Atlas of gross, histologic, and immunohistologic anatomy of the lymphoid system in Egyptian rousettes (Rousettus aegyptiacus).

Chiropterans exhibit remarkable immunological adaptations that facilitate their function as competent reservoir hosts for diverse high-consequence zoonotic viruses while maintaining apparent clinical health. The Egyptian rousette (Rousettus aegyptiacus), a frugivorous pteropodid species, represents the sole confirmed natural reservoir for Marburg and Ravn viruses (family Filoviridae), yet comprehensive characterization of its lymphoid system architecture remains absent from the literature. This investigation presents the first systematic gross, histological, and immunohistochemical atlas of lymphoid tissues in this epidemiologically critical species, encompassing primary (bone marrow, thymus) and secondary lymphoid tissues (lymph nodes, spleen, mucosa-associated lymphoid tissue). Tissues from fetal, juvenile, and adult specimens were systematically evaluated to characterize age-related variations in lymphoid architecture and cellular composition dynamics. Immunohistochemical analyses employing cross-reactive antibodies against CD3, CD79a, Iba1, and Granzyme B elucidated the spatial distribution and organizational patterns of key lymphocyte populations and antigen-presenting cells across distinct lymphoid compartments. While overall lymphoid tissue organization demonstrated conservation relative to other mammalian species, several distinctive morphological features were identified, including prominently developed splenic marginal zones, extensive gut-associated lymphoid tissue networks, and organized nasopharyngeal and bronchial lymphoid structures. These anatomical specializations likely represent species-specific evolutionary adaptations underlying viral tolerance mechanisms and immunological equilibrium maintenance during persistent viral exposure. This comprehensive atlas provides an essential foundational resource for veterinary pathologists, comparative immunologists, and infectious disease investigators conducting research with R. aegyptiacus bat populations. By establishing anatomically and immunologically detailed reference standards for lymphoid tissues in this critical reservoir species, this work supports interpretation of experimental infection studies and may help guide future investigations of host-pathogen interactions, reservoir competence, and zoonotic spillover dynamics at wildlife-human interfaces.