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

## TL;DR

A new vaccine using the TRP19 protein significantly reduced the severity of Ehrlichia canis infection in dogs, showing promise for future canine vaccines.

## Contribution

The study demonstrates that the recombinant TRP19 protein is a promising vaccine candidate for preventing Ehrlichia canis infection in dogs.

## Key findings

- Vaccination with rTRP19 induced a strong and sustained antibody response in dogs.
- The higher 100-µg dose of rTRP19 significantly reduced bacterial load and fever in vaccinated dogs.
- Both vaccinated groups showed complete elimination of the bacteria by day 14 after exposure.

## Abstract

Canine monocytotropic ehrlichiosis (CME), a tick-borne disease caused by the bacterium Ehrlichia canis, is a major global health concern for dogs. Currently, there is no effective commercial vaccine available. This study aimed to evaluate a new vaccine candidate, the recombinant TRP19 protein (rTRP19), for its ability to protect dogs against experimental E. canis infection. We vaccinated beagles with two different doses of rTRP19 and then exposed them to the infectious agent. The results showed that vaccination with rTRP19 elicited a strong and sustained antibody response in the dogs. Crucially, the vaccine led to a significantly lower rickettsial load (bacterial numbers), particularly at the higher 100-µg dose, compared to the unvaccinated control group. Fourteen days after exposure, both vaccinated groups showed a complete absence of the bacteria. The higher dose of the vaccine also helped reduce fever, a key clinical sign of acute ehrlichiosis. These findings are important because they demonstrate that rTRP19 is a promising vaccine prototype that can induce a measurable immune response against CME in dogs. This is a crucial step toward developing a much-needed tool that can mitigate the impact of monocytotropic ehrlichiosis on canine health worldwide.

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.