The objective of this study was to evaluate the safety and efficacy of a killed Mycobacterium paratubercolosis vaccine (Silirum®, Pfizer Animal Health) using a bovine experimental infection model. Twelve newborn Holstein male calves were randomly assigned to one of two groups. The vaccinated calves (n=6) received a single dose of vaccine, administered subcutaneously in the side of the neck, at 14 days of age. Control calves were sham vaccinated with placebo. All calves were given an oral challenge of 10⁹ CFU live field strain Mycobacterium paratuberculosis (MAP), administered on days 35 and 36 of age. Body temperature and injection site diameter were measured periodically following vaccination. Blood samples were collected at various time points for measurement of antigen specific release of Interferon-gamma by peripheral blood cells (Bovigam®, Prionics) and for ELISA testing for detection of serum antibodies against MAP (Paracheck®, Biocor). Calves were euthanized at 98 days of age and 32 tissues collected for culture of MAP, using both solid media (Herrold's Egg Yolk Media) and liquid media (MGIT®, Becton Dickinson). Total number of tissues that were culture positive, as well as the total number of CFU/calf on HEYM were compared for the two groups. For liquid culture, time to signal positive (TTP) was compared for the two groups, with a more rapid TTP indicating a higher concentration of MAP in the original sample. MAP-induced IFN-gamma release by prescapular lymph node cells cultured in-vitro was measured.

Following vaccination, there was a transient rise in body temperature (approximately 1 degree F), significantly different from control, on Days 1 and 2 following vaccination, with a return to baseline on Day 3. Vaccinated calves had visible swelling at the injection site that persisted throughout the study, but swellings were not painful and did not develop drainage. Vaccination sites were culture-negative for MAP at the conclusion of the study.

ELISA testing for serum antibodies to MAP gave negative results for all calves in both groups. Vaccinated calves had significantly higher IFN-gamma release by peripheral blood cells, and by prescapular lymph node cells, compared with controls.

Vaccinated calves had significantly reduced colonization of tissues by MAP, compared with control calves, whether measured by CFU/calf in the HEYM system or TTP in the liquid media system. There were on average, 22 tissues positive per calf in the vaccinated group compared with 28 per calf in the control group. There was a greater than seven-fold reduction in CFU/gram tissue in vaccinated calves compared with control, and TTP in liquid culture was 10 days longer on average for vaccinated calves. All of the above differences were significant at P<0.05. We conclude that Silirum® vaccine was associated with reduced tissue colonization by MAP when administered to calves.