Tropical Diseases

Front matter

TROPICAL DISEASES

Pellagra. An English case, diagnosed by Dr. Sambon. (Reproduced by permission of the "Wellcome" Bureau of Scientific Research.)

Tropical Diseases

Appendix : Some of the Commoner Parasitic Protozoa of Vertebrates and Invertebrates

Appendix

SOME OF THE COMMONER PARASITIC PROTOZOA

OF VERTEBRATES AND INVERTEBRATES

RECENT discovery has clearly demonstrated the importance of the Protozoa as factors in the etiology of disease, especially of tropical disease.

Manifestly the subject is still in its infancy. Many as yet unknown species of blood protozoa will doubtless be added to the already considerable list of those attacking man and the lower animals. As regards man, it is doubtful if we have as yet completed the list even of the much-studied malaria parasites, much less determined their respective life histories and zoological relationships. Moreover, there are certain genera of blood protozoa which, although occurring in other mammals, have, so far, not been found, or, at all events, recognized in man, Babesia and Hæmogregarina for example. It is not unreasonable to conjecture that, although hitherto unrecognized, representatives of these genera do occur in man, and that ere long they will be discovered. The investigator must not lose sight of this probability.

The entire subject is one of the utmost importance to the medical man in the tropics, and, whether he would perfect himself in existing knowledge or seek to add to it, he should study it practically and from the standpoint of comparative pathology. I would therefore strongly recommend him to avail himself of the abundant opportunities he is sure to enjoy to study the protozoa not in man only but in other mammals, in birds, in fishes, and in reptiles. As a guide and help in such studies I append a brief summary, for which I am indebted to Dr. Philip Bahr, of our present knowledge of this important class of parasites, so far as it has a bearing on human pathology. As to classification, still a quœstio vexata, the one adopted in Prof. Minchin's " Introduction to the Protozoa " (1912) has been for the most part followed.

DEFINITION OF THE PROTOZOA

Protozoa are unicellular organisms possessing a nucleus distinct from the cytoplasm. They belong to the Protistenreich of Haeckel, a kingdom which unites the simplest and most primitive forms of life, and which may be considered equivalent in systemic value to the animal and vegetable kingdoms.

The protozoal cell constitutes an entire individual; it may exist singly, or combined in the form of cell colonies.

The body is a mass of protoplasm sometimes enclosed in a limiting envelope. Such organs as are present are also protoplasmic, and are used for the purposes of locomotion and capture of food. Metabolism is generally of the animal type.

Reproduction takes place by fission. Sexual reproduction generally occurs throughout the group, and, as compared with other Protista, the developmental cycle may be a very complicated one in which the same organism reappears in a totally different form at different periods of its development.

There are certain forms of development in the blood protozoa which it is necessary to define and for which certain terms are used. Their application, however, when loosely used has given rise to much confusion. The protozoa parasitic in blood of the vertebrate host undergo a vegetative or non-sexual multiplication, the process being then known as schizogony; but in order to ensure their further existence, and their passage from one vertebrate host (intermediary host)* to another, a further cycle in a cold-blooded invertebrate host (definitive host) is necessary, during which the sexual cycle or sporogony takes place.

In the vertebrate blood the young parasite is known as a schizont. The products of the asexual fission are known as merozoites; these merozoites at some period of their existence develop into forms known as sporonts, destined to pass into the alimentary tract of the inveitebrate host. The term trophozoite is applied to the young schizont or sporont which, is absorbing nourishment direct from the cell in which it is lodged.

Parthenogenesis is not recognized as occurring in the protozoa, but was supposed by Schaudinn to take place in the case of the malaria parasite and to be responsible for the relapses of that infection.

These sporonts, still in the blood of the vertebrate host, develop into sexually differentiated cells (gametocytes), which are quiescent until they are removed from the blood; when this occurs both male and female cells extrude two polar bodies preparatory to the sexual process, and are then known as gametes. The flagella or microgametes extruded by the male cell function as spermatozoa, enter and fertilize the female cell (macrogamete), thereafter known as a zygote.

