Encyclopaedia Britannica, 11th Edition, "Gyantse" to "Hallel" / Volume 12, Slice 7

by Various

From Lankester’s Treatise on Zoology.
Fig. 3.—Haemogregarina bigemina, Laveran, from the blood of blennies.(After Laveran, magnified about 1800 diameters.)
a, The form of the parasitefound free in the blood-plasma. b, Parasite within a blood-corpuscle,preparing for division;the nucleus has alreadydivided. c, The parasite has divided intotwo rounded corpuscles,which assume the form ofthe free parasite, as seen ind, e and f.	N, Nucleus of the blood-corpuscle. n, Nucleus of the parasite.The outline of the blood-corpuscleis indicated by a thick black line.

Various types of form are to be met with among the Haemosporidia. In one, characteristic of most (though not of absolutely all) parasites of warm-blooded Vertebrates, the trophozoites are of irregular amoeboid shape; hence this section Comparative Morphology; variations in the life-cycle where known. is generally known as the Haemamoebidae. In another type, characteristic of the parasites of cold-blooded Vertebrates, the body possesses a definite, vermiform, i.e. gregariniform shape, which is retained during the intracorpuscular as well as during the free condition; this section comprises the Haemogregarinidae. Allied to this latter type of form are the trophozoites of Piroplasma, which are normally pear-shaped; they differ, however, in being very minute, and, moreover, exhibit considerable polymorphism, rod-like (so-called bacillary) and ring-forms being of common occurrence. It is important to note that in a certain species of Haemogregarina (fig. 3) the young trophozoites markedly resemble Piroplasma in their pyriform appearance; and a further point of agreement between the two forms is mentioned below. Lastly there is the Avian genus Halteridium, the trophozoites of which are characteristically bean-shaped or reniform. True Haemogregarines also differ in other slight points from “Haemamoebae.” Thus the young endoglobular trophozoite does not exhibit a ring (vacuolar) phase; and the cytoplasm never contains, at any period, the characteristic melanin pigment above noted. In some species of Haemogregarina the parasite, while intracorpuscular, becomes surrounded by a delicate membrane, the cytocyst; on entering upon an active, “free” period, the cytocyst is ruptured and left behind with the remains of the corpuscle. A very interesting cytological feature is the occurrence, in one or two Haemosporidia, of nuclear dimorphism, i.e. of a larger and smaller chromatic body, probably comparable to the trophic and kinetic nuclei of a Trypanosome, or of the “Leishman-Donovan” bodies. Schaudinn was the first to notice this character, in Piroplasma canis, and his observation has since been confirmed by Lühe.[4] Moreover, Brumpt has also noticed nuclear dimorphism in the ookinete of a species of Haemogregarina in a leech (as the Invertebrate host)—a highly important observation.

As regards the life-history, the endogenous (schizogonous) cycle is known in many cases. Sometimes schizogony takes the primitive form of simple binary (probably) longitudinal fission; this is the case in Piroplasma (fig. 4) and also in Haemogregarina bigemina just referred to. From this result the pairs of individuals (“twins”) so often found in the corpuscles. In addition, however, at any rate in Piroplasma, it is probable that multiple division (more allied to ordinary schizogony) also takes place; such is the case, according to Laveran, in P. equi, and the occurrence at times of four parasites in a corpuscle, arranged in a cruciform manner, is most likely to be thus explained. Labbé has described schizogony in Halteridium danilewskyi as taking place in a rather peculiar manner; the parasite becomes much drawn-out and halter-like, and the actual division is restricted to its two ends, two clumps of merozoites being formed, at first connected by a narrow strand of unused cytoplasm, which subsequently disappears. Some doubt, however, attaches to this account, as no one else appears to have seen the process. For the rest, schizogony takes place more or less in the customary way, allowing for variations in the mode of arrangement of the merozoites. It remains to be noted that in Karyolysus lacertarum, according to Labbé, two kinds of schizont are developed, which give rise, respectively, to micromerozoites and megamerozoites, in either case enclosed in a delicate cytocyst. This probably corresponds to an early sexual differentiation (such as is found among certain Coccidia (q.v.), the micromerozoites producing eventually micro-gametocytes, the others megagametocytes.

From Lankester’s Treatise on Zoology.
Fig. 4.—Development and schizogony of Piroplasma bigeminumin the blood-corpuscles of the ox. (After Laveran and Nicolle.)
a, Youngest form. b, Slightly older. c and d. Division of the nucleus.	e and f, Division of the body ofthe parasite. g, h, i, j, Various forms of thetwin parasite. k and l, Doubly infected corpuscles.

