Influence of species of negative control sera on results of a brucellosis fluorescence polarization assay

Authors: Olsen, Steven; CRAWFORD, LAUREN - Orise Fellow; FUENTES, ANTONIO - Montana State Diagnostic Laboratories; KOSTOVIC, MILADIN - Diachemix Corporation; Boggiatto, Paola

Submitted to: Journal of Veterinary Diagnostic Investigation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2020
Publication Date: 11/19/2020
Citation: Olsen, S.C., Crawford, L., Fuentes, A., Kostovic, M., Boggiatto, P.M. 2020. Influence of species of negative control sera on results of a brucellosis fluorescence polarization assay . Journal of Veterinary Diagnostic Investigation. 33:67-72. https://doi.org/10.1177/1040638720970888.
DOI: https://doi.org/10.1177/1040638720970888

Interpretive Summary: Brucellosis is a zoonotic bacterial disease for which animals serve as the primary source for transmission of infection to humans. Control of brucellosis in animals is primarily by use of serologic tests to detect infection. The florescence polarization assay is a serology test with high sensitivity and specificity that is widely used in the United States for brucellosis testing. This diagnostic test, which was initially developed for use in cattle, uses cattle sera for defining positive and negative responses. However, use of cattle sera may influence results when samples from other species are evaluated. In this study, we evaluated species differences in test performance and demonstrated that the source of the background sera can influence responses on the FPA test. However, because positive responses in infected animals demonstrate a greater magnitude change in FPA values than differences that occur between background sera, we found that test interpretation as positive, suspect or negative was not influenced by species differences in background . We also demonstrated that RB51 vaccination or persistent infection with RB51 does not cause positive responses on the FPA diagnostic test. This data will be of interest to regulatory officials and livestock producers as it clarifies the performance characteristics of the FPA assay for detecting brucellosis infection.

Technical Abstract: Responses of sera from cattle, bison, elk, and swine representing control, early vaccination (4 to 8 weeks), late vaccination (21 to 28 weeks) or booster vaccination, early after experimental challenge (2 to 4 weeks), and late after experimental challenge (8 to 21 weeks), were evaluated in the brucellosis fluorescence polarization assay (FPA; n=10/species/trt) using negative sera from cattle, bison, elk, and swine as background (n=5/species). Sera from cattle shedding B. abortus strain RB51 in milk were also evaluated against the background sera. Our data indicated that species of background sera could influence (P<0.05) delta mP on the FPA. However, in general, the species of background sera did not alter interpretation of FPA responses in control, vaccinated, or infected animals. Even after repeated RB51 vaccinations in bison, cattle, or elk, or in cattle shedding RB51 in milk, serologic responses on the FPA assay remained negative. Species differences in FPA responses were noted as elk develop robust humoral responses very quickly after infection that result in strong positive responses on the FPA assay. In cattle and bison, humoral responses appear to develop over a longer period of time and positive responses with the FPA appear to be more definitive at later times after infection. Specificity of the FPA assay was greatest for elk in early challenge samples and bison in late challenge samples. Data from the current study and others suggests that for natural hosts of B. abortus, the FPA assay is a sensitive and specific test for detecting infected animals.