After this act the zygote in some forms, especially the Plasmodidæ, becomes an elongated motile body (oökinete or travelling vermicule), which may remain free or become encysted (oöcyst). The oöcyst gives rise to secondary bodies in its interior (sporoblasts), which in turn develop into a number of minute rods known as sporozoites. Usually these sporozoites continue to enter the mouth parts of the insect, and by this route they once more gain the blood of the vertebrate host.

The Protozoa are generally divided into four main classes.

Class I.-SARCODINA

Protozoa in which the protoplasmic body has no limiting envelope in the form of a stiff cortical layer, thus tending to a spherical shape in the floating forms, or an irregular, ever-changing shape in the creeping forms. Organs serving for locomotion and capture of food are known as pseudopodia; a skeleton or shell may be present. Encysted resistant forms occur which may give rise to a number of daughter cells.

Sexual conjugation (gametogamy} is the usual process of reproduction, though autogamy within the cyst wall has been described.

EXAMPLES: Amœba limax, Entamœba coli. Entamœba histolytica.

Class II. MASTIGOPHORA

Protozoa in which the organs of locomotion and for food capture are flagella, that is to say, long slender filaments capable of performing lashing movements.

EXAMPLES: Trichomonas, Lamblia, Trypanosoma.

Class III.-SPOROZOA

Protozoa occurring as blood or intracellular parasites of other organisms. They possess no definite organs for locomotion or for ingestion of food. The typical reproduction takes place by the formation of spores and minute germs called sporozoites.

EXAMPLES: Gregarina, Coccidium, Plasmodium.

Class IV. INFUSORIA

Protozoa in which small filaments called cilia serve as organs of locomotion; these are distinguished from flagella by their much smaller size and in being present in great numbers, forming a fine covering over the whole of the body. The body protoplasm is always corticate.

EXAMPLE: Balantidium.

Class I. SARCODINA

This class, sometimes termed the Rhizopoda, includes parasitic amœbæ as well as free-living forms such as the Radiolaria and Foraminifera; these latter are of little interest to the student of tropical medicine. The distinctive characters of the amœbæ parasitic in man have already been given (p. 511). These are capable of independent amœboid locomotion by the alternate protrusion and retraction of protoplasmic processes called " pseudopodia."

Entamœba coli reproduces itself by binary fission in the ordinary way; this is called the vegetative or multiplicative phase. In addition to this a cystic or resting stage occurs, when conditions under which the parasite exists become adverse.

A sexual process, called autogamy, occurring within the cyst wall was described by Schaudinn, though recent authorities are inclined to doubt it. The process, as described, is as follows: The cyst contains at first but one nucleus, which divides into two. Each nucleus is then said to break up into chromidia and disappear, but to reappear later as a smaller and less distinct secondary nucleus on each side of the cell. Each secondary nucleus again divides into three; two of these again degenerate as reduction nuclei, while the third persists as a sexual or gameto-nucleus. The gameto-nuclei divide into four pronuclei, then two of each side conjugate and fuse, forming two nuclei or " synkarya" which subsequently divide into eight nuclei, in this manner forming eight amœbulæ within the cyst.

The end-product of the cystic stage of Entamœba histolytica possesses but four nuclei, and is known as the Entamœba tetragena of Viereck. The process of cyst formation by a budding process of the vegetative form, as described by Schaudinn, is now held to be incorrect.

The vegetative forms of the pathogenic and non -pathogenic species are further distinguished by differences in their protoplasmic contents. In E. histolytica the endoplasmic zone is granular, the ectoplasmic hyaline, while in E. coli no such differentiation occurs. The nuclei of the two forms have also distinctive differences. E. coli has a large and distinte nucleolus (karyosome) and a distinct nuclear membrane; whereas the E. histolytica nucleus is not so markedly differentiated (see p. 514).

Amœba limax and A. terricola are free-living forms of amœbæ occurring in water and earth. Their cystic stages possess a hard and refractile cyst wall. They are of importance to the tropical pathologist in that they may be ingested and recovered from the stools of dysenteries and others in culture on Musgrave and Clegg's medium. The parasitic amœbæ proper to man have not been so cultivated.

