Co-infecting Haemoproteus species (Haemosporida, Apicomplexa) show different host tissue tropism during exo-erythrocytic development in Fringilla coelebs (Fringillidae)

An Overview of the Neglected Modes of Existence in Avian Haemosporidian Parasites

Haemosporidian parasites (Apicomplexa, Haemosporida) are diverse obligatory heteroxenous protists, which infect all major groups of terrestrial vertebrates and use dipterous blood-sucking insects as vectors. These pathogens are responsible for various diseases, including malaria, which remains an important human and animal illness. In the wild, haemosporidians are particularly diverse in reptiles and birds in tropical countries, where they are flourishing. Avian haemosporidians have been particularly extensively investigated, especially due to their high prevalence and global distribution, including the countries with cold climates. The general scheme of the life cycle of haemosporidians is known, but the details of development remain insufficiently investigated or even unknown in most of the described parasite species, suggesting the existence of knowledge gaps. This attracts attention to some recent observations, which remain fragmentary but suggest the existence of formerly neglected or underestimated modes of the haemosporidians' survival in vertebrates. Such findings are worth discussion as they indicate the novel directions in wildlife haemosporidian research. This article overviews some recent findings, which call for broadening of the orthodox views on modes of existence of haemosporidian parasites in avian hosts. Among them are the role of blood merogony in the long-lasting persistence of malaria parasites in birds, the role of gametocytes in the long-lasting survival of Haemoproteus species in vertebrates, the possible reasons of undetectable avian Haemoproteus infections due to peculiarities of exo-erythrocytic development, and the plausible factors driving the narrow vertebrate host specificity of Haemoproteus species.

Leucocytozoon infections in tits (Aves, Paridae): blood and tissue stages investigated using an integrative approach

Leucocytozoon species are cosmopolitan and prevalent avian parasites, with some infections being lethal, mainly due to the exo-erythrocytic development of the parasite in bird tissues. The patterns of exo-erythrocytic development in Leucocytozoon spp. infections in wild birds remain poorly studied. This study investigated the development of Leucocytozoon spp. tissue stages in tits (Paridae). Great tits (Parus major), Blue tits (Cyanistes caeruleus), and Coal tits (Periparus ater) were screened for infections using an integrative approach that consisted of microscopic analysis of thin blood smears, histological techniques, chromogenic in situ hybridization (CISH), PCR-based methods, and phylogenetic analysis. In total, 41 individuals were analyzed (eight naturally infected that were selected and euthanized, and 33 found dead in the wild and opportunistically sampled). Among the naturally infected birds, all individuals that were microscopically positive for Leucocytozoon species were also PCR-positive for these parasites. Co-infections with Plasmodium spp. and Haemoproteus spp. were commonly found, mainly among the opportunistically sampled birds. Two morphotypes were identified, Leucocytozoon majoris (Laveran, 1902) and Leucocytozoon fringillinarum Woodcock, 1910. Tissue stages were present in three birds sampled exclusively during the non-breeding season, two of them with meronts developing in the kidneys and liver, and one individual with a megalomeront in the heart. All the exo-erythrocytic stages were confirmed to be Leucocytozoon spp. by CISH using a Leucocytozoon genus-specific probe. Phylogenetic analysis placed parasite lineages with different morphotypes in separate clades. The developmental patterns of exo-erythrocytic stages of Leucocytozoon spp. in naturally infected passerines are poorly understood, requiring further research.

Parasite spillover rather than niche expansion explains infection of host brain by diplostomid eye flukes

