Exo-erythrocytic development of Leucocytozoon parasites (Haemosporida, Leucocytozoidae) in song thrushes Turdus philomelos /

Leucocytozoon parasites (Haemosporida, Leucocytozoidae) are haemosporidians whose diversity, exo-erythrocytic development and potential vectors are the least studied. The knowledge about their exo-erythrocytic development and pathogenicity is fragmentary, resulting in an incomplete comprehension of the impact of these parasites on avian hosts. For a long time, Leucocytozoon infections were considered benign to wild birds, even though they were virulent in poultry and responsible for some wild bird population declines. This study aimed to investigate the presence of Leucocytozoon species exo-erythrocytic stages in song thrushes Turdus philomelos using conventional histological techniques (sections stained by H&E) and chromogenic in situ hybridization (CISH). Tissues from ten birds (seven naturally infected and three opportunistic samplings) were examined using both methods. Parasite lineages were identified from blood samples using PCR-based techniques. Leucocytozoon species meronts were found in five individuals (in four birds using H&E staining protocol, and in three in CISH-treated histological sections). Meronts were found mainly in the kidneys, but some meronts were also present in the lungs. It was possible to observe different maturation stages of meronts in the same bird individual, indicating an asynchronous development. Cytomeres were readily visible in developing meronts. One megalomeront-like structure was present close to a blood vessel in the heart. It was covered with a prominent capsular-like wall. No inflammatory reaction or necrosis was seen in the tissues surrounding the meronts or the megalomeront-like structure. We could confirm the transmission of three Leucocytozoon lineages (lTUPHI14, lSTUR1 and lTUPHI13) in Europe, and add evidence of the transmission of two Plasmodium lineages, including Plasmodium circumflexum (pTURDUS1), and Haemoproteus asymmetricus (hTUPHI01). We call for further research to better understand Leucocytozoon parasite exo-erythrocytic development.