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%.