Human monocytic ehrlichiosis, an emerging tick-borne disease, is caused by Ehrlichia chaffeensis. Infections with the pathogen are also common in the canine host. Our previous studies demonstrated that functional disruption within the E. chaffeensis phage head-to-tail connector protein gene results in bacterial attenuation, creating a modified live attenuated vaccine (MLAV). The MLAV confers protective immunity against intravenous and tick transmission challenges one month following vaccination. In this study, we evaluated the duration of MLAV protection. Dogs vaccinated with the MLAV were challenged with wild-type E. chaffeensis via intravenous infection at 4-, 8-, and 12-months post-vaccination. Immunized dogs rapidly cleared the wild-type pathogen infection and tested positive for bacteremia less frequently than unvaccinated controls. While immune responses varied among dogs, vaccinees consistently mounted IgG and CD4+ T-cell responses specific to E. chaffeensis throughout the assessment period. Our findings demonstrate that MLAV-mediated immune protection persists for at least one year against wild-type bacterial infection, marking a major advancement in combating this serious tick-borne disease. The data presented here serve as the foundation for further studies, elucidating the molecular mechanisms underlying virulence and vaccine development and aiding in preventing the diseases caused by E. chaffeensis and other tick-borne rickettsial pathogens.

Ehrlichia and Anaplasma are important tick-borne obligate intracellular bacteria that cause clinically and economically significant infections in humans and animals. Despite their importance, vaccine development for these pathogens remains limited, and the overall progression of vaccine-related research in this field has not been quantitatively characterized. In this study, we conducted a comprehensive bibliometric analysis to characterize the trajectory and the research landscape of vaccine-related research for these pathogens. The literature was retrieved from the Web of Science Core Collection using the search terms (Ehrlichia OR ehrlichiosis OR Anaplasma OR anaplasmosis) AND (vaccine OR vaccines). A total of 483 publications published between 1965 and 2025 were included in the analysis. Bibliometric analyses were conducted using Biblioshiny (Version 4.3.3) and VOSviewer (Version 1.6.20) to assess publication trends, citation patterns, leading countries, institutions, and major research themes. The literature showed gradual growth over time, with more consistent publication output from the late 1990s onward. Scientific output was concentrated in a relatively small number of countries and institutions, led by the United States, South Africa, Brazil, Spain, and France. Keyword frequency and co-occurrence analyses showed that the field has been shaped mainly by bovine anaplasmosis, heartwater, and tick-associated vaccine research, with Anaplasma marginale and Ehrlichia ruminantium emerging as dominant pathogen-focused themes. Major surface proteins, msp1a, subolesin, and Bm86, appeared frequently, indicating continued interest in both pathogen-derived and tick-associated vaccine targets. In contrast, human and canine pathogens such as Ehrlichia chaffeensis, Anaplasma phagocytophilum, and Ehrlichia canis were less represented. Our analysis maps the research landscape of Ehrlichia and Anaplasma vaccine development, quantifying how research activity has been distributed across pathogens, geographic regions, institutions, collaborations, and vaccine-related themes. It also identifies areas that remain comparatively less represented in the published literature and provides a quantitative baseline for future comparisons with independent measures of disease burden and research funding data.