Chapter 44

The effects of a Coccidian infection upon the host as a whole depend largely upon the extent to which endogenous multiplication of the parasites takes place. On the one hand, schizogony may be so limited in extent as not to cause appreciable injury to the host. This seems to be often the case in forms infecting Molluscs and Arthropods. On the other hand, where schizogony is rapid and prolonged, the results are often serious. For, although any one individual only causes the death of a single host-cell, yet the number of the parasites may be so enormously increased by this means, that the entire affected epithelium may be overrun and destroyed. Thus are occasioned grave attacks of coccidiosis, characterized by severe enteritis and diarrhoea, which may end fatally. In the case of the Vertebrates, secondary causes, resulting from the stoppage of the bile ducts, also help to produce death. There is, however, one factor in the endangered animal's favour. Schizogony cannot go on indefinitely; it has a limit, dependent upon the supply of host-cells, and consequently of nutriment, available. As this shows signs of becoming exhausted, by the rapid multiplication of the parasites, the latter begin to make preparations for the exogenous cycle, inaugurated by gametogony. When conjugation has taken place and sporogony is begun, the danger to the host is at an end. So that, if the acute stage of the disease is once successfully passed, the regenerative capacity of the epithelium may be able to restore something like equilibrium to the deranged metabolism in time to prevent collapse.

Morphology and life-history.

_Coccidium schubergi_, parasitic in the intestine of a centipede (_Lithobius forficatus_), may be taken as an example of a Coccidian life-history (see Schaudinn, 1900): some of the more important variations exhibited by other forms will be noted afterwards. The trophozoite, or actively-growing parasite, is an oval or rounded body (fig. 3, I.). The general cytoplasm shows no differentiation into ectoplasm and endoplasm; it is uniformly alveolar in character. The nucleus is relatively large, and possesses a distinct membrane and a well-marked reticulum in which are embedded grains of chromatin. Its most conspicuous feature is the large deeply-staining karyosome, which consists of the greater part of the chromatin of the nucleus intimately bound up with a plastinoid basis. When fully grown, the trophozoite (now a schizont) undergoes schizogony. Its nucleus divides successively to form a number of nuclei, which travel to the periphery, and there become more or less regularly disposed (fig. 3, II. and III.). The protoplasm in the neighbourhood of each next grows out, as a projecting bud, carrying the nucleus with it. In this manner are formed a number of club-shaped bodies, the merozoites, which are at length set free from the parent-body (IV.), leaving a certain amount of residual cytoplasm behind. By the rupture of the disorganized host-cell,[2] the fully-formed merozoites are liberated into the intestinal lumen, and seek out fresh epithelial cells. Each is more or less sickle-shaped, and capable of active movements. Once inside a new host-cell, the merozoite grows to a schizont again.

After this course has been repeated several times, gametogony sets in, the trophozoites growing more slowly and becoming the parent-cells of the sexual elements (gametocytes), either male individuals (microgametocytes) or female ones (megagametocytes). A microgametocyte (fig. 3, VI. [mars]) is characterized by its dense but finely reticular or alveolar cytoplasm, very different from the loose structure of that of a schizont. The male elements (microgametes) are formed in a manner essentially comparable to that in which the formation of merozoites takes place. Although the details of the nuclear changes and divisions vary somewhat, the end-result is similar, a number of little nuclear agglomerations being evenly distributed at the surface (VII. [mars]) Each of these elongates considerably, becoming comma-shaped and projecting from the gametocyte. Nearly all the body of the male gamete (VIII. [mars]) consists of chromatin, the cytoplasm only forming a very delicate zone or envelope around the nucleus. From the cytoplasm two long fine flagella grow out, one of which originates at the anterior end, the other, apparently, at the hinder end, acting as a rudder; but it is probable that this also is developed at the anterior end and attached to the side of the body. By means of their flagella the numerous microgametes break loose from the body of the microgametocyte and swim away in search of a female element.

