Immunization with Recombinant TRP19 Reduces Clinical Severity of Experimental Ehrlichia canis Infection in Dogs

Canine monocytotropic ehrlichiosis (CME), caused by the intracellular bacterium <i>Ehrlichia canis</i>, is a significant tick-borne disease in dogs that requires effective vaccination strategies. This study aimed to evaluate recombinant TRP19 (rTRP19), a highly conserved immunodominant antigen, as a promising vaccine candidate against experimental <i>E. canis</i> infection in dogs, following its success in a mouse model. Fifteen <i>E. canis</i>-negative beagles were immunized intramuscularly with either 50-µg or 100-µg of rTRP19 in alum adjuvant or a PBS control, on days 0, 30, and 60. All dogs were then exposed intravenously to <i>E. canis</i> on day 90 and monitored for 120 days for clinical signs, hematological changes (platelet count, hematocrit, and body temperature), and antibody titers. The rTRP19 vaccine prototypes induced strong antigen-specific humoral responses. They caused a significant reduction in rickettsial load, with complete elimination observed in the 100-µg group by day 7 and in both vaccinated groups by day 14 of exposure. Furthermore, the mean body temperature in the 100-µg rTRP19 group was significantly lower than that of the control group, suggesting that the higher-dose vaccine mitigated febrile response. Collectively, these results suggest that rTRP19 vaccine prototypes hold promise in overcoming the clinical signs and hematological abnormalities of <i>E. canis</i> infection by inducing a strong antigen-specific antibody response.