Bi-directional highways and super-seeder tissues underpin parasite dissemination in experimental visceral leishmaniasis.

Visceral leishmaniasis (VL) is a life-threatening parasitic disease caused by Leishmania donovani and L. infantum. Although VL is characterised by organ-specific immunopathology and parasitism of the spleen, liver and bone marrow, other tissues may harbour parasites without overt pathology. The mechanisms governing patterns of within-host dissemination and tissue tropism are largely unknown. We used a barcoded library of isogenic L. donovani and an ecological analysis framework to evaluate parasite population diversity across tissues and to map parasite dissemination in a murine infection model. We reveal an unexpected high degree of inter-connectivity between parasite populations demonstrating: i) continual bi-directional dissemination between viscera and skin; ii) the existence of "super-seeder" sites that act as hubs fuelling systemic spread; and iii) rerouting of dissemination "highways" following immune perturbation. These findings change our understanding of L. donovani pathogenesis, providing critical insights into infection dynamics with direct implications for relapse, treatment failure and transmission.