Exo-erythrocytic development of avian malaria and related haemosporidian parasites

Abstract

Background: Avian malaria parasites (Plasmodium spp.) and related haemosporidians (Haemosporida) are responsible for diseases which can be severe and even lethal in avian hosts. These parasites cause not only blood pathology, but also damage various organs due to extensive exo-erythrocytic development all over the body, which is not the case during Plasmodium infections in mammals. However, exo-erythrocytic development (tissue merogony or schizogony) remains the most poorly investigated part of life cycle in all groups of wildlife haemosporidian parasites. In spite of remarkable progress in studies of genetic diversity, ecology and evolutionary biology of avian haemosporidians during the past 20 years, there is not much progress in understanding patterns of exo-erythrocytic development in these parasites. The purpose of this review is to overview the main information on exo-erythrocytic development of avian Plasmodium species and related haemosporidian parasites as a baseline for assisting academic and veterinary medicine researchers in morphological identification of these parasites using tissue stages, and to define future research priorities in this field of avian malariology.

Methods: The data were considered from peer-reviewed articles and histological material that was accessed in zoological collections in museums of Australia, Europe and the USA. Articles describing tissue stages of avian haemosporidians were included from 1908 to the present. Histological preparations of various organs infected with the exo-erythrocytic stages of different haemosporidian parasites were examined.

Results: In all, 229 published articles were included in this review. Exo-erythrocytic stages of avian Plasmodium, Fallisia, Haemoproteus, Leucocytozoon, and Akiba species were analysed, compared and illustrated. Morphological characters of tissue stages that can be used for diagnostic purposes were specified.

Conclusion: Recent molecular studies combined with histological research show that avian haemosporidians are more virulent than formerly believed. The exo-erythrocytic stages can cause severe disease, especially in non-adapted avian hosts, suggesting the existence of a group of underestimated malignant infections. The development of a given haemosporidian strain can be markedly different in different avian hosts, resulting in significantly different virulence. A methodology combining the traditional histology techniques with molecular diagnostic tools is essential to speed research in this field of avian malariology.

Keywords: Akiba; Avian malaria; Exo-erythrocytic development; Fallisia; Haemoproteus; Leucocytozoon; Pathology; Plasmodium.

Fig. 1

Exo-erythrocytic stages of avian malaria parasites: cryptozoites (a–c), metacryptozoites (d) and phanerozoites (e–l). Plasmodium cathemerium in wing skin of a domestic canary Serinus canaria approximately 2.5 days after inoculation of sporozoites (note small size of maturing cryptozoite, a). Plasmodium fallax in culture of brain cells from a turkey Meleagris gallopavo embryo (note five mature cryptozoic meronts and numerous mature dispersed merozoites, b). Plasmodium lophurae in culture of brain cells from a turkey embryo (note numerous small mature cryptozoic meronts located in groups, c). Plasmodium garnhami in histological section of liver of the hoopoe Upupa epops experimentally infected by sporozoites (note the plentiful basophilic cytoplasm and prominent different size nuclei in three metacryptozoic meronts in different stages of growth, d). Growing phanerozoic meront of Plasmodium gabaldoni in the spleen of the pigeon Columba livia infected by inoculation of infected blood (note the plentiful basophilic cytoplasm and prominent nuclei, e). Mature merozoites of Plasmodium gabaldoni in a brain smear of turkey experimentally infected by inoculation of infected blood (note prominent irregular shape nuclei in mature merozoites, f). Mature phanerozoic meront of Plasmodium durae in spleen of turkey experimentally infected by inoculation of infected blood (note small size of the mature meront and a deformed host cell nucleus, g). Plasmodium elongatum in a smear of bone marrow of a European greenfinch Carduelis chloris infected by inoculation of infected blood (note three intracellular parasites in different stages of growth in stem cells and one extracellular meront, h). Plasmodium huffi in a smear of bone marrow of a toco toucan Ramphastos toco experimentally infected by inoculation of infected blood (note one mature meront and several growing meronts, which markedly displace and deform host cell nuclei, i). Elongate phanerozoic meronts of Plasmodium gallinaceum blocking cerebral capillaries (note two meronts in different stages of growth, j). Mature phanerozoic meronts of Plasmodium pinottii in a brain smear of experimentally infected domestic pigeon Columba livia (k, l): note numerous roundish merozoites (micromerozoites, k) and elongate merozoites (macromerozoites, l) in cerebral capillaries. Simple long arrows meronts, triangle wide shot arrows host cell nuclei, simple arrowheads merozoites. Scale bars 10 μm

Fig. 2

Cerebral malaria in a chicken (a) and a man (b) due to Plasmodium gallinaceum (a) and Plasmodium falciparum (b) infections, respectively. Note numerous developing and mature phanerozoites in endothelial cells of brain capillaries (a) and numerous infected red blood cells, which adhere to the endothelial cells of a brain capillary (b). In both malaria cases, the circulation interrupts due to blockage of the capillaries by the parasites, and the ischaemic brain changes occur, resulting in severe diseases and similar clinical symptoms, which mechanisms are different. Simple long arrows phanerozoites, simple short arrows wall of capillaries, triangle arrowheads red blood cells. Scale bars 10 μm

