Bible Encyclopedias
Trypanosomes

Historical

The first observation of a trypanosome is usually ascribed to Valentin (55), who in 1841 announced his discovery of certain amoeboid parasites in the blood of a trout. In the two or three years following several other observers recorded the occurrence of similar haematozoa in various fishes. The generic name of Trypanosoma was conferred by Gruby in 1843 upon the wellknown parasite of frogs. E. Ray Lankester (18) subsequently described this same form (under the name of Undulina ranarum) A B (From Lankester.) FIG. 1.2 - Undulina ranarum, Lankester, 1871. In B the nucleus is shown.

and was the first to indicate the presence of a nucleus in the cell-body. To Mitrophanow (1883-1884) and Danilewsky (1885-1889) we owe the first serious attempts to study the comparative anatomy of these haematozoa. Trypanosomes were first met with in cases of disease by Griffith Evans, who in 1880 found them in the blood of horses suffering from surra in India. In 1894 (Sir) David Bruce discovered the celebrated South African parasite (T. brucei) in cattle and horses laid low with nagana or the tsetse-fly disease; and this worker subsequently demonstrated, in a brilliant manner, the essential part played by the tsetse-fly in transmitting the parasites. The credit for first recognizing a trypanosome in human blood, and describing it as such, must undoubtedly be assigned to G. Nepveu (1898). Trypanosomes were next seen in human blood 1 Trypanophis, although lacking (so far as is known) a haemal habitat, is included here, since it is undoubtedly closely related to Trypanoplasma. 2 The illustrations in this article are from H. M. Woodcock's " Trypanosomes," in the Quarterly Journal of Microscopical Science. in Senegambia in 1901, in a European suffering from intermitterit fever. Forde discovered the parasites, but was uncertain of their nature; he shewed them to E. Dutton, who (11) gave this form the name of Trypanosoma gambiense. A year later A. Castellani (6) found the organisms (most probably the same species) in the cerebro-spinal fluid of patients suffering from sleeping-sickness in Uganda; and it has since been conclusively proved by Sir David Bruce and D. Nabarro (4) that they are the true cause of that dreadful malady.

More important, from the standpoint of protozoology, than these interesting medical discoveries have been the investigations by A. Laveran and F. Mesnil (20-24), L. Leger (30-35), S. Prowazek (47), F. Schaudinn (50) and others, upon numerous tolerated (i.e. non-pathogenic) forms; these researches supply, indeed, practically all the material facts on which to base an account of the Haemoflagellates at the present day.

Trypanosomes are harboured by members of all the chief classes of vertebrates with the exception of cyclostomes. By far the greater number of hosts are furnished by fishes, birds and mammals. Among batrachians the parasites have been found, up till now, only in frogs; and among reptiles their occurrence has only been observed in one or two solitary instances (T. damoniae, fig. 3 J). Data with regard to the frequency with which individual species occur, in any kind of host, are as yet somewhat scanty; in one or two cases the parasites are fairly common, T. lewisi, for example, being met with in a considerable percentage of sewer-rats throughout the world.

In considering the occurrence of Trypanosomes in mammals, careful distinction must be drawn between natural or true hosts, which are tolerant of the parasites, and casual ones, which are unaccustomed and unadapted to them. A Trypanosome usually produces markedly harmful effects upon gaining an entry into animals which have never been, by their distribution, liable to its invasion previously. Such a state of affairs is produced by the march of civilization into the " hinterlands " of the various colonies, when man, together with the numerous domesticated animals which accompany him, is brought into proximity to big game, &c., and, what is equally important, into the zone of the particular blood-sucking insects which prey upon the same.

Very many of the common domestic mammals can be successfully infected (either thus accidentally or else on purpose) with different " pathogenic " Trypanosomes, to which they succumb more or less readily, but they cannot be regarded as the natural hosts of those Trypanosomes. In dealing with disease-causing forms, the more narrowly the original source of the parasite concerned is defined, the closer do we get to the true vertebrate host or hosts. In the case of the naganaparasite, various Antilopidae (e.g. the gnu, bushbuck and koodoo) can certainly lay a strong claim to the honour. The capybara, again, is most probably the native host of T. equinum of mal de caderas of horses in South America. Similarly with regard to the many other pathogenic Trypanosomes now known, there is undoubtedly, in each case, some indigenous wild animal tolerant of that particular form, which serves as a " latent source of supply " to strange mammals.

