Abstract
Background Passerine birds are frequently infected with diverse haemosporidian parasites. While infections are traditionally considered benign in wild birds, recent studies demonstrated mortalities of passerine species due to exo-erythrocytic development of the parasites, which can damage organs in affected hosts. However, exo-erythrocytic development remains insufficiently investigated for most haemosporidian species and thus little is known about the virulence of tissue stages in wild passerine birds. The aim of the present study was to investigate natural haemosporidian infections in deceased Eurasian blackbirds (Turdus merula) and song thrushes (T. philomelos) and to determine parasite burden and associated histological effects.Methods For molecular analysis, blood and tissue samples from 306 thrushes were screened for Plasmodium, Haemoproteus and Leucocytozoon parasites by nested PCR. For the detection of parasite stages in organ samples, tissue sections were subjected to chromogenic in situ hybridization using genus- and species-specific probes targeting the rRNAs of parasites. Exo-erythrocytic parasite load was semi-quantitatively assessed and histological lesions were evaluated in haematoxylin-eosin-stained sections.Results 179 of 277 Eurasian blackbirds and 15 of 29 song thrushes were positive for haemosporidians. Parasites of all three genera were detected, with Plasmodium matutinum LINN1 and P. vaughani SYAT05 showing the highest prevalences. CISH revealed significant differences in exo-erythrocytic parasite burden between lineages in Eurasian blackbirds, with P. matutinum LINN1 frequently causing high parasite loads in various organs that were associated with histological alterations. Song thrushes infected with P. matutinum LINN1 and birds infected with other haemosporidian lineages showed mostly low parasite burdens. Two Eurasian blackbirds infected with Leucocytozoon sp. TUMER01 showed megalomeronts in various organs that were associated with inflammatory reactions and necroses.Conclusion This study suggests that P. matutinum LINN1, a common lineage among native thrushes, regularly causes high exo-erythrocytic parasite burdens in Eurasian blackbirds, which may result in disease and mortalities, indicating its high pathogenic potential. The findings further illustrate that the same parasite lineage may show different levels of virulence in related bird species which should be considered when assessing the pathogenicity of haemosporidian parasite species. Finally, the study provides evidence of virulent Leucocytozoon sp. TUMER01 infections in two Eurasian blackbirds caused by megalomeront formation.
Experimental infection by microparasites affects the oxidative balance in their avian reservoir host the blackbird Turdus merula
