CLUSTERING OF PIGS BASED ON THE ANTIBODY RESPONSES TO SALMONELLA SPP.

been exposed, since Salmonella is commonly subclinical in pigs. 1 Although a cut-off can be based on the antibody responses from either gnotobiotic or SPF pigs 5 , these Salmonella-free pigs may not represent the Salmonella-negative pigs of the target population, since pigs in commercial farms may have very different immune stimulation than gnotobiotic or SPF pigs. One method that may overcome this limitation is cluster analysis, a method that can be used to set empirically derived breakpoints from the distribution of OD values. 4 Cluster analysis may be also useful for differentiating subpopulations of pigs with regard to different exposure status to Salmonella spp. Additionally, cluster analysis may be applied to select appropriate control sera. This study was conducted to describe natural clustering patterns of antibody responses to Salmonella among commercially produced pigs, to select field -based control sera, and to examine the possibility of setting cut -off values using the output of cluster analysis. Materials and Methods Serum samples were collected at slaughter from 300 pigs from among approximately 20 herds. At a commercial slaughter plant, eve ry tenth pig was selected on the slaughter chain over a three -hour period each day for two days. A multiple -antigen ELISA was performed to detect antibodies against Salmonella spp. by using a modification of an existing protocol. 3 Briefly, this assay used four strains of Salmonella spp. to coat microtiter plate wells, test sera were incubated, and a alkaline-phosphatase conjugate system was used to develop color. Optical densities were recorded.