A Longitudinal Study of Mycobacterium avium Subspecies paratuberculosis Infections in Cattle Using Lipid Antigens and Providing Evidence for Age-Associated Antibody Dynamics

The problem: Johne’s disease, caused by infection with Mycobacterium avium subspecies paratuberculosis (MAP), can be widespread in cattle herds. As a consequence of the slow progression of the infection and the delayed appearance of a humoral response, at any time few of the infected animals may be detected as positive with current serological tests. Many disease control initiatives have focused on removing infected animals by using serology to detect antibodies, principally to a purified MAP extract. However, the delayed humoral response to MAP extract, which can be preceded by shedding MAP in the faeces, limits the value of this control method. The approach: We used a longitudinal study to evaluate the timing and magnitude of antibody responses to lipid antigens. The method: Thirty breeding females from a beef cow herd with a history of Johne’s disease were recruited at 10–12 months old. Blood (serum) and faecal samples were collected at six-month intervals over five years. The serum samples were evaluated by enzyme-linked immunosorbent assay (ELISA) using six synthetic lipid antigens, each from a different class of chemical compound that are components of the mycobacterial cell wall. The responses were correlated with MAP infection status as determined by faecal culture, a commercial serum antibody ELISA (ID Screen®), and interferon-γ release assays. Nineteen animals were positive using either culture or ID Screen® at one or more time points; eleven animals were never positive in either assay. Results: In the lipid ELISA, the median responses for all animals were significantly higher at time point 5 (when the animals were three years old) than those at earlier time points. The timing of this response is compatible with, but does not prove, a shift in immune-response profile. For the antigens MOD171 and RT237F2, the responses for each individual animal were higher at time point 5 than at time point 4. Outcome: The increase in response to lipid antigens MOD171 and RT237F2 between the ages of two and four years may indicate infection with MAP at an earlier stage than is achieved with the established serology tests.