Fig. 3

Exo-erythrocytic stages of Haemoproteus parasites: meronts (a, b) and megalomeronts (c–h). Small mature meront of Haemoproteus palumbis in lungs of a naturally infected nestling of the English wood-pigeon Columba palumbis (a). Group of maturing meronts of Haemoproteus attenuatus in lungs of a naturally infected European robin Erythacus rubecula (note numerous irregular-shaped cytomeres with developing merozoites, b). Numerous developing megalomeronts of Haemoproteus passeris in a section of liver in a naturally infected House sparrow Passer domesticus (note that megalomeronts tend to group, and each megalomeront is covered by a wall and contains several cytomeres, c). A maturing megalomeront of H. passeris (the same preparation as in c; note that the parasite is covered by a thick wall and contains seven well-defined cytomeres, which are covered by a thin membrane and contain numerous irregularly shaped subcytomeres with developing merozoites, d). Two maturing cytomeres of H. passeris (the same preparation as in c; note that the cytomeres separate from each other before maturation, and nuclei locate on the edge of subcytomeres, e). Mature oval megalomeront of Haemoproteus mansoni (syn. Haemoproteus meleagridis) in the section of the pectoral muscle of a turkey Meleagris gallopavo (note that megalomeront is surrounded by thick wall and is overfilled with roundish merozoites; the muscle fibres surrounding the parasite are swollen and pale-stained, f). Rupturing mature roundish megalomeront of H. mansoni (the same preparation as in f; note the swollen tissue surrounding the parasite, g). Mature lobular-shape megalomeront of H. mansoni (the same preparation as in f; note irregular outline of the parasite, h). Simple long arrows meronts, simple wide long arrows megalomeronts, triangle long arrows parasite nuclei, triangle wide long arrows cytomeres, simple wide short arrows subcytomeres, simple short arrows megalomeront wall. Scale bars 10 μm (a, b, e), 50 μm (c, d, f–h)

Fig. 4

Exo-erythrocytic stages of Leucocytozoon parasites: meronts (a, b) and megalomeronts (c–h). Hepatic meronts of Leucocytozoon simondi in a liver section of an experimentally infected Pekin duck Anas platyrhynchos domestica (note numerous meronts and the much haemorrhagic and disorganized liver tissue, a). Maturing hepatic meront of L. simondi (the same preparation as in a; note numerous cytomeres, in which division of parasite nuclei occurs and non-changed size of the infected hepatocyte nucleus, in comparison to nuclei size in the adjacent non-infected hepatocytes, b). Megalomeronts of Leucocytozoon sakharoffi in different stages of growth in spleen of a naturally infected the hooded crow Corvus cornix (note numerous roundish megalomeronts in different stages of growth, c). Maturing megalomeront of L. sakharoffi (the same preparation as in c; note a markedly enlarged host cell nucleus or the ‘central body’ of megalomeront, capsular-like wall surrounding the parasite, and numerous cytomeres, d). A fragment of the L. sakharoffi megalomeront (the same preparation as in c; note cytomeres containing numerous roundish subcytomeres, in which nuclear division occurs, a well-evident ‘central body’ and capsular-like wall surrounding the megalomeront, e). Megalomeront of L. simondi in hart of a naturally infected mallard Anas platyrhynchos (note numerous cytomeres, a well-evident ‘central body’, and capsular-like wall, f). Megalomeront of Leucocytozoon sp. in a section of spleen in a naturally infected Australian magpie Cracticus tibicen (note a large ‘central body’ and numerous roundish cytomeres in a parasite developing in a vessel, g). Megalomeront of Leucocytozoon sp. in a section of skeletal muscle of a naturally infected tawny frogmouth Podargus strigoides (note a large ‘central body’ and numerous roundish developing cytomeres; the muscle fibres surrounding the parasite are swollen, h). Simple long arrows meronts, simple wide long arrows megalomeronts, triangle wide long arrows cytomeres, simple wide short subcytomeres, simple short arrows megalomeront wall, triangle wide short arrow a nucleus of infected hepatocyte, simple arrowhead a nucleus of non-infected hepatocyte, stars host cell nucleus or ‘central body’ of megalomeront. Scale bars 10 μm (b), 50 μm (a, d–h), 200 μm (c)

Fig. 5

Developing megalomeronts of Leucocytozoon (Akiba) caulleryi in a kidney section of an experimentally infected domestic chicken Gallus gallus domesticus (a, b). Note numerous maturing extracellular cytomeres; developing megalomeronts are enclosed within a well-defined envelope (a). Host-cell nuclei are slightly enlarged and, if seen, locate on edge of the cytomere masses (b). Simple wide long arrows megalomeronts, triangle wide long arrows cytomeres, simple short arrow megalomeront envelope, star host cell nucleus of megalomeront. Scale bars 50 μm