Abstract
BackgroundBrucellosis is a zoonosis whose incidence is not declining worldwide despite the global effort to control the disease. Accurate and precise diagnosis is a crucial step in any prophylaxis program but single tests to unequivocally detect animals infected with Brucella spp. are currently unavailable. In Italy, serological diagnosis of bovine brucellosis is performed with two official tests: a rapid agglutination test (i.e., Rose Bengal Plate test, RBPT) and a complement fixation test (CFT) that detect antibodies directed mainly to the smooth lipopolysaccharide (S-LPS). Neither of the two tests is able to avoid the detection of false positive serological reactions (FPSRs) caused by bacteria sharing S-LPS components with Brucella spp. and responsible for the single reactors (SR) phenomenon. A B. melitensis R strain-based ELISA showed a good diagnostic performance in unravelling FP animals; however, since a limited number of animals were analyzed in that study, a large field study was conducted here to discriminate between Brucella-infected from FP animals, with the final aim of reducing the unnecessary slaughter of the latter. An ELISA based on a R strain of Brucella, i.e., Brucella melitensis B115, was employed to measure specific IgG responses in a collection of bovine sera (n = 648). Sera were obtained from 180 farms (either officially brucellosis-free or not brucellosis-free) recruited during an extended period of time (2014–2018) and were preliminarily assayed with the official tests by the Italian Reference Centers and then subjected to the ELISA.ResultsNegative sera, when subjected to the ELISA, gave O.D. values below the cutoff; SR sera, i.e. RBPT positive and CFT negative, as well as double positive (DP) sera, i.e. RBPT and CFT positive, gave O.D. values that were below the cutoff. All positive sera, i.e. from Brucella-infected animals, were RBPT positive and CFT positive (ICFTU ranging from 20 to 1280) and gave ELISA O.D. values above the cutoff.ConclusionsThe B. melitensis B115-based ELISA systematically unravelled all false positive (FP) sera while confirming the diagnosis in Brucella-infected animals. Thus, the test employed in the present study may complement the official assays to avoid the costly slaughter of FP animals.
Highlights
Brucellosis is a zoonosis whose incidence is not declining worldwide despite the global effort to control the disease
Both tests are routinely performed by the Italian Reference Centers (Istituti Zooprofilattici, IZS) and use whole bacteria as antigen, i.e., Brucella abortus (S 99 strain), to detect antibodies directed against the immunodominant O-chain of smooth lipopolysaccharide (S-LPS) of Brucella [10]
Negative sera (n = 259; group A, Table 1) were from officially brucellosis-free farms, they tested both Rose Bengal Plate test (RBPT) and complement fixation test (CFT) negative and when subjected to the ELISA they all gave an O.D. value below the cutoff value (O.D. 0.143) which was previously determined by a ROC analysis [22] and was not significantly different from that calculated in the present study (O.D. 0.141; p > 0.05)
Summary
Brucellosis is a zoonosis whose incidence is not declining worldwide despite the global effort to control the disease. In Italy, serological diagnosis of bovine brucellosis is performed with two official tests: a rapid agglutination test (i.e., Rose Bengal Plate test, RBPT) and a complement fixation test (CFT) that detect antibodies directed mainly to the smooth lipopolysaccharide (SLPS). Diagnosis of bovine brucellosis is based on two official serological tests: a rapid agglutination test (i.e., Rose Bengal Plate test, RBPT) and a complement fixation test (CFT) [9] Both tests are routinely performed by the Italian Reference Centers (Istituti Zooprofilattici, IZS) and use whole bacteria as antigen, i.e., Brucella abortus (S 99 strain), to detect antibodies directed against the immunodominant O-chain of smooth lipopolysaccharide (S-LPS) of Brucella [10].
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