Sensitivity of serological methods for detecting antibody in vaccinated and non-vaccinated Brucella-infected cows

Field evaluation of an indirect ELISA for detection of brucellosis in lowland Bolivia

Bovine brucellosis exists endemically at an estimated prevalence of 10% in the developing dairy industry of Santa Cruz in tropical Bolivia. This paper describes field testing of an FAO/IAEA indirect ELISA for brucellosis, as a possible replacement confirmatory test for the complement fixation test (CFT). The ELISA and CFT were compared on sera from 3 cattle populations: a non-vaccinated negative population, an S19-vaccinated negative population, and a brucellosis-positive population of unknown vaccination status. The CFT and ELISA showed excellent specificities of 100% and 98% respectively against the negative non-vaccinated group. The CFT maintained a specificity of 98% against the S19-vaccinated negative group, but ELISA specificity fell to 83% using a cut-off of 20% of positive control, and 94% using a cut-off of 40% of positive control. Against sera from the positive population, the ELISA gave many more positive reactions than the CFT, probably a combination of both higher sensitivity and lower specificity. It is concluded that as Santa Cruz is entering a phase of brucellosis control rather than eradication, the extra sensitivity of the ELISA is not valuable enough to risk a higher level of false positive reactions, especially as S19 vaccination is being increasingly used.

Antibody isotype response in adult cattle vaccinated with Brucella abortus S19

In a chronological study of sera collected from eight adult cattle vaccinated with 3 X 10(-10) cfu of Brucella abortus S19, antibody of each of the four major isotypes was measured by indirect enzyme immunoassay (ELISA) and by direct and modified complement fixation tests (CFT). Six of the cattle gave antibody responses to the vaccine strain that commenced between days 5 and 8 for all the isotypes in the ELISA, peaked by 1 to 4 months and then declined to low levels by 10 months. Direct CFT and modified CFT titers were measurable by 7 or 8 days post-vaccination, and peaked by 1 month; direct CFT titers disappeared by 5 months while the modified CFT titers lingered for 10 months. Two animals gave cyclical direct CFT and modified CFT antibody responses, a cyclical IgG1 response, a low IgG2 and an elevated IgA response. The amplitude of the cycles was uniform over three cycles while the wavelength increased with time. A year post-vaccination, B. abortus S19 was isolated twice from milk from one of the animals (no attempt was made to culture B. abortus from the other). Sera from B. abortus naturally infected cattle were analysed for comparison.

A review of enzyme immunoassay for detection of antibody to Brucella abortus in cattle

Enzyme immunoassay has gained wide acceptance for serological diagnosis of bovine brucellosis because of its ability to detect antibody of all isotypes unlike the conventional tests. The indirect enzyme immunoassay, however, presents several parameters that require careful analysis. These parameters include the choice of antigen and antiglobulin-enzyme conjugate reagents for use in the assay, dealing with the large amount of data the semi-automatic or automatic assay can generate and the inter- and intralaboratory standardization and quality control. This review considers the various methods described in the literature and, briefly, how some of the problems have been overcome or how they might be dealt with.