Class II. MASTIGOPHORA

The Mastigophora are divided into three subclasses, of which only one subclass, that of the Flagellata, containing the more typical forms, deserves our attention. The subclass Flagellata is divided into four orders:

i. Pantasomina.

ii. Protomonadina.

iii. Polymastigina.

iv. Euglenoidina.

The second and third only of these orders contain parasitic genera.

Order ii. Protomonadina

The Protomonadina are mostly flagellates of a small size with a principal and one or more subsidiary flagella. They are either saprophytic or parasitic.

Suborder 1.— Cercpmonas is a frequent parasite in the intestine of man; it is oval in shape, with a single nucleus and well-marked nucleolus, and has two flagella, one passing forwards, and the other backwards over the surface of the body to the hinder end, which is frequently drawn out into a protrusion resembling a tail.

Suborder 2.— Bodo (Fig. 225) is similar in shape and general structure to Cercomonas, but has two flagella, one directed forwards, the other backwards as a trailing flagellum; it occurs both as free-living forms and as parasitic in animals, for the most part in the digestive tract.

Suborder 3.— Prowazekia.— Prowazekia resembles Bodo in the arrangement of its flagella, but in the possession of a tropho- and a kinetonucleus it more resembles Trypanoplasma. It differs from Trypanoplasma in that the backward-directed flagellum is free from the body and devoid of an undulating membrane. Several species have been described from human fæces, and one, P. cruzi, is considered to be the cause of a diarrhœa in China.

Suborder 4.—The Hæmoflagellata and allied forms are an important suborder of the Protomonadina, and under this term are grouped a number of forms having a characteristic but not invariable parasitic habitat in the blood of vertebrates and in the digestive tract of invertebrates.

The group comprises a number of heterogeneous forms conveniently grouped together, their chief morphological feature being the possession of two nuclei, a tropho- and a kinetonucleus (blepharoplast). For this reason they were included by Hartmann as a distinct order of the Flagellata and termed the "binucleata."

The following five genera represent the more important types: (1) Trypanosoma, (2) Trypanoplasma, (3) Crithidia, (4) Herpetomonas, (5) Leishmania.

Fig. 225.—Bodo. N., Nucleus; Rh., rhizoplast; Fl., flagella of unequal length.

1. The Genus Trypanosoma.—The structure of the trypanosome body is of a uniform type, though subject to variation in minor details. The body is long and sinuous; the anterior end tapers gradually to a narrow point, while the posterior end generally terminates more bluntly.

The principal nucleus (trophonucleus) is situated centrally; the kinetonucleus is usually placed posterior to the nucleus, but is sometimes closely approximated to it. Certain exceptions to this rule are known, namely, the recently described T. rhodesiense (p. 185), and some multiplicative forms of T. lewisi. The flagellum arises from a centriole (blepharoplast) closely connected with the kinetonucleus, and passes forwards as the margin of the undulating membrane; in some cases it may end with the undulating membrane at the anterior extremity of the body, but more usually it projects forwards as a free lash. The arrangement of the centriole and kinetonucleus varies; sometimes the former is lodged within the latter, sometimes connected with it by a delicate rhizoplast. In one species, T. equinum (Fig. 226), the cause of "mal de caderas" in South America, the kinetonucleus is very minute.

Trypanosomes occur as blood parasites in all classes of vertebrates. Many wild animals harbour small numbers of these parasites in their blood. Certain are specific and probably harmless to their particular vertebrate host.

Trypanosomes of cold-blooded animals include T. rajæ, T. damoniæ, T. granulosum, and T. rotatorium (Figs. 227-30).

Though showing but slight morphological differences as a general rule, members of this genus have been differentiated on morphological as well as on pathological grounds.

Fig. 227.—T. rajæ. (After Laveran and Mesnil.) A trypanosome of fish (the skate). | Fig. 228.—T. damoniæ. (After Laveran and Mesnil.) A trypanosome of the tortoise (Damonia revesi). | Fig. 229.—T. granulosum. A ttrypanosome of the eel.

