Evaluation of a serological Salmonella mix-ELISA for poultry used in a national surveillance programme.

A Mix-ELISA using lipopolysaccharide antigens from Salmonella enterica serotype Enteritidis and Typhimurium was evaluated using samples collected over time in the Danish salmonella surveillance programme for poultry. Serological samples (n = 42,813) taken from broiler-breeder flocks after a year of bacteriological monitoring with negative results were used for calculating the flock and individual test specificities, which were 0.997 and 0.999, respectively. Layer flocks from the table egg sector were used for calculation of positive predictive values. In the survey, flocks were examined for salmonella by Mix-ELISA and by faecal culture, and in case of a positive result in either of these a repeated, serological testing was performed, and 60 animals were organ-cultured. If one of these samplings was positive, the flock was declared salmonella infected. In a period of 3 months, 35 flocks were found to be positive in the routine samples. Of these, 32 were serologically positive, 2 both serologically and faecally positive and 1 flock only faecally positive. For flocks serologically positive in the surveillance programme, a positive-predictive value of 0.62 for organ culture positivity was found, and while considering serological follow-up samples, the value was 0.95.

Use of muscle fluid as a source of antibodies for serologic detection of Salmonella infection in slaughter pig herds.

Fluid drained from a muscle tissue sample was used as an alternative to serum for the detection of specific anti-Salmonella antibodies in an indirect LPS enzyme-linked immunosorbent assay (ELISA). In the first study, serum and muscle fluid from 3 pigs experimentally infected with Salmonella typhimurium showed parallel dilution-response relationships when ELISA optical density (OD) values were plotted against sample dilution. ELISA results obtained with serum diluted 1:400 corresponded to those from muscle fluid diluted 1:30. In a second study, using the predetermined dilutions of individually paired serum and muscle fluid samples from 103 pigs, a high degree of concordance between the serum ELISA and the muscle fluid ELISA was observed. Limits of agreement between the 2 methods were calculated as -8.9 to 12.3 OD%, which was considered acceptable. The muscle fluid ELISA had specificities of 0.91-1.0 and sensitivities of 0.80-0.89 at various cutoff values as compared with the serum ELISA. Muscle fluid is a useful postmortem alternative to serum when used with an ELISA to detect anti-Salmonella antibodies.