Evaluation of the potential virulence of Mycobacterium avium subspecies paratuberculosis isolates from cattle in Uganda

Understanding the pathogenesis of paratuberculosis (PTB), caused by Mycobacterium avium subspecies paratuberculosis (MAP), is essential in the management of this economically important disease. MAP strains are known to differ genotypically, which could contribute to differences in strain virulence and disease outcomes. The behaviour of MAP inside the host and the mechanisms utilized by MAP to overcome the host defence system are not entirely understood. This study aimed at evaluating the potential virulence of different MAP isolates from cattle in Uganda. This was achieved by analysing the cytokine expression profiles of RAW 264.7 macrophages when infected with four different MAP isolates: MAP1, MAP2, MAP3 and MAP4. Specific real-time PCR was used to determine the expression of different cytokines, probable virulence genes of MAP as well as MAP quantities inside the macrophages at different time points (3 h, 24 h, 48 h) post-infection (pi). There were considerable variations in the cytokine expressions among macrophages infected with the different MAP isolates. The expression of IL-6 in macrophages infected with MAP1 at 48 h pi was noticeably high compared to the other isolates; and this is a known strategy for MAP survival inside macrophages. The regulation of the probable virulence genes: kdpC, katG, papA2, impA, umaA1, and the anti-apoptotic (BCL-2) gene also varied considerably among the isolates. Isolates MAP1, MAP2 and MAP3 demonstrated higher virulent behaviour regarding intracellular replication and survival compared to MAP4 isolate. The behaviour of these MAP isolates clearly indicates variations in virulence and further research needs to focus on how this can be exploited in the design of PTB diagnostic and control strategies.