Serologic reactions against Salmonella in samples from broiler parent stock with and without preceding colibacillosis: A case-control study

In the Danish Salmonella Control Program, eggs from broiler parent flocks a re surveyed by serologic analysis every 4 wk for antibodies against Salmone lla lipopolysaccharide O-antigens 1, 4, 5, 9, and 12 (Mix-enzyme-linked imm unosorbent assay [ELISA]) and 6 and 7 (Infantis-ELISA). The antibody respon se is measured in percentage optical density (OD%) of a strong positive rea ction, and the cutoff value has been determined to be 40 OD%. Two or more r eactors above 40 OD% will place the parent flock under suspicion. There has been concern about possible cross-reactions between Salmonella spp. and ot her Enterobacteriaceae, e.g., Escherichia coli, because a high specificity of a Salmonella antibody test is desirable. Moreover, false-positive Salmon ella results have economic consequences and impede planning the production. A case-control study based on cases of clinical E. coli infections (colibac illosis) from two Danish hatcheries, supplying about 62% of the Danish broi ler production, is described. In order to eliminate a possible bias from ag e and season, the controls were matched on age of the birds and on time of submitting the samples. This study shows that flocks with preceding colibac illosis did nor have higher salmonella reactions than matched flocks withou t a preceding colibacillosis. This observation was confirmed in longitudina l studies.