Transovarial transmission of <i>Babesia ovis</i> in <i>Rhipicephalus bursa</i>, confirmed by multi-generational experiments

Babesia ovis is a tick-transmitted intraerythrocytic apicomplexan parasite that causes ovine babesiosis, a disease with high economic impact in endemic regions. Although Rhipicephalus bursa is a known biological vector of B. ovis, the dynamics of vertical (transovarial) transmission are poorly understood. In the present study, the transovarial transmission of B. ovis was investigated experimentally in four continuous generations of R. bursa under laboratory controlled conditions. A total of 11 sheep were used during the study. An initial stabilate co-infected with B. ovis and Theileria ovis was inoculated into a splenectomized sheep, followed by an infestation with Babesia-free adult R. bursa. Study of dead tick bodies and larval pools by molecular methodology confirmed the elimination of T. ovis and the vertical transmission B. ovis. Rhipicephalus bursa ticks derived vertically from F₁ to F₄ generations kept high levels of infection (98% in F₃ adults), and consistently led to severe clinical babesiosis in both immunosuppressed and immunocompetent sheep. Although all feeding stages became infected while feeding on their hosts, only adult ticks were able to transmit the parasite to vertebrate hosts. Our findings demonstrate that B. ovis can be transmitted vertically through several tick generations, while preserving its ability to cause severe disease, even without selective pressure. The virulent, mono-infected B. ovis strain developed in this study will provide an infectious challenge model for future vaccine and pathogenesis studies under field-relevant conditions.