Hepatocystis is a genus of parasites transmitted by midges of the genus Culicoides. Hosts include Old World primates, bats, hippopotamus and squirrels. This genus is not found in the New World. The genus was erected by Levaditi and Schoen, 1932, as Hepatocystes.
The type species is Hepatocystis kochi. There are currently 25 recognised species in this genus.
The first observation of malaria in bats dates to the end of the 19th century when Dionisi described gametocytes in the blood of a bat in 1898. The type species of this genus (Hepatocystis kochi) was described by Laveran in 1899 from the blood of a primate and he named it Plasmodium kochi.
The protozoa are transmitted by the bite of the insect vector. The sporozoites migrate to the liver where they typically form merocysts within the liver parenchyma. These may also develop in the spleen and lungs. Macroscopically these may appear as white to gray nodular foci within the tissue. These lesions within these organs are characterized by well-circumscribed focal fibrosis, accumulation of eosinophils and histiocytes, hemorrhage or hemosiderosis with the loss of normal architecture.
The cystlike exoerythrocytic schizonts are found within the merocysts. These are found around the edges of the merocytes in huge numbers. The central core of the cyst consists of a non-cellular granular material.
Released merozoites either invade other hepatocytes or erythrocytes. Within the erythrocytes the merozoites first become ring forms, then trophozoites and finally gametocytes. The gametocytes are huge and fill the entire erythrocyte. Like those of Plasmodium and unlike those of Hepatozoon their nuclei are Feulgen negative. Haemozoin may be found within the erythrocytes.
Infected erythrocytes are then taken up when the insect vector feeds on the host. Within the vector the sporogonic cycle lasts between 5–6 days. The oocysts develop in the haemocoele chiefly in the head near the brain and eye.
The mature merocysts are visible to the naked eye on the hepatic surface. These appear as raised, grayish-white to translucent foci with a central accumulation of fluid. Multiple, depressed areas of fibrosis with calcification representing healed lesions may also be found.
Microscopically the liver cysts have an irregular central space filled with faintly eosinophilic, acellular, flocculent material. Surrounding this is a peripheral rim of myriad, round, lightly basophilic merozoites which measure ~1.0–2.0 mm in diameter. The cysts themselves are delineated by a thin, convoluted, eosinophilic, hyaline capsule. A significant inflammatory response to the merocysts may or may not be present. If a reaction is present, it is typically granulomatous with an admixture of eosinophils and lymphocytes.
The first appearance in the blood is normally two months after infection. The only stage seen in the blood are gametocytes. These double in number for the first three days and mature. Having reached maturity, they gradually decline in number and normally disappear after a month. Secondary episodes may occur at variable intervals.
The early gametocytes consist of minute dense spots of chromatin with a tiny loop of cytoplasm. As the parasite grows, the chromatin tends to spread in a semicircle or into multiple dots. There is no stippling of the erythrocyte.
Mature gametocytes are larger than a normal erythrocyte stain poorly compared to other protozoa. In both male and female gametocytes, the nucleus consists of two portions.
The macrogametocyte is a steel blue color measuring 9.5 micrometres in diameter. The nucleus has a pale pink area with dense chromatin in the middle and is much smaller than that of the microgametocyte.
The microgametocyte stains less densely, is biscuit-colored and measures 9.0 micrometres in diameter. The nucleus is a large, oval, pale pink area occupying one-third to one-half of the parasite.
In endemic areas, the incidence can range from 24 to 75% in nonhuman primates. Although infection with Hepatocystis does not normally cause fever it may cause anaemia in the host.
Some versions of the Duffy antigen have been associated with protection from Hepatocytis infection in yellow baboons (Papio cynocephalus).[1]
Primates and bats are the most commonly identified host of these parasites. H. bouillezi, H. cercopitheci, H. kochi and H. simiae infect African monkeys. H. semnopitheci and H. taiwanensis infect Asian monkeys. These species may lie in different phylogenetic clades but this is not yet known.
Known vectors belong to the genus Culicoides. Species of Streblidae may also be able to act as vectors but this is not yet certain.
Hepatocystis is a genus of parasites transmitted by midges of the genus Culicoides. Hosts include Old World primates, bats, hippopotamus and squirrels. This genus is not found in the New World. The genus was erected by Levaditi and Schoen, 1932, as Hepatocystes.
ヘパトシスティスは、アピコンプレックス門に属する寄生性の原生生物である。媒介者はヌカカであり、哺乳類の肝細胞や赤血球内に寄生する。分類学的にはヘパトシスティス属(Hepatocystis)におよそ25種が知られている。
肝臓で直径1mmから数mm程度のメロシストと呼ばれる小結節となり、肉眼でも肝臓表面に観察することができる。顕微鏡的にはメロシストの周囲に好酸球やマクロファージが集結しており、局所的に線維化や出血が認められる。成熟メロシストの中央部はエオジン好染性の液胞が大きく発達し、周縁部には大量の娘虫体（メロゾイト）が生じる。
赤血球中ではマラリア原虫のものと良く似たリング体となるが、マラリア原虫と違って無性生殖を行わずに常に生殖母体を形成する。生殖母体は赤血球よりやや大きく、雌性生殖母体のほうがやや大きいが、細胞核は雄性生殖母体の方が大きい。
中間宿主は哺乳類で、狭鼻猿（旧世界ザル）やコウモリが多く、それ以外にカバやリスが知られている。終宿主はヌカカのなかでもCulicoides属である。感染から血液中に見出されるまでの潜伏期間は2ヶ月程度で、1ヶ月程度で自然に消失する。ふつう感染動物に発熱は認められないが、貧血を起こす場合がある。
ヌカカが哺乳動物を吸血した際にスポロゾイトが唾液と共に侵入する。まず肝細胞に感染し、1ヶ月から2ヶ月ほどかけてメロシストを形成するが、脾臓や肺で発育する場合もある。メロシストから放出されたメロゾイトは他の肝細胞に再感染するほか、赤血球に感染してリング体を形成する。リング体は栄養体を経て雌雄の生殖母体となり、それがヌカカに吸血されて配偶子となり有性生殖を行う。オーシストはおもに頭部の血体腔に生じる。
アピコンプレックス門無コノイド綱住血胞子虫目に属し、研究者によりプラスモジウム科またはヘモプロテウス科に所属させる。なお分子系統解析の結果から、ヘパトシスティスは哺乳類を宿主とするマラリア原虫の一部が特殊化したものと推定されており、ヘモプロテウス科とする場合には生物の系統を反映できない[1]。
以下にヘパトシスティスに属する種とその宿主を例示する。
この項目は、原生生物に関連した書きかけの項目です。この項目を加筆・訂正などしてくださる協力者を求めています（ポータル 生き物と自然／プロジェクト 生物）。 
