Transitions in diagnostic tests used for detection of Mycobacterium avium subsp. paratuberculosis infections in cattle.

Diagnosis of infections with Mycobacterium avium subsp. paratuberculosis (MAP) is difficult due to a long incubation period and lack of tests which can accurately predict the future status of animals. Early detection of infectious animals is necessary to reduce transmission of MAP. The objective of this study was to determine the time from first detection of MAP-antibodies in milk ELISA to start of MAP shedding, for animals with various shedding patterns. An observational longitudinal study was carried out over 3 years. A total of 24,076 milk and 10,074 faecal samples were obtained from 1906 cows and tested using ELISA and FC, respectively. Cows were classified into 5 shedding groups based on their repeated FC: non-shedders (NS; n=1512 cows, 79.3% of total), transient (TS; n=36, 1.9%), intermittent (IS; n=137, 7.2%), low (LS; n=143, 7.5%), and high shedders (HS; n=78, 4.1%). Results showed that 5% of TS, 30% of IS, 60% of LS and 70% of HS were ELISA-positive at the date of first positive FC, and many HS (28%) and LS (14%) were positive >or=1 year prior to first detection of shedding. FC confirmed shedding within the first year after the positive ELISA in 10% of 328 cows with fluctuating ELISA compared with 35% of 445 cows with the last 2 or more ELISAs positive. To conclude, MAP-antibodies were generally detected prior to start of bacterial shedding, with difference between the various patterns of shedding, and a positive ELISA was useful for predicting that an animal would subsequently become infectious.