TIMOTHY UEARY

d Corner Book

PLATE I.

Entamoeba histolytica.

Chilomastix mesniii

omonas hominis.

Entamoeba coli

Some of the Intestinal Protozoa of Man, as they appear when alive and active.

THE INTESTINAL

PROTOZOA OF MAN

EY

CLIFFORD DOBELL, and F. W. O'CONNOR,

M.A., F.R.S., Protistologist to

the Medical Research Council,

National Institute for Medical

Research, London.

R.C.S., L.R.C.P., D.T.M. & H. Wandsworth Scholar, London School of Tropical Medicine.

TIMk,. uEARY

' ' O, wonder ! ' ' How many goodly creatures are there here " How beauteous mankind is ! 0 brave new world, " That has such people in't ! "

Shakespeare, Tempest, V. i.

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PREFACE.

The following treatise is addressed to all Zoologists and Medical Men who are interested in the Intestinal Protozoa of Man, but more especially to those whose professional duties demand an intimate practical know- ledge of these organisms. During the recent Great War the need for such a work became urgent : and although the War is now ended, and interest in the subject has waned, there must still be many workers, especially in the tropics, to whom a work of this character would be if properly executed of very great service.

It appeared to the present authors that such a book touching upon the two fields of Zoology and Medicine ought to be written jointly by a zoologist and a medical man : for by such collaboration many mistakes, due to the limited knowledge of either, might obviously be avoided. This consideration, and a mutual interest in the subject, prompted the authors of the following work Captain O'Connor and myself to enter into partnership. It was originally agreed between us that we should write the book together, though one of us should be specially responsible for the medical parts, the other for those parts which were purely protozoological.

Unfortunately, it proved impossible to carry out our original inten- tions. The work was first planned at the end of 191 8 : but in the autumn of the following year, when the book had only been sketched out and begun, Captain O'Connor left England on a scientific expedi- tion to the Gilbert and Ellice Islands. Further collaboration thus became impossible, and the completion of the work consequently devolved entirely upon me. As many papers on this subject have been published recently, and as I have continued my own researches during the last few years, it will be understood that the book, as it now appears, is in many ways very different from that originally planned.

For several sections of the book I am solely responsible. The most important of these are : the Introduction (Chap. I) ; the section dealing

PREFACE

with the coprozoic organisms— based largely upon hitherto unpublished researches ; the lists of synonyms, and keys for the determination of genera and species, together with all discussions of systematics and classification ; the references at the end of the volume, and the general bibliographic work throughout. I have also drawn all the illustrations, with the exception of figs. 97-102 (PI. VI), which I have merely redrawn from Captain O'Connor's originals— these having proved unsuitable for reproduction. Footnotes which contain my personal opinions are dis- tinguished by bearing my initials, whenever it has seemed desirable or necessary to indicate their authorship.

I have thought it right to narrate these particulars here. But my object in so doing is not that I may claim the greater share of credit— if any there be— for our joint performance, but to exonerate my partner from blame for the mistakes which have doubtless been made. During the last eighteen months, whilst I have been engaged in writing and revising the book, and in passing it through the press, I have been entirely deprived of his counsel. I have been unable to discuss with him any of the new work which has appeared. I have changed my views on various subjects as I have learned new facts, and I have had no means of ascertaining whether his views have undergone corres- ponding changes. Consequently, although Captain O'Connor permitted me in fact besought me to make any alterations which appeared to me necessary during the progress of the work, I feel that I have been compelled to take far greater liberties with his contributions than any ordinary collaborator would have a right to take. And while it is my hope that I have not, in the following pages, expressed any views from which Captain O'Connor would dissent, yet I feel it incumbent upon me to point out here that, for any mistakes which have been made, a far greater share of responsibility lies upon my shoulders than upon his. During the preparation of this work I have fortunately been able to consult Captain S. R. Douglas, I. M.S. (ret.), on medical matters outside my competence. He has also had the kindness to read through Chapters III, VII, and VIII, which have profited by his help and criticism. For these services we offer him here our sincere thanks. I wish also to thank Professor W. Bulloch, F.R.S., for supplying me with a number of references to works which I should else have overlooked. We are further indebted to the Editor of Parasitology, Professor G. H. F. Nuttall, F.R.S., for permission to republish fig. 28 (PI. Ill); and to Lieutenant-Colonel W. Byam, R.A.M.C., and the Oxford University Press, for allowing us to use figs. 27 (PI. Ill) and 109-111 (PI. VII),

PREFACE VII.

which were drawn originally for Byam and Archibald's forthcoming treatise on The Practice of Medicine in the Tropics.