Fig. 230.—T. rotatorium. (After Laveran.) A trypanosome of the frog (Rana esculenta).

The Lewisi group includes T. lewisi, T. cuniculi, T. duttoni of the mouse, T. microti of Microtus arvalis, all about equal in size (24-25 μ in length and 1-2 μ in breadth), and similar in general appearances; that is, the organisms are long and slender; both the flagellar and aflagellar extremities are markedly pointed and the nucleus situated towards the flagellar end. They are non-pathogenic, or only slightly pathogenic.

The Brucei group includes nearly all the pathogenic varieties: T. brucei (nagana), T. gambiense (sleeping sickness), T. rhodesiense (human trypanosomiasis in Rhodesia), T. evansi (surra), T. hippicum (murrina, a disease of mules in Panama), T. equinum (mal de caderas), T. equiperdum (dourine, or mal du coït), T. vivax vel cazalboui (souma disease of mules and cattle in the Soudan).

These trypanosomes are all about the same size (22-28 μ in length and 1-3 μ in breadth). The aflagellar extremity is blunted to a variable extent, the nucleus is centrally placed, and the protoplasm granular. For further details of the members of this group, see p. 194.

According to some authors, T. dimorphon, a parasite of horses in the Gambia, producing symptoms resembling nagana, represents a distinct type. As its specific name implies, it is found in two distinct forms—one long form, 22 μ in length; and a short, stumpy form, 1-5 μ in length. The aflagellar extremity is markedly blunted, and the body protoplasm is continued to the end of the flagellum.

T. nanum resembles T. dimorphon in general features, and is the smallest of all the mammalian trypanosomes, measuring 11-16 μ by 2·5 μ. It produces a chronic disease, marked by emaciation and anæmia, in cattle in the Soudan.

T. theileri (Fig. 231), 60-70 μ in length by 4 μ in breadth,

Fig. 231. T. theileri. (After Laveran.) | Fig. 232. T. lewisi. (After Laveran.) | Fig. 233. T. brucei. (After Laveran.)

one of the largest mammalian trypanosomes, is recognized by its large size and rapid movements. It is apparently non-pathogenic, and is transmitted, according to Theiler, by Hippobosca rufipes. T. ingens, found in cattle in Uganda, is still larger than T. theileri, measuring 72-122 μ in length, and the flagellum alone measuring 17 μ.

The transmission of these trypanosomes from one host to another is carried out, so far as is known, by the agency of some blood-sucking invertebrate. When such a host is of terrestrial habit the definitive host is an insect, when aquatic a leech; though in one species, T. equiperdum, the cause of "dourine" in horses, the trypanosomes may pass from one host to another during coitus. Hereditary transmission in trypanosomes has not yet been proved. One or more species of insect may serve as the definitive host; thus in T. lewisi the selective hosts are the rat-fleas (Ceratophyllus fasciatus and Xenopsylla cheopis), though it may also develop in a different genus, the rat-louse (Hæmatopinus spinulosus), and also in Pulex irritans and Ctenocephalus canis.

The life cycle in the vertebrate host is generally as follows: After infection (as in T. lewisi, Fig. 232) the trypanosomes appear in the blood about the fifth to the seventh day. Owing to the rapidity with which fission takes place, the forms are of great variety. After two or three months have elapsed the swarms of trypanosomes in the rat's blood have begun to diminish and eventually disappear. When this has taken place that rat is immune to that particular species of trypanosome.

No confirmation of Schaudinn's statement that some part of the development of trypanosomes in the blood is ultra-microscopic has been made, though nitrations of infected blood plasma through a bacterial filter have been experimented with. Fry and Ranken have observed a process of granule-shedding in trypanosomes under the dark-ground illumination (see p. 161).

In some instances trypanosomes appear in the peripheral circulation only at certain periods, as in the case of T. rhodesiense in man; and in others, as in the trypanosome of Athene noctxa, the little owl, they are said to be most numerous in the peripheral circulation during the night-time, and then only during the summer months; in the winter months they are found in the bone marrow.