A megagametocyte (VI. [venus]) is distinguished by its rather different shape, being more like a bean than a sphere until ripe for maturation, and by the fact that it stores up in its cytoplasm quantities of reserve nutriment in the form of rounded refringent plastinoid grains. Each female gametocyte gives rise to only a single female element (megagamete), after a process of nuclear purification. The karyosome is expelled from the nucleus into the cytoplasm, where it breaks up at once into fragments (VII. [venus]). Meanwhile the gametocyte is becoming spherical, and its changes in shape aid in setting it free from the shrivelled host-cell. The fragments of the karyosome, which are, as it were, squeezed out to the exterior, exert a powerful attraction upon the microgametes, many of which swarm round the now mature megagamete. The female nucleus (pronucleus) approaches the surface of the cell (VIII. [venus]), and at this spot a little clear cytoplasmic prominence arises (cone of reception). On coming into contact with this protuberance (probably attracted to it by the female pronucleus), a microgamete adheres. Partly by its own movements and partly by the withdrawal of the cone of attraction, the male penetrates into the female element and fertilization is accomplished. Only one microgamete can thus pass into the megagamete, for immediately its entry is effected a delicate membrane is secreted around the copula (zygote), which effectually excludes other less fortunate ones. This membrane rapidly increases in thickness and becomes the oocyst (IX.), and the copula is now ready to begin sporogony.

Sporogony goes on indifferently either inside the host or after the cyst has been passed out with the faeces to the exterior. The definitive nucleus of the zygote (resulting from the intimate fusion of the male and female pronuclei, by means of a somewhat elaborate "fertilization-spindle" [X.]) gives rise by successive direct divisions to four nuclei (XII.), around which the protoplasm becomes segregated; these segments form the four sporoblasts. Around each sporoblast two membranes are successively secreted (exospore and endospore), which constitute the sporocyst (XIII.); the sporocyst and its contents forming the spore. The nucleus of each spore next divides, again directly, and this is followed by the division of the cytoplasm. As a final result, each of the four spores contains two germs (sporozoites), and a certain amount of residual protoplasm (fig. 3, XIV.); this latter encloses a viscid, vacuole-like body, which aids in the subsequent dehiscence of the sporocyst. On being eaten by a fresh host, the wall of the oocyst is dissolved at a particular region by the digestive juices, which are thus enabled to reach the spores and cause the rupture of the sporocysts. As the result of instructive experiments, Metzner has shown that it is the pancreatic and not the gastric juice by which this liberation of the germs is effected. The liberated sporozoites creep out and proceed to infect the epithelial cells. The sporozoites (XV.) are from 15-20 µ long by 4-6 µ wide; they are fairly similar to merozoites in form, structure and behaviour, the chief point of distinction being that they have no karyosome in the nucleus (cf. above).

PLATE I.

PLATE II.

Comparing the life-cycle of other Coccidia with that just described, a greater or less degree of modification is frequently met with. In the process of schizogony two orders of division sometimes occur; the parent-schizont first divides up into a varying number of rounded daughter-schizonts (schizontocytes), each of which gives rise, in the usual manner, to a cluster of merozoites,[3] which thus constitute a second order of cells. Siedlecki (1902) has found this to be the case in _Caryotropha mesnilii_ (fig. 4), and Woodcock (1904) has shown that it is most probably really the same process which Smith and Johnson (1902) mistook for sporogony when originally describing their Coccidian of the mouse, _Klossiella_. In _Caryotropha_, a perfectly similar state of affairs is seen in the formation of microgametes from the microgametocyte; this is additionally interesting as showing that this process is neither more nor less than male schizogony.

Coming to the sexual generation, considerable variation is met with as regards the period in the life-history when sexual differentiation first makes its appearance. Sexuality may become evident at the very beginning of schizogony, as, e.g. in _Adelea ovata_ (Siedlecki, 1899), where the first-formed schizonts (those developed from the sporozoites) are differentiated into male and female (micro-and mega-schizonts) (see Plate II., fig. 5). Correspondingly, the merozoites, to which they give rise, are also different (micro-and mega-merozoites). In one or two cases sexuality appears even earlier in the cycle, and has thus been carried still farther back.

