Salmonella diagnosis in pig production: methodological problems in monitoring the prevalence in pigs and pork

Salmonellosis is an important foodborne infection in industrialized and developing countries. Especially for human Salmonellosis caused by Salmonella enterica serovar Typhimurium, pigs and pork are the major sources of infection. Mitigation and control strategies that result from surveillance programs attempt to reduce or even eradicate Salmonella in pork to lower consumers' risks. The methodology for Salmonella screening in pigs is generally based on antibody detection at slaughter with meat juice as the sample matrix. The instructions to most commercial enzyme-linked immunosorbent assay (ELISA) kits for the detection of Salmonella antibodies state that their product is suitable for antibody detection in meat juice and sera. In the present study, we show that it is essential to recalculate the percent optical density (OD%) data obtained from meat juice by the ELISA (IDEXX HerdCheck swine Salmonella) by the following regression equation: OD%sera = -70.5587 + 128.1490/ {1 + exp[(-18.8969 - OD%meatjuice)/27.6032]}(1.1771), r = 0.87, to compare results with those obtained from sera. By this regression equation, we were able to compare the Salmonella antibody levels (classified as <10, 10 to <20, 20 to <40, and > or =40 OD%) for sows, growers, and slaughter pigs. We identified significantly higher numbers of growers with lower OD% levels than for sows and slaughter pigs. Without a recalculation of the meat juice results, the higher fraction of samples with low OD% values led to an underestimation of the actual seroprevalence.