I would point out here that the figures have all been drawn unless the contrary is expressly noted from actual specimens, with the aid of the camera lucida. They are not diagrammatic. But the figures on the Frontispiece (PI. I), though not intended to appear schematic, were drawn from memory and imagination. They are composite pictures made accurately to scale, and as correct as possible in their details, but not copied from any particular specimens. It is impossible to draw an actively moving protozoon with the camera lucida ; and the artist who professes to depict such an organism " from life " must always, in reality, first observe it accurately, and then make his drawing from memory combining the thousands of changing images which have fallen upon his retina into a single fixed and lifeless picture. The figures on Plate VIII are frankly " diagrams " of the same sort, so drawn for a special purpose. They are attempts to show to others, as accurately as is possible by means of single images, the appearances which I have seen upon innumerable occasions. Not one of these figures has been copied from any particular specimen, but each is a general description with the brush instead of the pen of the thousands of similar individual objects which have passed before my eyes. It is really impossible to convey an exact impression of such objects by means of drawings ; and when such drawings have been more or less effectively executed, it is almost impossible to overcome the difficulties involved in the process of reproduction. The methods by which I have, in the present case, " faked " the figures into a semblance of reality, are too obvious to require comment.

It has been our aim, throughout this work, to be as brief and accurate as possible. We have made no attempt to treat the subject in an encyclopaedic manner, but have aimed rather at producing a practical handbook a book which will help the beginner, and at the same time assist more serious students in the prosecution of their studies. A work of this character would be of little use if it did not contain full and accurate references, and I have therefore devoted special attention to the bibliographic aspects of the subject. Every work cited has been consulted in the original, and every effort has been made to insure accuracy in quotations and references. Those who have any knowledge of the subject, and who are acquainted with the almost endless bibliographic errors in most works dealing with it, will realize the toil which this has entailed. The references represent,

Vlll. PREFACE

indeed, the intermittent labour of many years : but I think the time taken over them has not been mis-spent, for it has enabled us to avoid the repetition of many text-book traditions of the unfounded but long-lived type familiar to all students of scientific literature.

It has not been possible to take notice of many works which have appeared in the last few months, but an attempt has been made to incorporate at least a reference to every work of importance which has come to my notice up to the time of going to press. No effort has been made, however, to cite every work that has been written on the subject, since this would have made our references run into many thousands. Hundreds of references have, indeed, been weeded out in the final revision. Judicious selection, rather than compendious collection, has been aimed at in this respect.

It seems to me that it is the duty of every scientific worker to study and weigh what his predecessors and contemporaries have written, and that he should be as careful in quoting them as he is in making and recording his own observations. To neglect to notice the work of others, or to misquote it, is often something more than incivility : it easily leads an author to claim or to appear to claim as his own a discovery or observation to which he has no title. But in dealing with the works of others one must constantly note their errors no less than their good parts. To summarize without criticizing is not possible in a work which aims at being scientific. Error and truth cannot be added together. Consequently, criticism also is a duty to every collector of facts. I have often been taken to task, by reviewers of my previous publications, for the "severity" of my criticisms of the work of others. I wish, therefore, to make this explanation. All my criticisms are directed against opinions or interpretations not against persons. If a statement is true, it will withstand the severest criticism. If, on the other hand, it is false, it cannot be too severely condemned. I thus see no reason to reproach myself for the severity of any criticisms which I may have made, unless they have unwittingly been unjust to persons or unjustified in matters of fact.

But it is easy to destroy and hard to build, and I would therefore end with the words of the ingenious Dr. Edward Tyson,* who long ago excused himself to perfection upon a like occasion : " My design here," said he (and it is ours also), " is not the raising of any Hypothesis,

* See his once celebrated memoir on the Tape-worm, Phil. Trans. Roy. Soc, 1683. No. 146.

PREFACE IX.

but the enquiring into the truth of those of others. It being much easier to spy others faults, then to avoid them our selvs. In what I have said I have done the former ; but can no ways secure my self as to the latter. But in the whole, if I have not hit the mark ; I have fairly aimed for it, and it may be some help, and direction to others in the prosecution of this subject."

Clifford Dobell. London, April, 1 92 1.

