Haemamoeba (partim); Haemocystidium (partrim); Halteridium; Laverania (partim)

Haemoproteus parasites are classified as members of the phylum Apicomplexa (syn. Sporozoa), order Haemosporida, family Haemoproteidae.

Haemosporidian parasites with merogony (schizogony) in cells of fixed tissues of vertebrate hosts. Merogony is absent from circulating blood cells. Gametocytes develop in red blood cells and contain malarial pigment (hemozoin) (Fig. 1). Sexual process and sporogony take place in biting midges (Ceratopogonidae) and louse flies (Hippoboscidae). Widespread parasites of birds; several Haemoproteus species have been described in reptiles, but their relationships with bird parasites remain insufficiently studied.

Two subgenera, i.e., Parahaemoproteus and Haemoproteus, are widely accepted in genus Haemoproteus. The great majority of bird parasites belong to the subgenus Parahaemoproteus, which include ceratopogonid transmitted species with small oocysts (<20 μm in diameter). Both ends of sporozoites are approximately equally pointed in species of Parahaemoproteus (Fig. 2, 8). Species of the subgenus Haemoproteus are bird parasites transmitted by louse flies; these parasites produce large (>20 μm in diameter) oocysts (Fig. 2, 7). One end of sporozoites is more pointed than the other in species of Haemoproteus (Fig. 2, 9). Recent phylogenetic analyses based on mitochondrial gene sequences indicate that species of Parahaemoproteus and Haemoproteus are closely related sister groups, supporting the traditional subgeneric classification of these parasites.

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The life cycle of Haemoproteus parasites is obligate heteroxenous. The parasites develop in vertebrates (mainly birds) and vectors, which are blood-sucking dipterans belonging to the Ceratopogonidae and Hippoboscidae. Sexual process takes place in the vectors. Complete life cycles and details of development remain unknown for the great majority of described species. The exoerythrocytic development has been fragmentarily investigated, but knowledge on this issue is particularly important for better understanding pathology caused by these parasites.

During blood meal, the vectors inoculate sporozoites (Fig. 2, 8, 9) in vertebrate hosts giving rise to exoerythrocytic meronts (schizonts), which undergo asexual division in cells of the fixed tissues of the hosts (Fig. 3, 1–3). The merogony is absent from blood cells. Numerous uninuclear merozoites, which are asexual stages of spreading within the vertebrate host, develop in exoerythrocytic meronts. The latter develop mainly in the endothelial cells and probably in fixed macrophages, while in some species the meronts mature in myofibroblasts. There are several generations of the exoerythrocytic development, during which the parasite gradually adjusts to the host. Most frequently, meronts are found in the lungs (Fig. 3, 1, 3) and less often in the liver, spleen (Fig. 3, 2), kidneys, heart, skeletal musculature, and other organs. The meronts are markedly variable in shape and size; their length usually does not exceed 100 μm. During development, large exoerythrocytic meronts can split into individual parts (cytomeres, Fig. 3, 2) containing numerous nuclei.

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In some species (Haemoproteus handai, Haemoproteus mansoni), huge meronts (megalomeronts) develop in the endothelial cells of capillaries, in myofibroblasts of the skeletal musculature, in the heart muscle, and sometimes in other muscular organs. Megalomeronts are significantly larger than meronts developing in the lungs and other organs (Fig. 3, 4). The length of the largest megalomeronts exceeds 400 μm; they can be observed without microscope on surfaces of the damaged organs. Megalomeronts reach maturity between 2 and 3 weeks post exposure and rupture to release small numerous spherical merozoites. As a result of the exoerythrocytic merogony, there is an avalanche growth of the initial source of infection. Exoerythrocytic merozoites initiate a new cycle of the merogony and/or invade erythrocytes and develop into gametocytes (Fig. 1).