The cycle in the invertebrate host takes place mainly in the digestive tract. Thus, T. lewisi, after being imbibed by the flea, multiplies rapidly, chiefly in the stomach cells, subsequently passing into the hindgut and rectum. Here the parasites assume a leptomonas and crithidia appearance, and occur in large bunches attached by their flagellar ends to the epithelium, where they divide with great rapidity, or they may occur in cyst-like masses within degenerating epithelial cells. Eventually they give rise to the small infective trypanosomes first described by Swellengrebel and Strickland, which occur in vast numbers in the hind-gut and rectum of the flea. The latter becomes infective in four to seven days, and it may remain infective for forty -five days or longer. The flea does not infect the rat through its proboscis, and the parasites are not found in the flea's salivary glands. Theoretically infection can take place in three ways: (a) the flea harbouring the infective forms of the flagellates may be crushed and devoured by the rat; (b) the rat may lick its fur upon which an infected flea has just dejected; (c) the rat may lick and infect with flea dejecta the wound produced by the insect. Of these the second is probably the most common method; the other two are doubtful.

Many pathogenic trypanosomes in Africa are transmitted by tse-tse flies (Glossina), and probably these flies remain infective for life. Thus, T. gambiense of man is transmitted by Glossina palpalis, and perhaps by G. morsitans as well; T. brucei (T. pecaudi) (Fig. 233) of big game, and T. rhodesiense of man, by G. morsitans; T. vivax (probably identical with T. cazalboui) by G. morsitans, palpalis, longipalpis, and tachinoides; T. caprœ by G. morsitans; T. dimorphonby G. palpalis; T. grayi, the trypanosome of crocodiles, is also transmitted by this same glossina; T. evansi, of "surra," a disease of horses in India, by Stomoxys nigra, and probably by a tabanus as well. The trypanosomes of birds probably develop in mosquitoes; those of fishes and turtles in leeches.

Two methods of transmission of trypanosomes must be distinguished. The one, a mechanical method, can be performed by any blood-sucking insect for a few hours after a meal of infected blood; the other, indirect or cyclical, is a developmental cycle, and can only take place in the particular definitive host—that is, in most instances, some species of glossina—to which the particular species of trypanosome has become adapted.

The development of T. gambiense (Fig. 234) in G. palpalis, thanks to the researches of Kleine, Taute, Bruce and his collaborators, is now fairly completely known. Apparently the whole development takes eighteen to twenty-five days or more. Five to seven days after ingestion of infected blood the trypanosomes become scarce in the digestive tract of the fly, only to reappear in large numbers and in a variety of crithidial and other forms. Finally an invasion of the salivary glands by short, stumpy, trypaniform individuals takes place. In T. vivax (cazalboui) a completely different mode of development in glossina takes place, a method termed by Roubaud évolution par fixation directe; this occurs solely in the proboscis of Glossina palpalis. The trypanosomes assume a leptomonas shape and attach themselves by their flagellum to the labium or hypopharynx of the proboscis tube, multiplying in the salivary fluid; these flies remain infective for life. A third method of development occurs in the case of T. dimorphon—the évolution par fixation indirecte of Roubaud. In this case the trypanosomes multiply first in the digestive tract of the glossina, and then pass forwards into the proboscis.

In T. cruzi the development both in the intermediary and in the definitive host is of so different a character that Chagas has relegated it to a special genus, Schizotrypanum. In man the adult trypanosomes multiply not in the peripheral circulation but in the internal organs of the body. In the capillaries of the lungs the adult trypanosome loses its flagellum and occasionally the kinetonucleus as well, the body becomes round, and the mass breaks up into eight small individuals (merozoites). Some regard these merozoites as exhibiting a sexual differentiation. The merozoites enter the red blood-cells, and so gain the general circulation. Within the corpuscles they form into a normal trypanosome, which, when set free, either repeats the schizogonic cycle or enters into the invertebrate host, in which it develops still further.

A second type, and probably the normal method of multiplication in man, takes place within the hypertrophied endothelial cells of the lung and striped muscles, notably the cardiac muscle, but can apparently occur in any tissue. In this situation the parasite assumes the shape and appearance of a leishmania.