" E ben piu facile insegnare una veritk, che stabilirla sopra le rovine di un errore ; e ben piu facile l'aggiungere che il sostituire."

Leopardi.

CONTENTS.

Chapter I. Introduction. The Intestinal Protozoa of Man

II. The Intestinal Amoebae of Man

III. Amoebiasis

IV. The Intestinal Flagellates of Man. " Flagellosis : V. The Intestinal Coccidia of Man. Coccidiosis

VI. The Intestinal Ciliates of Man. Balantidiosis

VII. The Diagnosis of Intestinal Protozoal Infections

VIII. The Treatment of Intestinal Protozoal Infections

IX. The Coprozoic Protozoa of Human Faeces ...

References

Index ...

Plate I, Frontispiece. Plates II VIII, at end of Volume.

*9

40

5S 94 ic6

125 148 164 187 205

; Reade not to Contradict, and Confute ; Nor to Beleeve and Take for granted ; Nor to Finde Talke and Discourse ; But to weigh and Consider."

Bacon, Of Studies (ed. 1625).

THE INTESTINAL PROTOZOA

OF MAN.

CHAPTER I. INTRODUCTION. THE INTESTINAL PROTOZOA OF MAN.

TO speak of Man as a Microcosm " an abstract or model of the world," as Bacon has it is an ancient and familiar figure of speech. But the modern scientific writer can hardly stop short at this metaphor : he knows that, within this microcosm, there is a less poetic and still smaller world which has been revealed to the inquiring eye of the microscopist. Man's body is, indeed, itself a macrocosm for in- numerable micro-organisms ; and it is to one of the microscopic com- munities inhabiting one small province of this very little world the Protozoa living in the intestine of Man that the following treatise is devoted.

The object of this first chapter is to introduce the Intestinal Protozoa of Man to the reader. In the following chapters he will have an opportunity of cultivating their acquaintance more closely ; but this acquaintance cannot ripen into intimacy unless he combines the perusal of this book with a study of the organisms themselves.

Historic Note. In the year 1681, Antony van Leeuwenhoek, the illustrious Hollander who discovered the Protozoa and is rightly regarded as the Father of Protozoology, described a " little creature " which he had observed, with the aid of magnifying glasses, in his own stools.* This little creature was the flagellate protozoon now known as Giardia intestinalis, and its discovery marks the beginning of our knowledge of the Intestinal Protozoa of Man.

The discovery excited but a passing interest, and lay almost forgotten for over a century and a half. Then, in the year 1854, two

* See Dobell (1920), where these observations are considered in detail.

2 THE INTESTINAL PROTOZOA OF MAN

similar little animals were found in human stools by the French parasit- ologist Davaine, who subsequently named them " Cercomonas hominis A " and " C. hominis B." These have since been rediscovered, redescribed, and renamed Chilomastix and Trichomonas. Other forms belonging to the same group of organisms have also been found and studied by Davaine's followers down to the present day.

A much larger animal was found in human stools by the Swedish physician Malmsten in 1856. His organism differed so strikingly from those already mentioned that it clearly belonged to a different group. It is now called Balantidium and has been studied and redescribed by many later workers. About the year i860 another Swede, Kjellberg,* discovered yet another different kind of organism, this time living actually in the tissue of the human bowel. This was the first of the animals now called Coccidia to be described in the human gut ; and its discovery has been followed by the finding of several similar forms which have received the attention of many subsequent investigators.

Finally, a fourth kind of "little creature" was discovered t in human stools by two Anglo-Indian medical officers, Lewis and Cunningham, in the years 1870 and 1871. Soon afterwards in 1875 a similar discovery was made by Losch in Russia. The organisms which these observers studied are known as amoebae, and belong to a different group of animals from any of those previously noticed. They have now been very thoroughly studied by later workers, and their numbers have been augmented accordingly.

The discoveries briefly related above are all landmarks in the subject with which the present work deals. They mark the beginning of our knowledge of four different groups of microscopic animals which inhabit the human bowel : and the following up of these several discoveries has resulted in the accumulation of an immense mass of facts which now almost form a special science by themselves. It is now known that all these animals belong to one great group of the animal kingdom the Protozoa of which they form, however, an almost infinitely minute part. That they have attracted so much attention is due to the circum-

* The discovery was reported by Virchow in i860. See Dobell (1919) for further details.

f This discovery is usually incorrectly attributed to Lambl (i860). Cf. Dobell (1919 a, pp. 8-9, and 71 et seq.) where additional details will be found.

