Evaluation of two enzyme-linked immunosorbent assays for detecting Salmonella enterica subsp enterica serovar Dublin antibodies in bulk milk

Two enzyme-linked immunosorbent assays (ELISAs) for the detecting Salmonell a enterica subsp. enterica serovar Dublin antibodies in bulk milk were deve loped and evaluated for potential use in control programs. The ELISAs were based on either lipopolysacharide (LPS ELISA) or flagellar antigen (GP ELIS A). Sensitivity was determined with 79 case herds with a wide range of clin ical signs. Specificity was determined with 125 Dutch and 200 Swedish contr ol herds. The relation between antibodies in bulk milk, antibodies in serum , and the level of milk production of individual cows was studied with 61 c ase herds. The optimal optical density (OD) values of the LPS ELISA and the GP ELISA were determined to be 0.2 and 0.5, respectively. The sensitivitie s of the LPS ELISA and the GP ELISA were 54 and 63%, respectively, with, a specificity of 98% for both ELISAs with samples from the Dutch control herd s. The specificities for samples from the Swedish herds were 100% for the L PS ELISA and 95% for the GP ELISA. The sensitivity of the combination of te sts was 65% when samples were run in parallel, and the specificity was 100% when samples were run in series, irrespective of whether the samples came from Dutch or Swedish control herds. The variance (R-2) in the OD value for bulk milk samples could be explained by the percentage of sero positive la ctating cows in a herd with the LPS ELISA for 51% of the samples and with t he GP ELISA for 72%. The variance in the OD value was best explained by the combination of the percentage of seropositive lactating cows in the herd a nd the mean log(10) serum antibody titer for that herd (R-2 = 62% for the L PS ELISA and R-2 = 75% for the GP ELISA). Case herds more often tested nega tive by the ELISA with bulk milk when the percentage of seropositive lactat ing cows was less than 5%. It is concluded that both ELISAs with bulk milk can be used in control programs to distinguish between infected and noninfe cted herds. Specificity can be increased by using the two tests in combinat ion. Sensitivity was relatively low for both single tests and both tests co mbined.