Abstract
Control of paratuberculosis in dairy herds is based on preventing the transmission of Mycobacterium avium subsp. paratuberculosis (Mptb) from cows to calves by management measures, supported by removal of cows excreting these bacteria by the fecal route (Mptb shedders). Fecal culture is the most accurate test for identifying Mptb shedders, but this technique is expensive and takes up to 16 weeks for results to be available. Serologic tests are inexpensive, rapid, and easy to perform. Of serologic tests, the complement fixation test (CFT) and absorbed enzyme-linked immunosorbent assay (ELISA) are the serologic tests used most frequently; the CFT is considered less accurate than the ELISA with respect to sensitivity and specificity. The commonly accepted absorbed ELISA is from the Australian Central Serum Laboratory. However, a European supplier has marketed a second ELISA that is supposed to be more sensitive in detecting Mptb shedders. These 2 absorbed ELISAs, designated ELISA-A and ELISA-B, and an in-house CFT were compared with data from 2 serum panels. The Mptb shedding panel consisted of sera from 198 culture-positive cows from 53 infected herds. The method used for culture of fecal samples was a modified Jørgensen method on individual samples. The Mptb shedder detection rate by the 3 serologic tests ranged from 29.8% to 39.4%. Detection rate for ELISA-A was lower than that for ELISA-B and CFT. For all 3 tests, detection rate was dependent on the level of Mptb shedding and the age of the animals. Detection rates increased as cattle age increased to 4 years. The specificity panel was initially composed of sera from 811 cows randomly selected from 41 herds without clinical paratuberculosis that were negative for Mptb based on whole-herd fecal culture. The modified Jørgensen method for culture was used on pooled fecal samples. Serologic test specificity ranged from 93.4% to 99.8%. The specificity of ELISA-A was higher than that of ELISA-B and CFT. Specificity of ELISA-B between herds was 75–100%. Specificity of CFT between herds was 62–100%. The low specificity of ELISA-B and CFT could not be explained by a higher sensitivity for Mptb-infected cows before onset of shedding, because in the 19 herds with 8 more subsequent negative whole-herd fecal cultures in the 4 years after sampling, specificity was not improved. The insufficient specificity of ELISA-B was not corrected sufficiently by heightening the cutoff value because Mptb shedder detection rate was lowered to 28.9%, equal to that of ELISA-A, and specificity only rose to 97%, much lower than that of ELISA-A. Taking into account the different test characteristics, serologic tests are a cost-effective alternative to fecal culture in high-prevalence herds. For certification programs, only ELISA-A is recommended because in a large number of nonsuspect herds specificity remained almost 100%.