Infections with Mycobacterium avium subsp. paratuberculosis(MAP) are characterised by long incubation periods. Diagnosis using repeated ELISA testing is preferred in the Danish control programme to detect infectious animals. Therefore, the time from testing positive by ELISA to MAP shedding was studied.

Repeated ELISA and FC results were available from 1892 dairy cows. The cows were divided into five shedding groups based on the FC results: Non-shedders (NS, n_NS=1507); potential transient shedders (TS, n_TS=40); intermittent shedders (IS, n_IS=116); low shedders (LS, n_LS=142); and high shedders" (HS, n_HS=87). Cows were defined as TS if a positive FC was followed by three negative tests; IS if had more than one positive FC, but no successive positive samples and the last sample was negative; LS and HS if had a series of positive samples, and FS if had a minimum of one sample with > 50 CFU / g faeces.

The time between entering the shedding group to testing positive by ELISA was assessed by a generalised additive model.

The results showed that 15 to 20% of cows in the three shedding groups IS, LS and HS were positive by ELISA one year prior to entering the shedding group. Among IS, 50% were ELISA positive when shedding was detected, whereas among low and high shedders 60% were ELISA positive when entering the shedding group. One year after detection of MAP shedding, 80 to 90% in the three groups had positive ELISA results. Only 9 (10%) of the HS cows were shedding > 50 CFU prior to being ELISA-positive. Among the TS-group, 40% had positive ELISA-reactions, of which most occurred 0 to 3 years after entering the TS-group. The latter result indicates that many cows that were classified as transient shedders were probably infected and should have been classified as intermittent shedders. In conclusion, although many shedding cows are detected by ELISA prior to shedding, a large proportion may only become ELISA-positive after shedding has started, but the amount of bacterial shedding is at low levels until the animal become ELISA-positive.