INTRODUCTION 3

stance that sonic of them like the organisms found by Malmsten and Losch are associated with human diseases : and although only a few- can claim this unenviable distinction, it has inevitably invested the others also with a particular human interest. Some conception of the present magnitude of this branch of Protozoology can be formed from the scope and size of the present volume.

Having made the foregoing brief allusion to the history of our subject, by way of introduction, we shall attempt in the rest of this chapter to define, very briefly, the Protozoa : to survey, very rapidly, the forms which live in the human intestine : and to point out how these various forms live in this environment. Detailed descriptions will be given in later chapters.

The Protozoa. The Animal Kingdom is usually divided into two main groups, or sub-kingdoms Protozoa and Metazoa. The latter group comprises all the animals whose bodies are built up of the morphological units called " cells," and may accordingly be defined as consisting of all the multicellular animals. The former group is usually defined, in contrast, as comprising the "unicellular" animals. For reasons discussed elsewhere (Dobell, 191 1) the term "unicellular" appears objectionable and misleading ; for it implies that the body of an individual protozoon is homologous with a single cell in the body of a metazoon, and not with a whole metazoal individual. If we regard the whole organism as an individual unit, then a whole protozoon is strictly comparable with a whole metazoon, and not with a part of it. But the body of a protozoon, though it often shows great complexity of structure, is not differentiated internally into cells like the body of a metazoon. Consequently, it differs from the latter not in the number of its cellular constituents, but in lacking these altogether. We therefore define the Sub-kingdom of the Protozoa as the group which contains

ALL NON-CELLULAR ANIMALS.

This is not the place to define " cell" and "animal " : and we shall therefore entrust the comprehension of the foregoing definition to the common sense of the reader.

Classification of the Protozoa. The Protozoa are classically subdivided into four main groups, which are generally called Classes, but which probably correspond more closely, in systematic status, to the groups called Phyla among the Metazoa. Various names have been proposed for these main groups, but we shall follow the usual conven-

4 THE INTESTINAL PROTOZOA OF MAN

tion and call them (i) Rhizopoda, (2) Mastigophora, (3) Sporozoa, (4) Ciliophora.

These four groups, or Phyla, of the Protozoa, can be roughly dis- tinguished by means ot the characters supplied by the external organs of locomotion of the animals placed in them. These characters have been used for classifying the Protozoa ever since 1773, when they were first used for this purpose by the Danish zoologist O. F. Miiller. Modern protozoologists have found such simple characters inadequate, when used alone. Nevertheless, they will suffice for our present purpose, and will enable us to distinguish the four main groups as follows :

(1) The Phylum Rhizopoda comprises those Protozoa whose external organs of locomotion are typically pseudopodia temporary prolongations or extensions of the protoplasm of the body, familiar to everyone as the means of movement in Amoeba.

(2) The Phylum Mastigophora consists of all those Protozoa which move, in their fully developed and typical condition, by means of whip-like filaments or flagella familiar to all who have studied Euglena, or any other common flagellate.

(3) The Phylum SPOROZOA contains a number of exclusively para- sitic forms, which in their motile stages when present move without the aid of any special external locomotory organs. The several common species of Monocystis, parasitic in earthworms, supply familiar examples with their slow, worm-like motions, performed by the body as a whole.

(4) The Phylum Ciliophora contains all the Protozoa which move, in their typical active stages, by the agency of many little hair-like threads or cilia exemplified in the familiar Paramecium and other common ciliates.

Each of these Phyla contains a vast array of species, variously collected into genera, families, orders, and higher groups. It will be unnecessary, however, to discuss their classification in detail here, and we shall limit ourselves to a consideration of the systematic position of those species alone with which the present work is concerned. It will suffice to note the general grouping of our forms, and their more obvious relations to one another.

The human intestine harbours protozoa belonging to all the four Phyla just enumerated. As these groups contain organisms as different

INTRODUCTION 5

from one another and as distantly related as the members of different Phyla among the Metazoa, it will be clear that many of the intestinal protozoa of man have little but their habitat in common. Among themselves they show great diversities, which are expressed by placing them in different systematic groups. The Rhizopods in the human gut, for example, are closely related to the Rhizopods in the guts of other vertebrates and to those leading an independent existence in water : they are but remotely related to the Ciliates found in man, though they chance to share the same habitation. In other words, the organisms with which we have to deal form, as a whole, an "unnatural" group- in the systematist's sense and are treated together merely because Nature has assembled them in a common domicile.