The transmission of the parasites from one vertebrate individual to another is effected, in the great majority of cases,' by a blood-sucking invertebrate, and by this means alone. The " carrier " of a Trypanosome of warmblooded vertebrates is, in all instances so far described, an insect, generally a member of the Diptera; in the case of parasites of cold-blooded vertebrates the same role is usually played by an ichthyobdellid leech (piscine forms), but possibly, now and again, by an Ixodes (amphibian or reptilian forms).

Until lately it remained quite uncertain, however, whether the invertebrate merely conveys the Trypanosomes or whether 1 Trypanosoma equiperdum, the cause of dourine in horses and asses, is apparently only conveyed by the act of coitus. This direct mode of transmission is most likely a secondary acquirement.

it is a true alternate host, one i.e. in which definite stages of the parasite's life-cycle are undergone. Schaudinn (50), who investigated certain avian Trypanosomes, considered the latter view to be correct, and believed that the carrier - in this instance a gnat - is indeed the definitive host, i.e. the one in which sexual conjugation occurs. Many other workers have since studied the subject and, so far as the parasites of fishes are concerned, there can be little doubt, thanks to the researches of E. Brumpt (5a), L. Leger (32, 33) and others, that leeches are true alternate hosts for these forms, in which certain phases of the life-cycle are normally undergone.

We cannot write quite so confidently with regard to the relation of the various pathogenic Trypanosomes to Tsetseflies (Glossinae). In the first place experiment has shown that biting-flies, other in all probability than the true, natural hosts, may at times transmit the parasites - as it were - accidentally, if, after feeding on an infected animal, they are allowed to bite a fresh one within a limited time. One very helpful factor in determining which is the principal carrier of any form is the coincidence of the zone of a particular insect with that of any disease. By this means it has been ascertained with practical certainty that, among the family of Tsetseflies (Glossinae) for instance, at least four species are the natural carriers of different Trypanosomes. Of these perhaps the best-known is G. palpalis, of Equatorial Africa, whose bite transmits the human parasite (T. gambiense). Nevertheless, the fact, commented upon by several observers, that even here an infected fly is only infectious for a comparatively short period suggests that this species of fly, at any rate, is not the true alternate host in which the life-cycle of that particular Trypanosome is completed. However, indications furnished by Koch (16a) point in this connexion to G. fusca. Lastly, before leaving this interesting and important subject, F. Stuhlmann's work (54a) on developmental phases of T. brucii, the nagana parasite in G. fusca and G. tachinoides, does render it probable that the pathogenic forms also have true invertebrate hosts.

Schaudinn had fully described the relations of certain avian Trypanosomes to their invertebrate host, Culex pipiens (females). The distribution of the parasites in the gnat is closely connected with the process of digestion. The Trypanosomes ultimately overrun practically all parts of the body, sometimes not even the ova escaping. Thus true hereditary infection of a succeeding generation of gnats may be brought about. The life of the parasites while in the insect is characterized by an alternation of active periods, during which multiplication goes on, with resting-periods, when the Trypanosomes become attached to the epithelial cells of the host. According to S. Prowazek (47), the behaviour of T. lewisi 'in a' louse (Haematopinus) is, in its main features, similar.

On gaining an entry into the blood of a vertebrate the organisms pass rapidly into the general circulation, and are thus carried all over. Considering them first in a tolerant host, the trend of observation is to show that they are never abundant, but on the contrary usually somewhat scarce. One reason for this scarcity is to be sought in connexion with the fact that multiplicative stages are very rarely met with, at any rate in the general circulation. The parasites are frequently more numerous in the spleen, bone-marrow, kidneys, &c., than elsewhere, and it has been found that multiplication goes on rather more actively in the capillaries of these organs.

The Trypanosomes, in the active phase, are of course always free in the blood plasma (interglobular). In the majority of cases it is very uncertain whether they actually come into relation with the blood corpuscles or not. Schaudinn has stated, however, that Trypanomorpha becomes, in certain phases, attached to a red blood-corpuscle (ectoglobular), and, in others, penetrates inside one and eventually destroys it (endoglobular); while his other avian parasite, Trypanosoma ziemanni, apparently draws up into itself the white corpuscle (leucocyte) to which it becomes attached. In addition, there are two or three obervations to hand which shew that piscine, amphibian and mammalian Trypanosomes may also become attached. Probably most forms possess a resting, attached phase at some period or other, in the invertebrate, if not in the vertebrate host.