Parasites often occupy specific sites within their host, which has important implications for host performance and parasite transmission. Nonetheless, parasitic infections can occur beyond their typical location within a host, significantly altering host-parasite interactions. Yet, the causes behind the atypical tissue tropism are poorly understood. Here, we focus on a ubiquitous group of diplostomid parasites that form diverse communities in fish eyes. We used targeted DNA metabarcoding (cytochrome c oxydase subunit 1, COX1, 250 bp) to evaluate potential mechanisms underlying eye parasite atypical tissue tropism to the brain of two widespread fish species (Eurasian perch and common roach). We found that the most common eye-infecting species (Tylodelphys clavata, Diplostomum baeri) are present in the brains of perch but not in roach. The bipartite network comprising 5 species and 24 mitochondrial haplotypes revealed no brain-specific haplotypes, indicating an apparent lack of genetic divergence between brain- and eye-infecting parasites. Instead, the prevalence, intensity and diversity of eye infections were positively correlated with brain infections. Thus, our results suggest that the most parsimonious mechanism underlying brain infection is density-dependent spillover rather than parasite divergence-driven niche expansion. We anticipate that 'off-target' infections are likely to be severely underestimated in nature with important ecological, evolutionary and medical implications.

Unravelling the patterns of exo-erythrocytic development of Haemoproteus parasites (Haemoproteidae, Haemosporida), with a case of abortive tissue stages in a naturally infected bird

Haemoproteus species (Haemosporida, Haemoproteidae) are cosmopolitan blood parasites that affect bird fitness and health. Recent discoveries based on the application of molecular markers showed that exo-erythrocytic or tissue stages of haemoproteids damage various internal organs including the brain. However, the patterns of exo-erythrocytic development remain unclear for most of the described species. This study aimed to understand the exo-erythrocytic development of Haemoproteus parasites in naturally infected Thrush nightingales Luscinia luscinia (Muscicapidae). Infections were confirmed in eight bird individuals by microscopic examination and PCR-based methods. Organs were examined using histology and in situ hybridization, which applied genus-specific and lineage-specific oligonucleotide probes targeting the 18S rRNA of the parasites. Exo-erythrocytic meronts of Haemoproteus attenuatus (lineage hROBIN1) were found and described for the first known time in this avian host. Most meronts were seen in the lungs, with a few also present in the liver, heart, and pectoral muscle. The available data suggest that this parasite produces only meronts, and not megalomeronts. However, numerous megalomeronts at different stages of development were observed in the gizzard and the heart of one individual. Based on the morphology, location in organs, and diagnostics using the lineage-specific probes, the megalomeronts were attributed to Haemoproteus majoris (lineage hWW2). Two cases of empty capsular-like walls of megalomeronts were seen in the gizzard, indicating that the megalomeronts had already ruptured and degenerated. The extensive microscopic examination did not reveal gametocytes of H. majoris, obviously indicating an abortive development. Abortive haemosporidian infections were often speculated to occur in wildlife but have not been documented in naturally infected birds. This study recognised patterns in the exo-erythrocytic development of H. attenuatus, and is to our knowledge the first documentation of abortive Haemoproteus infection in a naturally infected bird during exo-erythrocytic development.

A new blood parasite of the accentor birds: description, molecular characterization, phylogenetic relationships and distribution

Haemoproteus bobricklefsi sp. nov. (Haemosporida, Haemoproteidae) was found in the dunnock Prunella modularis and represents the first blood parasite described in accentor birds of the Prunellidae. The description is based on the morphology of blood stages and includes information about a barcoding segment of the mitochondrial cytochrome b gene (lineage hDUNNO01) and the full mitochondrial genome, which can be used for identification and diagnosis of this infection. The new parasite can be readily distinguished from described species of haemoproteids parasitizing passeriform birds due to markedly variable position of nuclei in advanced and fully grown macrogametocytes. Illustrations of blood stages of the new species are given, and phylogenetic analyses based on partial mitochondrial cytochrome b gene sequences and the full mitochondrial genome identified the closely related lineages. DNA haplotype networks showed that transmission occurs in Europe and North America. This parasite was found in the dunnock in Europe and several species of the Passerellidae in North America. It is probably of Holarctic distribution, with the highest reported prevalence in the UK. The parasite distribution seems to be geographically patchy, with preference for areas of relatively cool climates. Phylogenetic analysis suggests that H. bobricklefsi sp. nov. belongs to the Parahaemoproteus subgenus and is probably transmitted by biting midges belonging to Culicoides (Ceratopogonidae). The available data on molecular occurrence indicate that this pathogen is prone to abortive development, so worth attention in regard of consequences for bird health.

