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

Canine monocytotropic ehrlichiosis (CME), caused by the intracellular bacterium Ehrlichia canis, 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 E. canis infection in dogs, following its success in a mouse model. Fifteen E. canis-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 E. canis 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 E. canis infection by inducing a strong antigen-specific antibody response.