum ether-chloroform-phenol yielded a protein fraction free of detectable lipopolysaccharide, in which group 3 OMPs (28,500 apparent molecular weight [28.5K], 27.0K, and 25.5K) represented 81% of the total. Group 1 OMPs and 67.0K, 22.5K to 21.5K, and 19.5K to 18.0K proteins were also detected. Adsorption of immune sera with whole bacteria suggested that group 3 OMPs and 67.0K, 22.5K to 21.5K, and 19.5K to 18.0K proteins had antigenic determinants exposed on the surfaces of both B. ovis and rough B. melitensis cells but not on smooth B. melitensis cells. Antibodies to group 3 OMPs and the 67.0K protein in the sera of 93 and 87%, respectively, of B. ovis-infected rams were found by immunoblotting. Antibodies to other proteins were present in 67% of these animals. Compared with B. ovis-infected rams which had not developed lesions, rams with epididymo-orchitis had antibodies to a larger variety of proteins. Although ewes infected with B. melitensis also showed antibodies to OMPs, the immunoblot reactions were less intense.

Comparison of the dot-immunobinding assay with the complement fixation test for the detection of Brucella antibodies in sheep

A dot-immunobinding assay (DIA), using as antigen a sonic extract of Brucella abortus dotted on nitrocellulose bound to a plastic strip, was employed for the detection of Brucella antibodies in 666 sheep sera. The results were compared with the complement fixation test (CFT). All the 242 sera belonging to two flocks were found to be negative by DIA. CFT was negative in 239 cases, whereas three samples showed anti-complementary activity. Of the 424 sera from the remaining three flocks, 98 were positive by both tests and six were positive in DIA, but negative in CFT. In addition, 14 of the 19 anti-complementary sera were also positive by DIA.

Swine brucellosis in Indonesia

Brucella suis biotype 1 was isolated from 13.1% of the pigs slaughtered in Kapuk Jakarta, West Java and from 15.09% of the pigs slaughtered in Surabaya, East Java. The prevalence of B. suis by means of the Rose Bengal Plate Test, was 22.3% for West Java and 14.9% for East Java. The Rose Bengal Plate Test detected more B. suis infected animals (73% of the infected animals) than did the Complement Fixation Test (41%) and the Serum Agglutination Test (54.5%). The high infection rate is a potential health danger for the abattoir workers.

Comparison of lipopolysaccharide and outer membrane protein-lipopolysaccharide extracts in an enzyme-linked immunosorbent assay for the diagnosis of Brucella ovis infection

Brucella ovis hot saline extracts and petroleum ether-chloroform-phenol lipopolysaccharide were compared in an enzyme-linked immunosorbent assay for the diagnosis of B. ovis ram epididymitis. Hot saline extracts detected greater numbers of infected rams. Chemical characterization of the antigens showed that, although both contained lipopolysaccharide, hot saline extracts also contained outer membrane proteins. These proteins were active as antigens in Western blot tests with sera of infected rams, and therefore they explained the better diagnostic results obtained with hot saline extracts. However, compared with lipopolysaccharide, hot saline extracts showed a higher degree of cross-reactivity with sera from smooth B. melitensis-infected animals. This observation might be explained by the presence of B. ovis outer membrane proteins in hot saline extracts which lack the specificity necessary for serological identification of the Brucella species present.

Effect of passively-acquired antibodies and vaccination on the immune response to contagious ecthyma virus

Lambs which received colostrum from ewes vaccinated with contagious ecthyma (CE) virus and other lambs vaccinated with CE virus were compared for serum anti-CE immunoglobulin (Ig)G levels, delayed-type hypersensitivity (DTH) responses to CE viral antigen, and protective immunity to challenge with CE virus. Ewes vaccinated 3-4 weeks prior to parturition transferred CE antibody to lambs via colostrum. Although these lambs had higher levels of antibody at challenge than lambs vaccinated when 1-4 days old, only the vaccinated lambs were protected against challenge with CE virus at 1 month of age. Furthermore, the presence of colostrum-derived maternal antibody prevented an active antibody response in lambs to vaccination and/or challenge with CE virus, except where pre-inoculation titres were low. In contrast, the DTH response to CE viral antigen and induction of protective immunity by CE vaccination were not impaired by passively-acquired antibody. Actively immunised lambs could be distinguished from those only receiving passively-acquired antibody by the DTH response to heat-killed CE viral antigen.