Brain parasites and misorientation of migratory birds

Haemoproteus blood parasites of birds are thought to be relatively benign. Recent findings show that infections may develop in the brain of birds, possibly distorting their orientation sense. Misdirected migration may lead migrants outside their range where they are recognized as vagrants and can transmit parasites to novel hosts.

First experimental observation on biology of the avian malaria parasite Plasmodium (Novyella) homonucleophilum (lineage pSW2), with remarks on virulence and distribution

Species of subgenus Novyella remain most fragmentarily studied amongst avian malaria agents. Transmission of the recently described Plasmodium (Novyella) homonucleophilum (lineage pSW2) occurs broadly in the Old World, including Europe, however biology of this pathogen remains insufficiently investigated. This study provided the first data on the development of P. homonucleophilum in the experimentally infected Eurasian siskins Spinus spinus exposed by inoculation of infected blood. The parasite strain was isolated from a naturally infected song thrush Turdus philomelos, multiplied in vivo, and inoculated to six Eurasian siskins. The same number of birds were used as negative controls. All exposed birds were susceptible, and the controls remained uninfected during the entire study (172 days). Prepatent period was 8-12 days post exposure (dpe). Maximum parasitaemia reached 50-90 % of infected erythrocytes between 20 and 44 dpe. Then, parasitaemia decreased but remained relatively high during the entire observation. Three of six exposed birds died, indicating high virulence of this infection. The parasitaemia increase coincided with a decline of haematocrit value, indicating anaemia. Polychromasia was evident in all infected birds but not in controls. Body mass of exposed birds increased, coinciding with increased food intake. The latter probably is an adaptation to compensate energy loss of hosts due to the long-lasting parasitism. Exo-erythrocytic stages were not found, suggesting that long-lasting parasitaemia was entirely due to erythrocytic merogony. The lineage pSW2 has been reported broadly in the Old World and is likely a generalist infection. Neglected avian Novyella malaria parasites are worth more attention of researchers due to their cosmopolitan distribution and high virulence.

Unexpected absence of exo-erythrocytic merogony during high gametocytaemia in two species of Haemoproteus (Haemosporida: Haemoproteidae), including description of Haemoproteus angustus n. sp. (lineage hCWT7) and a report of previously unknown residual bodies during in vitro gametogenesis

Neglected avian blood parasites of the genus Haemoproteus (Haemoproteidae) have recently attracted attention due to the application of molecular diagnostic tools, which unravelled remarkable diversity of their exo-erythrocytic (or tissue) stages both regarding morphology and organ tropism levels. The development of haemoproteids might result in pathologies of internal organs, however the exo-erythrocytic development (EED) of most Haemoproteus species remains unknown. Seven individual birds - Curruca communis (1) and Phylloscopus trochilus (6) - with high gametocytaemia (between 1% and 24%) of Haemoproteus angustus n. sp. (hCWT7) and Haemoproteus palloris (lineage hWW1) were sampled in Lithuania, and their internal organs were examined extensively by parallel application of histology and chromogenic in situ hybridization methods. Tissue stages were apparently absent, suggesting that the parasitaemia was not accompanied by detectable tissue merogony. Haemoproteus angustus n. sp. was described and characterized morphologically and molecularly. Sexual process and ookinete development of the new species readily occurred in vitro, and a unique character for Haemoproteus parasites was discovered - the obligatory development of several tiny residual bodies, which were associated with intracellular transformation of both macrogametocytes and microgametocytes before their escape from the host cells and formation of gametes. A DNA haplotype network was constructed with lineages that cluster in one clade with the lineage hCWT7. This clade consists of lineages mostly found in Curruca birds, indicating specificity for birds of this genus. The lineage hCWT7 is mainly a parasite of C. communis. Most reports of this lineage came from Turkey, with only a few records in Europe, mostly in birds wintering in Africa where transmission probably occurs. This study highlights unexpected difficulties in the research of EED even when using sensitive molecular diagnostic tools and extends information about transformation in early stages of gametogenesis in haemosporidian parasites.