The developmental cycle of this trypanosome takes place in a bug, Lamus megistus, and is shortly as follows: The trypanosomes taken up into the bug's stomach change in about six hours; having lost their flagellum they assume a leishmania form and multiply by fission. After a while these forms assume a crithidial shape and pass into the intestine, eventually gaining the body cavity and salivary glands of the insect as small trypanosomes, and in this manner pass into the vertebrate host.

The changes undergone by trypanosomes in culture on N.N.N. medium (p. 158) are diverse and striking; they then exhibit a series of forms quite distinct from those seen in the blood of the vertebrate, but bearing more resemblance to those seen in the invertebrate host. In such cultures they assume crithidial, leptomonas, and even leishmania forms.

2. The Genus Trypanoplasma (Fig. 235).—The species of this genus parasitic in blood are only known as yet to occur in fresh-water fishes, and are transmitted by leeches. A number of species are parasitic in the organs of snails, such as T. helicis, found in the receptaculum seminis of Helix pomatia. The principal feature is the presence of two flagella arising together from two blepharoplasts at the anterior extremity. One flagellum projects forwards, passing down the side of the body to the hind end, forming the free edge of the undulating membrane and projecting freely backwards as a free flagellum.

Fig. 236.—Crithidia. Fl., Flagella; M., undulating membrane; Bl., blepharoplast; N., nucleus.

3. The Genus Chithidia (Fig. 236).—The distinctive feature of this genus is the relatively short undulating membrane and flagellum arising from the centre of the body at a point in close proximity to the kinetonucleus. The shape of the body varies considerably. Crithidial stages are commonly found as developmental forms of trypanosomes. There remain, however, a number of forms which are parasitic in the intestinal tract of insects, notably the Chironomidæ and Pulicidæ.

4. The genus Herpetomonas (Leptomonas).—Intestinal parasites of insects, especially muscidæ and hemiptera. In most cases they are parasites of insects alone, infection being brought about by the ingestion of cyst-infected fæces. Some species are parasitic in the latex of plants, and are taken up by the appropriate plant hemiptera. The distinctive characters (Fig. 237) are the possession of a single flagellum arising from a single kinetonucleus, itself situated in the anterior portion of the body, and the absence of an undulating membrane. Uniflagellate and biflagellate forms occur. Some authorities regard the latter as the result of precocious formation of a new flagellum in anticipation of longitudinal

Fig. 237.—Herpetomonas muscæ domesticæ. (After Prowazek.) Fl., Flagella; Bl., blepharoplast; N., nucleus; Ax., axostyle.

division. They have shown that trypanosome forms, but lacking the well-developed undulating membrane so characteristic of the mammalian trypanosome, also occur, the changes in form being in all probability brought about by the migration of the nuclei; all stages being found between the non-flagellate leishmania, through leptomonad, crithidial, to the trypanosome form (Fig. 240).

Fig. 238.—Leishmania donovani. (After Leishman.)

5. The genus Leishmania includes human parasites, L. donovani (Fig. 238), the cause of a general disease, kalaazar, and L. tropica, the cause of oriental sore—a purely local infection. L. donovani, in the Mediterranean area, is found especially in children, and is associated with a similar disease occurring in dogs and caused by the same parasite. The type of parasite found in the vertebrate host is a small oval body multiplying by fission and contained within cells—endothelial cells or macrophages; it is very uniform in size. Each parasite possesses a tropho- and a very distinct kinetonucleus (blepharoplast).

In cultures on the N.N.N. medium the leishmania increase in size, and develop a flagellum, finally assuming an elongated leptomonas form. It is thought that this latter represents a stage of the development of the parasite in the invertebrate host; such a development, however, has not yet been followed. Patton believes that in the case of L. donovani the parasite completes its developmental cycle in Cimex rotundatus, and in the Mediterranean area there is a certain amount of evidence to incriminate Ceratophyllus fasciatus, the dog-flea, as being the definitive host. In the case of L. tropica, it has been pointed out that since the sores, as they occur in man, are situated in some uncovered part of the body, the invertebrate host is more likely to be some biting fly or mosquito, or even the sand-fly (Phlebotomus).

