Bayet, M.; Nielsen-Leroux, C.; Rodrigues, V.; MEYER, D. F.

Ehrlichia ruminantium, the causative agent of heartwater disease, is an obligate intracellular bacterium that poses significant economic threats to livestock production in endemic regions. Current research models present substantial ethical, logistical, and economic constraints, particularly for studying host-pathogen interactions within arthropod vectors. Here we establish Galleria mellonella larvae as a tractable invertebrate infection model for Ehrlichia ruminantium, enabling experimental investigation of pathogen persistence and host-pathogen interactions in an arthropod system. Following infection, G. mellonella proved susceptible to E. ruminantium with moderate mortality and remarkable bacterial persistence. Using rhodamine-labeled bacteria and fluorescence microscopy, we tracked bacterial dissemination from injection sites to systemic distribution in characteristic segmental patterns throughout the larval body. Critically, we confirmed intracellular localization of E. ruminantium within hemocytes, the primary immune cells of G. mellonella. Quantitative PCR analysis revealed stable bacterial loads over the study period, indicating bacterial persistence within the host. These findings demonstrate that E. ruminantium can hijack the innate immune system of G. mellonella, similar to its behavior in natural hosts. The segmental bacterial distribution suggests exploitation of hemolymph circulation and sessile hemocyte populations, providing new insights into potential mechanisms of pathogen persistence. This model offers significant advantages: ethical acceptability, cost-effectiveness, experimental tractability, and compatibility with high-throughput screening approaches. The G. mellonella system represents a valuable complement to existing mammalian models and provides a unique platform for investigating arthropod-specific aspects of E. ruminantium biology, screening antimicrobial compounds, and understanding mechanisms of immune evasion that may inform strategies for heartwater disease control.