Global Trends in Vaccine Research on Ehrlichia and Anaplasma: A Bibliometric Review

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.