Prepatent period for the majority of investigated Haemoproteus species varies between 10 and 17 days. Gametocytes develop in mature erythrocytes (Fig. 1, 1–8). More than one (sometimes up to 15) merozoites can penetrate into an individual erythrocyte. Multiple infection of one erythrocyte by several parasites is a function of the intensity of parasitemia. Usually, one or two parasites gain the maturity. Malaria pigment (hemozoin) is the product of digestion of host hemoglobin; it is well visible in gametocytes as discrete granules of golden-brown, brown, or black color (Fig. 1, 2, 5, 7). The number, form, and position of pigment granules in gametocytes are important characters used in taxonomy on species level. Together with the true malarial pigment (hemozoin), more or less compact gatherings of matter called volutin are often found in the gametocytes of certain species (Fig. 1, 3, 7). The granules of volutin, if stained by Giemsa, usually obtain various hues of the violet color and sometimes have azurophilic tints. They are usually readily distinguishable from pigment granules by their lesser light refraction. In some species, volutin granules are associated with pigment granules, making the latter difficult to visualize (Fig. 1, 4, 8). The nature and role of volutin granules are still not completely understood in Haemoproteus spp. Gametocytes capable of gametogenesis appear within 2–6 days after penetrating of merozoites in erythrocytes. Afterward, the capability of gametocytes to produce gametes decreases. A small number of viable gametocytes (often less than 1 per 1000 erythrocytes) is usually maintained in the blood of infected birds due to the weak merogony in internal organs during the period, when transmission occurs in nature.

Gametocytes possess the sexual potency (they produce gametes). Gametocytes, which produce macrogametes, are known as macrogametocytes (Fig. 1, 1–4), as distinct from microgametocytes (Fig. 1, 5–8), which produce microgametes. Macrogametocytes are usually easily distinguishable from microgametocytes due to their sexual dimorphic characters (compare Fig. 1, 1–4 with Fig. 1, 5–8), which are a more intense staining of the cytoplasm with Giemsa and a compact nucleus with clear margin (there are exceptions). Pigment granules (hemozoin) tend to gather at the ends of elongate gametocytes in microgametocytes (Fig. 1, 5, 8), which is not generally a characteristic of macrogametocytes (there are exceptions). Morphology of gametocytes and the patterns of their impact on infected erythrocytes are important taxonomic characters used for Haemoproteus species identification (Fig. 1). Gametocytes are infective for vectors.

The parasites persist in birds. Once infected and having survived the initial infection, a bird usually maintains the parasites for many years or even for the lifelong period, thus being the source of infection for vectors. A relapse of parasitemia occurs in most of Haemoproteus species during the reproduction period of vertebrate hosts; that facilitates infection of vectors and transfer of infection to the offspring. Mechanism of relapse remains insufficiently investigated. Gonadotrophic hormones are likely involved. Experimental evidence suggests that increases in photoperiod and subsequent physiological changes in levels of hormones such as melatonin, which regulate circadian rhythms, are important stimuli for initiating relapses among species of Haemoproteus in temperate regions.

Gametocytes undergo gametogenesis (Fig. 3, 1, 2) in the midgut of the vectors, resulting in a sexual process of the oogamy type (Fig. 3, 3). This process is extremely rapid and occurs within several minutes after ingestion of mature gametocytes. Main stimuli for the beginning of the gametogenesis are the fall in temperature and the change of the oxygen and carbon dioxide concentration, when the blood is transferred from the vertebrate host to the vector. Sexual process can be readily induced in vitro conditions when blood containing mature gametocytes is exposed to air. A macrogametocyte produces one rounded macrogamete (Fig. 3, 1), while microgametocytes undergo the exflagellation and form eight motile thread-like microgametes (Fig. 3, 2). Fertilization occurs extracellularly (Fig. 3, 3). Zygote (Fig. 3, 4) is transformed into an elongate motile ookinete (Fig. 3, 5). The latter penetrates through a peritrophic membrane and through the epithelial layer of the midgut. The length of microgametes, morphological features of zygotes, patterns of zygote transformation into ookinete, the sizes of ookinetes, and the rate of ookinete formation under standard conditions markedly differ in many Haemoproteus species. The ookinetes round up under the basal lamina and develop into oocyst, which is surrounded by a capsule-like wall built from the material of the host (Fig. 3, 6, 7). During oocyst development (sporogony), numerous uninuclear elongate bodies (sporozoites) develop (Fig. 3, 8, 9). After maturation of oocysts, the sporozoites get into the hemocoele and next penetrate into the salivary glands of the vector. Sporozoites are infective for birds. Transmission occurs when sporozoites are injected by the vector into the vertebrate hosts with salivary gland secretion during its blood meal.