Considering now the Trypanosomes in an unaccustomed, mammalian host, they may either remain infrequent or rare (sometimes, indeed, being unnoticed until shortly before death), or, on the other hand, they may soon become numerous and go on increasing (fig. 2). In the latter case the disease is acute and rapidly fatal; in the former it is more chronic and lasts much longer, often several months. The main features of trypanosomosis, or illness caused by a Trypanosome, show a general agreement, whichever variety is considered; one symptom may be, of course, more marked than another in any particular case. Death is due either to weakness and emaciation (in chronic cases), or to blocking of the cerebral capillaries by the parasites (where these are abundant), or to disorganization of the nervous system (paraplegic and sleepingsickness cases).

In post-mortem examination, the most obvious pathological lesion is hypertrophy of the spleen, which may be very pronounced; the lymphatic glands in the neck, inguinal region, &c., are also often greatly swollen. These are undoubtedly the organs which react most strongly to the parasites, and their enlarged condition is to a great extent due to their enhanced activity in elaborating blood-corpuscles and leucocytes to cope with the enemy. Ingestion and dissolution of the Trypanosomes by phagocytes has frequently been observed; and it is probable also that the haematopoietic organs secrete some substance which exerts a harmful action on the parasites, and causes them to undergo involution and assume weird-looking " amoeboid " and " plasmodial " forms.

A peculiar feature in the behaviour of the parasites, which is most probably caused by unfavourable biological conditions -in the host, is that known as agglomeration. The process is readily brought about artificially by the addition of sera or chemical solutions to blood containing the parasites. Agglomeration consists in the grouping or union together of several Trypanosomes around a common centre; this leads to the formation of rosette-like clusters, or even of large masses composed of several rosettes. The end by which the parasites join is typically, in the case of Trypanosoma, the non-flagellate (anterior) end. If a favourable change in the surrounding medium sets in, the Trypanosomes are able to undergo the reverse process, namely disagglomeration; the parasites liberate themselves and the rosette is dissolved.

Trypanosomes vary greatly with regard to size; even in one and the same species this variation is often noticeable, especially under. different conditions of life. The common Trypanosoma rotatorium of frogs (fig. 4, A and B) is, taking it all in all, one of the largest forms so far described. Its length (inclusive of the flagellum) varies from 40-60 while its greatest width (including the undulating-membrane) is from 8-30 ,u; in the very wide individuals breadth is gained more or less at the expense of length. Conversely, T. gambiense, the human parasite (fig. 3 C), is one of the smallest forms known, its average size being about 21-23 by 122 There is equally great diversity in respect of form. Typically, the body is elongated and spindle-shaped; it is usually more or less curved or falciform (fig. 3, A - D), and tends to be slightly compressed laterally. It may be, however, anything from extremely slender or vermiform (fig. 3, H) to squat and stumpy (fig. 3, G, 4, A). Moreover, apart from the fact that a full-grown adult, ready to divide, is in many cases much plumper than a young adult (cf.

T. lewisi, fig. 6, A and B), there can be no doubt that considerable polymorphism also sometimes occurs (e.g. T. rotatorium). In many cases, at any rate, this indicates a difference in sexuality; and it is particularly necessary to bear this factor in mind when considering the avian Trypanosomes, where, perhaps, the extremes of form are to be met with. That one and the same species may appear entirely different in different phases of the life-history is manifest on comparing, for instance, the chief " forms " of Trypanosoma FIG. 3. - Representative Mammalian, Avian and Reptilian Trypanosomes, to illustrate the chief morphological characters.

A, Trypanosoma lewisi, after Bradf. and Plimmer.

B, T. brucei, after La y. and Mesnil. (X2000.) C, T. gambiense (blood, T-fever), after Bruce and Nabarro.

D, T. equinum, after La y. and Mesnil. (X2000.) E, Trypanomorpha (Trypanosoma) noctuae, after Schaud.

F, Trypanosoma avium, after Lay. and Mesnil.

G, Hanna's Trypanosome from Indian pigeons.

H, T. ziemanni, after Schaud.

J, T. damonia, after Lay. and Mesnil. (X2000.) c.g, Chromatoid grains; v, vacuole; l.s, fold or striation.

ziemanni described by Schaudinn. The asexual or indifferent type (fig. 3, H) is extremely thread-like, greatly resembling, in fact, a Spirochaete; on the other hand, both male and female individuals have the form of a very wide spindle.

