In order to assess the performances of paratuberculosis diagnostic tests, 5 young calves (1-2 weeks old) were experimentally infected by oral route. The animals were infected with 10 mg (10⁸ CFU) of M. avium subspecies paratuberculosis (ATCC 19698) per day during 10 days. Samples were taken periodically. The Cell Mediated Immunity (CMI) was assessed by the Interferon gamma assay and the lymphoproliferative responses after in vitro stimulation of blood samples with avian and bovine PPD's. Humoral responses were detected by an absorbed ELISA. Detection of M. avium subspecies paratuberculosis in faeces was done by classical mycobacteriology (HEYM + Mycobactin). Our results can be summarized as follows:

1. the animals showed fluctuating CMI responses in both Interferon gamma and lymphoproliferation assays. It took more than 12 weeks post-infection before some animals were consistently classified M. avium subspecies paratuberculosis positive by the Interferon gamma;
2. no detectable humoral responses could be evidenced, so far;
3. we were unable to culture M. avium subspecies paratuberculosis from faecal samples of these animals, although all tests and reagents passed the quality control and
4. no paratuberculosis clinical signs could be seen, until now.

All together these results emphasises the difficulties of an early and specific detection of M. avium subspecies paratuberculosis infected animals although a combination of available tests was used. More, our results suggest that the absence of positive serology as well as the absence of fecal excretion of M. avium subspecies paratuberculosis at the group level, could jeopardise "paratuberculosis-free" certification programs based on those techniques.