Morphologically the leishmania is a leptomonas, but the fact that it occurs in a vertebrate host should justify its inclusion in a separate genus, Leishmania.

Fig. 239.—Diagram of types of flagellates. (After Wenyon.) a, Trypanosome; b, Crithidia; c, Leptomonas; d, Leishmania.

Nomenclature of the hæmoflagellates.—The generic terms given to these different parasites are also utilized to denote the different stages in their life cycles. Of this cycle the leishmania is taken to represent the most primitive, the trypanosome of vertebrates the most highly developed form (Fig. 239).

One and all these stages occur in the developmental forms of certain trypanosomes in culture, and in the gut of the definitive host the glossina, and even in the vertebrate, as in T. lewisi. The generic titles Trypanosoma, Herpetomonas, Crithidia and Leptomonas should, strictly speaking, be reserved for the parasite according to the highest developmental form it is capable of assuming. The accompanying diagram, taken from Wenyon (Fig. 240), illustrates this idea.

Parasites of Uncertain Zoological Position resembling Leishmania

Hæmocystozoon braziliense was found by Franchini in the blood of a man from Brazil. The parasites are sometimes oval, the size of a red blood-cell, containing a large nucleus, a variable amount of pigment, and one

Fig. 240.—Diagram of the classification of the trypanosomes and allied flagellates. (After Wenyon.)

or more centrosomes, and sometimes a terminal flagellum arising from a blepharoplast; sometimes non-flagellate and very minute forms were found in the red cells. In a liver puncture, round, clear and encysted forms occurred, enclosed exceptionally in lencocytes. The patient suffered from irregular fever and anæmia combined with a cystic enlargement in close proximity to the inferior cervical glands. Liver and spleen were also enlarged.

Histoplasma capsulatum was found by Darling in Panama in the tissues of one Chinaman and two negroes who died of some obscure malady characterized by wasting, irregular fever, enlarged spleen, and anæmia. In smears and sections of the lung small bodies were found within the epithelioid cells which bore a great resemblance to leishmania, but differed in possessing no kinetonucleus (blepharoplast). According to Rocha-Lima the parasite is really a yeast-like organism and has nothing to do with the protozoa, but resembles the Cryptococcus farciminosus, the cause of epizootic lymphangitis of horses in Senegal.

Order iii.—Polymastigina

Polymastigina possess three or more unequal flagella; a distinct mouth opening may be present or absent; in other points they resemble the foregoing. The Polymastigina are divided into two divisions or suborders—the Tetramitidæ and the Octomitidæ.

|

Fig. 241.—Trichomonas intestinalis. (After Martin and Robertson.) | Fig. 242.—Lamblia intestinalis. (After Wenyon.)

Bl., Blepharoplast; Ch. L., chromatinic line; M., mouth; Fl., ant. flagella; N., nucleus; Ch. B., chromatic body; Ax., axostyle; U. M., undulating membrane. | N., Nuclei; Ax., axostyles; Fl. 1-4, four pairs of flagella.

Suborder 1.—The Tetramitidæ have three or more flagella arising at the anterior end; the trailing flagellum, if present, may be united to the body by an undulating membrane.

Trichomonas intestinalis (Fig. 241), a common parasite in man, has the trailing flagellum united to the body by such an undulating membrane. It is a matter of controversy whether encystment occurs. Trichomonas, in addition to its motile powers, is capable of a certain amount of amœboid movement; it possesses a definite but small mouth cavity, and a stiff supporting rod known as the axostyle.

Tetramitus mesnili also occurs as a harmless parasite in human fæces; it resembles trichomonas in its general features, but differs in the lack of an undulating membrane and in the presence of a well-marked cystosome in which there is either a flagellum or a membrane; there are three anterior flagella as in tnchomonas.

Suborder 2.—The Octomitidæ, for the most part intestinal parasites, have six or eight flagella, generally arranged in pairs; the body is bilaterally symmetrical in structure.