In Trypanoplasma and Trypanophis there are two flagella, inserted into the body very close to the anterior end (fig. 4, F and G). One flagellum is entirely free and directed forwards; the other at once turns backwards and is attached to the convex or dorsal side of the body for the greater part of its length. In all other Trypanosomes there is only one flagellum, which is invariably attached to the body in the same manner as the posterior one of biflagellate forms. This flagellum, however, is most probably not to be considered homologous in all cases. (See Woodcock, loc. cit.) In Trypanomorpha (fig. 3, E), which is to be derived from a Herpetomonadine type, the single, anterior flagellum of the ancestral parasite has been drawn backwards along one side of the body and now originates in the posterior half. Hence in this genus the end bearing the free part of the flagellum is the anterior one. The genus Trypanosoma, in which are included at present the great majority of Trypanosomes, is rather to be regarded as derived from a Heteromastigine ancestor, such as Trypanoplasma, by the loss of the anterior flagellum. Hence in this type the single flagellum represents the posteriorly-directed one of Trypanoplasma, and the end at which it becomes free is the hinder end. The point of origin of the flagellum in Trypanosoma is usually near the anterior end, but may vary considerably (cf. figs.); and its free portion may be very short or lacking.

Along the dorsal side runs the characteristic fin-like expansion of the body, the undulating-membrane, which is the organella principally concerned in locomotion. This always begins at the place where the attached flagellum emerges from the body; and its free edge is really constituted by the latter, which forms a flageIlar border. The membrane is usually more or less sinuous in outline, and is sometimes thrown into broad folds (fig. 3, F and J). Distally it thins away concurrently with the body.

(After Doflein.) FIG. 2. - Trypanosoma equiperdum (of dourine), in the blood of a rat eight days after inoculation.

a, Parasites.

b, Blood-corpuscles.

The body appears to be in all cases naked. A differentiation of the peripheral cytoplasm in the form of an ectoplasmic layer has been described in one or two instances, and it seems probable that in most Trypanosomes there is such a layer, although only poorly developed, as a rule, around the body generally. On the other hand, the undulating-membrane is largely if not entirely an ectoplasmic development. This is usually much clearer and more hyaline than the general cytoplasm. In many forms deep-staining grains or granules, of a chromatoid nature and of var y ing size, are to be seen in the cytoplasm. In B.

only is there an intimate correspondence in this respect between the two principal organellae, but the flagellar apparatus itself is really of nuclear origin and remains closely connected with the kinetonucleus (cf. fig. 7). In most cases, however, little beyond the position and general appearance of the nuclei has been so far made known. The trophonucleus is usually situated somewhere about the middle of the body. The kinetonucleus is typically near the anterior end; but in a few instances it lies more centrally (e.g. T. inopinatum, T. rotatorium, fig. 4, A - C); in Trypanomorpha it is in the posterior half of the body (fig. 3, E).

In certain forms the occurrence of prominent myonemes or muscle-fibrillae has been described, and, moreover, a nuclear origin assigned to them also. In Trypanomorpha they are confined to the undulating-membrane (fig. 3, E), but in other cases - Trypanosoma ziemanni, T. lewisi, T. brucei, and T. soleae - they are arranged laterally, half running down each side of the body (fig. 4, J). In Trypanoplasma borreli there is only a single myoneme on either side.

All Trypanosomes are capable of binary longitudinal fission, and this appears to be the chief method of multiplication. The division of the nuclear appa ratus is the first to take place (fig. 5, A). The kinetonucleus more often leads the way, but sometimes either kinetonucleus or trophonucleus may do so indifferently. The duplication of the flagellum begins at its proximal end, that which is in relation with the kinetonucleus. Until recently the process has been considered as an actual longitudinal splitting of the flagellum, following upon the separation of the two daughter-kinetonuclei. Both Schaudinn (in the case of Trypanomorpha) and Prowazek (in the case of Trypanosoma lewisi and T. brucei), have found, however, that the new flagellum is developed quite independently and laid down alongside the old one. It is at present somewhat uncertain, therefore, in what cases actual splitting occurs. The same applies equally to the formation of the undulating-membrane. H If the flagellar border splits, the membrane doubtless divides also; but where the flagellum is a new formation the membrane will be too. The division of the cytoplasm in most forms is equal or sub-equal, and two approximately equal daughter-Trypanosomes result (fig. 5, C). In some instances (e.g. T. equinum, T. equiperdum) the longitudinal fission is apparently multiple, three or even four descendants being produced simultaneously.