It is worth noting that, contrary to the closely related malaria parasites of the genus Plasmodium, the gametogenesis in Haemoproteus parasites does not require additional stimuli, e.g., the presence of vector-derived xanthurenic acid and blood-derived factors. As a result, the extensive sexual process and ookinete development of Haemoproteus spp. occur in the digestive tract located not only in the midgut but also in the head (foregut) and thorax (thoracic midgut) areas of the fully engorged dipteran insects. That results in presence of ookinetes and sometimes even initial stages of oocysts’ development, throughout the body of blood-sucking insects. However, mature oocysts have been observed only in the midgut.

Sporogony of Haemoproteus species developing in biting midges and louse flies is different. The majority of the investigated species of bird hemoproteids develop in biting midges belonging to Culicoides. Small oocysts (<20 μm in diameter) with one germinative center and < 100 sporozoites develop in this case (Fig. 3, 6). The average length of sporozoites exceeds 10 μm; their ends are more or less approximately equally pointed (Fig. 3, 8). Sporogony in biting midges usually completes faster than in 10 days, which is the adjustment to a relatively short (7–10 days) gonadotrophic cycle of the midges. Sporogony of the species developing in louse flies is characterized by a longer development of oocysts (often ≥ 10 days), in which multiple germinative centers and several hundreds of sporozoites are formed (Fig. 3, 7). This type of development, which is not exactly synchronized with blood meal, is an adjustment to the mode of life of relatively long-living louse flies, which spend a long time on birds. The diameter of mature oocysts usually is > 20 μm. For example, oocysts of Haemoproteus columbae reach 40 μm or even more in diameter. The average length of sporozoites is usually less than 10 μm; one end of the sporozoites is more pointed than the other (Fig. 3, 9).

Mature oocysts usually burst, and sporozoites penetrate into the hemocoele of the vectors. Gradual release of sporozoites from oocysts was also observed in several species of Haemoproteus. A part of sporozoites gather in the salivary glands of blood-sucking insects, which become capable to infect birds during their next blood meal.

Species of Haemoproteus are widespread in birds in countries with temperate and tropical climates. They are absent from high arctic tundra. The diversity of species and their haplotypes is greatest in countries with warm climates, particularly in the tropics. Haemoproteus parasites usually are rare in birds on oceanic islands; exceptions are frigate birds and some species of seagulls. The latter two groups of birds are often prevalently infected with hippoboscid transmitted species of Haemoproteus. For unclear reasons, Haemoproteus parasites have been rarely reported in birds belonging to primitive orders (Sphenisciformes, Gaviiformes, Podicipediformes, Procellariiformes, Tinamiformes, Apterygiformes, Struthioniformes), but they are common and often prevalent among species of evolutionary relatively young orders, particularly of the Passeriformes and also of the Falconiformes, Strigiformes, Anseriformes, Gruiformes, Columbiformes, and Coraciiformes.

A combination of the microscopic examination of Giemsa-stained thin blood smears and the polymerase chain reaction (PCR)-based methods is the gold standard. The PCR-based diagnostic methods using general primers are sensitive in the detection of light parasitemia and the identification of species, particularly on tissue stages. However, this tool markedly underestimates coinfections of parasites belonging to the same and different genera of hemoproteids and also Plasmodium spp.; microscopic examination of blood films helps to minimize this shortcoming of the molecular detection.

Virtually nothing is known about immune mechanisms in hemoproteid infections. Domestic pigeons with chronic parasitemia of H. columbae acquire immunity (premunition) which is being lost while gametocytes disappear from the blood. Widespread coinfections of different species of Haemoproteus indicate that a cross immunity does not develop.

Prevention is based on the protection of birds from bites of vectors during periods when blood-sucking dipteran insects are abundant and active (usually the rainy warm seasons of a year). Small groups of birds or expensive individuals are put in cages or aviaries covered with fine-mesh bolting silk (a mosquito-net effect); that prevents birds from bites of vectors. Treatment has been insufficiently developed. Atebrine, plasmochin, chloroquine sulfate, primaquine, mefloquine, and buparvaquone are effective for reducing parasitemia, but these drugs usually do not affect tissue stages.