The Coccidia, as a whole, have not developed the phenomenon of association of the sexual individuals prior to gamete-formation which is so characteristic of Gregarines. Their method of endeavouring to secure successful sporulation, and thus the survival of the species, has been rather by the extreme specialization of the sexual process. In place of many female elements, which the primitive or ancestral forms may be assumed to have had,[4] there is always, save possibly for one exception,[5] only a single relatively huge megagamete formed, which offers a comparatively easy goal for one of the many microgametes. Nevertheless in the effort to render fertilization absolutely certain, a few Coccidia have acquired (secondarily) the power of associating; a state of things which enables those forms, moreover, to effect an economy in the number of male gametes, only three or four being developed. Instances are seen in _Adelea mesnili_ (Perez, 1903), _A. ovata_ (fig. 6), and _Klossia helicina_ (Siedlecki, 1899). It is very interesting to note that, in the two last cases, unless this association of the microgametocyte with the megagametocyte occurs, neither can the former produce male elements (microgametes) nor can the female individual maturate and become ready for fertilization. (Concerning this question of association see also GREGARINES.)

In sporogony, great variation is seen with respect to the number of spores and sporozoites formed; and, as in Gregarines, these characters are largely used for purposes of classification, under which heading they are better considered. Usually, the spores (fig. 7) are quite simple in outline, and not produced into spines or processes; exceptions are found, however, in a few instances (e.g. _Minchinia chitonis_). In one case (_Coccidium mitrarium_), the oocyst itself, instead of being spherical, is curiously shaped like a mitre.

The life-history as a whole is invariably undergone in a single host, i.e. there is no alternation of true hosts.[6] Schaudinn, in his work on the _Coccidia_ of _Lithobius_ (1900), showed that the oocysts expelled with the faeces may be eaten by wood-lice (Oniscus), but when this happens they pass through the intestine of the wood-louse unaltered, the latter not being an intermediate host but merely a carrier.

Classification.

The order Coccidiidea is divided into four families, characterized by the number of sporocysts (if any) found in the oocyst.

Fam. ASPOROCYSTIDAE, Léger. No sporozoites are formed in the oocyst, the sporozoites being unenclosed (gymnospores).

Genus, _Légerella_, Mesnil. This genus actually conforms to Aimé Schneider's original definition of _Eimeria_, which was founded on what were really the schizogonous generations of other forms, then thought to be distinct. In view of the great confusion attending the use of this name, however, Mesnil (1900) has suggested the new one here adopted. Two species known, _L. nova_ and _L. testiculi_, both from different species of _Glomeris_, a Myriapod; the former inhabits the Malpighian tubules, the latter the testis.

Fam. DISPOROCYSTIDAE, Léger. The oocyst contains 2 spores.

Genus 1. _Cyclospora_, A. Schneider. Spores dizoic, i.e. with two sporozoites. _C. glomericola_, from the intestinal epithelium of _Glomeris_, and _C. caryolytica_, from the intestinal epithelium of the mole, intranuclear.

Genus 2. _Diplospora_, Labbé. Spores tetrazoic. _D. lacazei_, from many birds, is the best-known species; and others have been described from different Sauropsida. _D. lieberkühni_ is an interesting form occurring in the kidneys of the frog, which it reaches by way of the circulation.

Genus 3. _Isospora_, Schn. Spores polyzoic. Founded for _I. rara_, parasitic in the black slug (_Limax cinereo-niger_). Many authors consider that Schneider was mistaken in attributing many sporozoites to this form, and would unite with it the genus _Diplospora_.

Fam. TETRASPOROCYSTIDAE, Léger. The oocyst contains 4 spores.

