Leucocytozoon spp. exo-erythrocytic and blood stages in naturally infected Great tits Parus major. Individual H44/21R with Leucocytozoon spp. (molecular co-infection) (A–C) presented blood stages in oviduct (A) and meront development found in the kidneys (B, C). Tissue stages visualized in H&E-stained preparations were confirmed by chromogenic in situ hybridization (CISH) using a Leucocytozoon genus-specific probe (Leuco18S) indicated by a purple signal in the inserts of panels. Individual H58/21R infected with Leucocytozoon sp. (cytochrome b lineage lPARUS16) (D–F) had blood stages in the liver vessels (D) as well as meronts in the liver (E) and kidneys (F). All meronts seemed to be growing, as merozoites were not recognizable. Individual H83/21R infected with Leucocytozoon sp. (lPARUS22) (G–I) had blood stages in the spleen (G) and a megalomeront developing in the heart (H, I). The megalomeront, covered by a thick eosinophilic wall, showed growing cytomeres. Black arrowheads: gametocytes. Black triangle arrowheads: meronts. Wide black arrow: enlarged host cell nucleus. Simple black arrow: megalomeront-wall. Scale bars: A, D, G, I 20 μm. B–C, E–F, H 50 μm. (insert I, 50 μm. All remaining inserts, 20 μm).