It has now been recognized for some time that the sexual (exogenous) part of the life-cycle of all the Haemamoebidae takes place in an Invertebrate (Insectan) host, and is fundamentally similar to that above described in those cases where it has been followed. In contradistinction to the malarial parasites, this host, in the Avian forms (Haemoproteus and Halteridium)[5] is a species of Culex and not of Anopheles; in other words, gamete-formation, conjugation and subsequent sporozoite-formation in these cases will only go on in the former. On the other hand, in the case of the Haemogregarines, it was thought until quite lately that the entire life-history, including conjugation and sporogony, went on in the Vertebrate host; and only in 1902 Hintze described what purported to be the complete life-history of Lankesterella (Drepanidium) ranarum undergone in the frog. This view was rendered obsolete by the work of Siegel and Schaudinn, who demonstrated the occurrence of an alternation of hosts and of generations in the case of Haemogregarina stepanovi, parasitic in a tortoise, and in Karyolysus lacertarum; the Invertebrate hosts, in which, in both cases, the sexual process is undergone, being respectively a leech (Placobdella) and a tick (Ixodes). With this discovery the main distinction (as supposed) between the Haemosporidia of warm and of cold-blooded Vertebrates vanished. It was further acknowledged by Schaudinn (under whom Hintze had worked) that the latter had been misled by Coccidian cysts and spores, which he took for those of Lankesterella. The gametogony and sporogony of Haemogregarina stepanovi in the leech agree in essential particulars with the process above described. The microgametes are extremely minute, and the sporozoites, which are developed in the salivary glands, where the motile ookinetes finally come to rest, are extremely “spirochaetiform”—the full significance of this latter fact being, perhaps, not appreciated.

Christophers recently described some remarkable phases which he regarded as belonging to the cycle of Haemogregarina gerbilli (one of the few Mammalian Haemogregarines known) in a louse (Haematopinus). In a private communication, however, the author states that he has probably mistaken phases in the development of an ordinary gregarine parasite in the louse for part of the life-cycle of this Haemogregarine.

The Mammalian parasite Piroplasma is the one about whose life-history our knowledge is most vague. Besides the typical and generally occurring forms, others have also been observed in the blood, but it is doubtful how far these are to be looked upon as normal; for instance, Bowhill and Le Doux have described, in various species, a phase in which a long, slender pseudopodial-like outgrowth is present, with a swelling at the distal end. It is, moreover, quite uncertain which are the sexual forms, comparable to gametocytes. Doflein regards large pear-shaped forms as such (megagametocytes?), which become spherical when maturing; and Nocard and Motas have figured amoeboid, irregular forms, with the nucleus fragmented and possessing flagella-like processes (possibly microgametes?). The Invertebrate host is well known to be, in the case of all species, a tick; thus bovine piroplasmosis (P. bigeminum) in America is conveyed by Rhipicephalus annulatus (Boophilus bovis), canine piroplasmosis (P. canis) in South Africa by Haemaphysalis leachi (and perhaps Dermacentor reticulatus), and so on. The manner in which the infection is transmitted by the tick varies greatly. In some cases (e.g. P. bigeminum and P. canis) only the generation subsequent to that which receives the infection (by feeding on an infected ox) can transmit it back again to another ox; in other words, true hereditary infection of the ova in the mother-tick is found to occur. The actual period in the life of the daughter-tick at which it can convey the infection apparently varies. On the other hand, in the case of East African coast-fever, Theiler found that hereditary infection does not occur, the same generation transmitting the parasite (P. parvum) at different periods of life. Little is certainly known regarding the phases of the parasite which are passed through in the tick. Lignières has observed a kind of multiple fission in the stomach, several very minute bodies, consisting mostly of chromatin, being formed, which may serve for endogenous reproduction. Koch has published an account of certain curious forms of P. bigeminum, in which the body is produced into many stiff, ray-like processes, giving the appearance of a star; according to him fusion of such forms takes place, and the resulting zygote becomes rounded, perhaps transitional to the pear-shaped forms.

The classification and nomenclature of the Haemosporidia are in a very unsettled condition. For an account of the various systems and modifications hitherto adopted, the article of Minchin (see under [Sporozoa]: Bibliography) should be consulted. Classification. With the realization that the life-history in the case of the “Haemamoebae” and the Haemogregarines is fundamentally similar in type, the chief reason for grouping them as distinct suborders has disappeared. It is most convenient to regard them as separate, but closely allied families, the Plasmodidae (“Haemamoebidae”) and the Haemogregarinidae. The Piroplasmata, on the other hand, constitute another family, which is better placed in a distinct section or sub-order. In addition there are, as already noted, two or three genera whose systematic position must be considered as quite uncertain. One is the well-known Halteridium of Labbé, parasitic in various birds; the type-species is H. danilewskyi (Gt. and Fel.). Another is the much-debated parasite of white blood-corpuscles (leucocytes), originally described in birds by Danilewsky under the name of Leucocytozoon, a form of which has been recently observed in Mammals.

In conclusion, the chief members of the above-mentioned families may be enumerated.

Fam. Plasmodidae (“Haemamoebidae”).