Rhipicephalus microplus and Its Impact on Anaplasma marginale Multistrain Infections in Contrasting Epidemiological Contexts

Bovine anaplasmosis is a disease caused by <i>Anaplasma marginale</i>, a tick-borne bacterial pathogen with global distribution, primarily determined by the range of its vector. In Argentina, <i>Rhipicephalus microplus</i> is the main species associated with <i>A. marginale</i> transmission, even though this bacterium can also be mechanically transmitted. We studied complex infections (more than one <i>A. marginale</i> variant) in naturally infected bovines from two different epidemiological contexts: a region with the tick vector and a tick-free region. In the tick-free area, symptomatic infections were associated with a single <i>A. marginale</i> genotype, while asymptomatic bovines from the same herd remained chronically infected with a low number of genotype variants. By contrast, in the region where <i>R. microplus</i> is present, the only symptomatic bovine showed highly diverse infections, with 19 distinctive genotypes. Additionally, <i>A. marginale</i> genotypes were also detected in tick tissues. These findings, together with previous data, indicate that <i>R. microplus</i> harbors <i>A. marginale</i> populations that are maintained through tick generations by means of transovarial transmission. Furthermore, this tick species is responsible for maintaining <i>A. marginale</i> diversity in the bovine host over time through coinfection and superinfection events.