Genus 1. _Coccidium_,[7] Leuckart. The spores are dizoic and the sporocysts rounded or oval. A very large number of species are known, mostly from Vertebrate hosts. _C. cuniculi_ (= _C. oviforme_) from the rabbit (intestine and diverticula), but also occurring sometimes in other domestic animals; C. falciformis, from the mouse; _C. faurei_ from sheep; and _C. schubergi_, from _Lithobius_ (a centipede), are among the best-known forms. All of them may cause disastrous epidemics of coccidiosis.

Genus 2. _Paracoccidium_, Laveran and Mesnil. This genus is distinguished from _Coccidium_ by the fact that the sporocysts become dissolved up in the oocyst, thus leaving the 8 sporozoites unenclosed, recalling the condition in _Légerella_. _P. prevoti_, unique species, from the frog's intestine.

Genus 3. _Crystallospora_, Labbé. Spores also dizoic, but having the form of a double pyramid. _C. crystalloides_ from a fish, _Motella tricirrata_.

Genus 4. _Angeiocystis_, Brasil. Apparently 6 sporozoites, but the only species, _A. audouiniae_, has only been briefly described; from a Polychaete (_Audouinia_).

Fam. POLYSPOROCYSTIDAE, Léger. The oocyst contains numerous spores.

There are several genera with monozoic spores, characterized by variations in the form and structure of the sporocysts, e.g. _Barroussia_, Schn. (fig. 8), _Echinospora_, Léger, and _Diaspora_, Léger; most of these forms are from Myriapods.

Genus _Adelea_, Schn. Dizoic spores; sporocysts round or oval, plain. Several species are included in this well-known genus, among them being _A. ovata_, _A. mesnili_, _A. dimidiata_; most of them are parasitic in Insects or Myriapods.

Genus _Minchinia_, Labbé. Dizoic spores; the sporocysts are produced at each pole into a long filament. _M. chitonis_, from the liver of _Chiton_ (Mollusca).

Genus _Klossia_, Schn. The spores are tetrazoic (or perhaps polyzoic). _K. helicina_ from the kidney of various land-snails is the best-known form. Usually said to have 5 to 6 spores, but Mesnil considers that the normal number is 4, as is the case in another species, _K. soror_.

Genus _Caryotropha_, Siedlecki. Many spherical spores (about 20) each with 12 sporozoites. _C. mesnilii_, unique species, from the spermatogonial (testis) cells of _Polymnia_ (a Polychaete). An interesting point in the schizogony is the formation of schizontocytes (see above).

A Coccidian parasitic in the kidneys of the mouse has been described by Smith and Johnson (1902) and named by them _Klossiella_, on the ground that it possessed many spores, each with about 20 sporozoites. Woodcock has shown, however, that the authors were in all probability dealing with a similar modification of schizogony to that which obtains in _Caryotropha_. The sporogony of this form (and hence its systematic position) remains at present, therefore, quite unknown.

There are several doubtful or insufficiently known genera, e.g. _Bananella_, _Goussia_, _Hyaloklossia_, _Gonobia_, _Pfeifferella_ and _Rhabdospora_, many of which probably represent only schizogonous generations of other forms. (For information concerning these see Labbé, 1897.)

Lastly it remains to mention the extremely interesting forms parasitic in Cephalopods. For some years these have provided a fruitful source of discussion to systematists. Here it may be stated simply that their systematic position and nomenclature were thought to have been finally settled by the researches of Jacquemet (1903) and Lühe (1902) in the following terms:--

Genus _Eucoccidium_. Lühe (syn. _Légerina_ Jacq.), Coccidia possessing polysporous oocysts and lacking schizogony, parasitic in Cephalopods. Two well-known species: _E. eberthi_ (Labbé), (=_Benedenia_ seu _Klossia e._ seu _octopiana_), parasitic in _Sepia_, which is tri- or tetra-zoic; and _E. octopianum_ (Schn.), (syn. _Benedenia_ seu _Klossia o._) from _Octopus_, which is polyzoic, having 10 to 12 sporozoites. In both forms cysts containing megaspores and megasporozoites, and others containing microspores and microsporozoites are found, considered as representing sexual differentiation thrown back to the very earliest stages of the life-cycle.