Haemosporidioses in wild Eurasian blackbirds (Turdus merula) and song thrushes (T. philomelos): an in situ hybridization study with emphasis on exo-erythrocytic parasite burden

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.

Geographic and host distribution of haemosporidian parasite lineages from birds of the family Turdidae

“…Phanerozoites of P. giovannolai develop almost exclusively in the spleen of canaries but were never seen in the brain, whereas those of P. matutinum are found in numerous organs [ 1 ]. A comprehensive study on haemosporidioses of T. merula and T. philomelos confirmed that phanerozoites of P. matutinum pLINN1 develop in multiple organs [ 86 ], but tissue stages of pLINN1 were not yet studied in canaries. Additional experimental infections of canaries with pLINN1 could help confirm differences between P. giovannolai and P. matutinum pLINN1 regarding the location of tissue stage formation.…”

Effects of blood parasite infections on spatiotemporal migration patterns and activity budgets in a long‐distance migratory passerine

“…As Plasmodium infections (known to occur with lower average infection intensities than Haemoproteus parasites) and infections with both haemosporidian genera in particular led to such delays indicates that it is not infection intensity per se but rather a systemic effect of certain stages in the life cycle of parasites belonging to the genus Plasmodium . Studies on differential pathogenicity of haemosporidians revealed that specific lineages of Plasmodium parasites can be much more pathogenic than Haemoproteus infections, despite their relatively low parasitaemia (Himmel et al., 2020; Ilgūnas et al., 2016). Unfortunately, our sample size is insufficient for a lineage‐specific comparison.…”

A citizen science-based survey of avian mortality focusing on haemosporidian infections in wild passerine birds

“…For example, signals found in the lumen of blood vessels measuring up to the size of erythrocytes were regarded as blood stages, whereas signals located in other host cells and exceeding the size of erythrocytes were regarded as exoerythrocytic meronts. Detailed descriptions and examples on how CISH signals were distinguished can be found in [ 50 ]. In addition, corresponding HE-stained sections were checked to confirm presence or absence of exoerythrocytic stages.…”

“…Exploiting such archives of tissue samples, previous retrospective studies in Austria revealed an association of blackbird mortalities with the presence of widespread haemosporidian lineages [ 30 , 50 ]. Notably, severe parasite loads were linked to infections with Plasmodium matutinum LINN1, pointing towards its pathogenic role in these birds’ mortalities.…”