A combination of tests based on detection of cellular and humoral responses were evaluated in a naturally exposed calf cohort over 8 months and compared to both fecal culture and PCR. The objectives of this study were:
- To evaluate the validity of intradermal tuberculin tests, gamma interferon and ELISA in a cohort of calves with reference to fecal culture and PCR results.
- To compare the findings from CMI or antibody-based tests in calves born from ELISA positive, suspect, and negative dams.
- To explore the association between the PTB status of the dams (determined by serology, fecal culture, PCR in milk and feces) and the status of their respective heifers (determined by humoral and CMI-based tests, and fecal culture/PCR).
Two dairy herds with 11% and 2% seroprevalence of PTB infection were the source of study heifers. A cohort of heifer calves born from sero-positive, suspect and negative dams was followed up until 30 months of age with the purpose of characterizing the cellular and humoral immunity of naturally exposed individuals. Enrollment of heifers started in November 2000 and is currently on going. Preliminary descriptive results on 308 dams and heifers indicate the following:
- Skin tests reactions in calves are more numerous when testing is performed between 4 and 6 months of age.
- The majority of the skin test reactions correspond to the cervical injection site rather that the caudal fold site.
- The ELISA in calves remains negative despite the repetitive intradermal injection of M. avium subsp. paratuberculosis PPD.
- There is a significant loss of heifers born from ELISA positive dams due to perinatal death or premature culling because of repetitive pneumonia or delayed growth.
- Skin test reactions in calves seem not to be associated with the dam's ELISA status.