The Rhizopoda are represented in the human intestine by five species of amoebae belonging to four different genera (1) Entamoeba, with two species E. coli and E. histolytica ; (2) Endolimax, with one species E. nana ; (3) Iodamoeba, and (4) Dientamoeba, each also with but a single species /. biltschlii and D. fragilis respectively. All these belong to the Class called Amoebaea, which comprises all the naked rhizopods resembling the well known Amoeba and its allies.

Among the Mastigophora, we find five distinct species each be- longing to a different genus and several other doubtful forms which require further investigation. All of these belong to the Class Flagellata a very large group containing many families and genera. The genera and species found in the human gut are : (1) Trichomonas hominis, with several varieties, (2) Chilomastix mesnili, (3) Giardia intestinalis, (4) Embadomonas intestinalis, (5) Enteromonas hominis. To these some still uncertain forms may ultimately have to be added.

The Sporozoa of the human bowel all belong to the group known as theCocciDiA, and are represented by four species placed in two different genera: (i) Eimeria, with the species E. wenyoni, E. oxyspora, and E. snijdersi, and (2) Isospora, with the single species /. hominis.

The Ciliophora found in man all belong to the Ciliata, a very large Class containing numerous species. Those of man belong to the genus Balantidinm, represented by the species B. coli and B. miniitum (some- what doubtful), and possibly by others also. A species of another genus Nyctotherus has also been described, but its existence appears still rather uncertain.

All the organisms just mentioned will have to be considered in detail

6 THE INTESTINAL PROTOZOA OF MAN

in the ensuing chapters : but it will be convenient here to notice certain general characters which all the members of our " unnatural " group have in common. These concern chiefly their lives and habits, their distribution, and their relations to man.

Life-histories. So many wonderful life-histories have been de- scribed— and even proved to occur in the Protozoa, that the mere mention of the name often leads the less instructed to expect some marvellous revelation. It will be well to state at the outset, therefore, that the intestinal protozoa of man all lead so far as we know at present comparatively simple lives which can be understood, in their main outlines, by anybody. Most of them develop in a straightforward manner, and their development can be described without the use of numerous technical terms. Many of the exciting doings which have been attributed to these animals are now known to rest upon mal- observation, misinterpretation, and unscientific use of the imagination ; and no excuse will be needed, therefore, for ignoring these mistakes at this point, and omitting to use some of the superfluous terms which they have introduced into biological language.

From the most general standpoint, the life of an intestinal protozoon consists typically of two main periods a period of freedom or activity (often curiously called a "vegetative" stage) and a period of rest. It may be noted, in passing, that the first period can hardly be called one of "freedom" and "activity " in the case of the Coccidia ; for during the corresponding stages in these organisms, the individuals are intracellular and sedentary only the young forms being free and motile. But it is characteristic of all the intestinal protozoa that during this first period of relative freedom and activity they feed, grow, and multiply actively multiplication being effected always by a process of simple or multiple fission. This period is, moreover, invariably passed in the case of the organisms under consideration within the human bowel. On the other hand, the resting period is always passed outside the human body, within a special protective capsule or cyst.

The " free " forms, living and multiplying in the body of man, give rise to the condition called infection : while the resting or encysted forms, capable of external existence, serve to convey infection from one man to another. Infection with any intestinal protozoon is, in nature, always acquired through the mouth, by swallowing a living cyst containing the resting form of the particular organism. In ordinary

INTRODUCTION 7

circumstances the free forms cannot live outside the body for more than a very short time, and they die if swallowed in other words, they are non-infective.

The two periods or cycles of development alternate, more or less regularly, with one another. When a cyst is ingested, it passes intact through the stomach into the intestine. Here it hatches and liberates its contained organism (or organisms), which seeks its appropriate place in the bowel and there begins its development as an active or free form. After living and multiplying for some time in this form, its offspring secrete cysts round themselves, and then pass out of the intestine with the stools. The cycle of events is repeated if these cysts are fortunate enough to get swallowed again by a human being.

The above is a brief outline of the life of each of the intestinal protozoa of man. Each has its own peculiar structure and mode of life, and each its own characteristic encysted form, which can be recognized in the faeces by the trained microscopist. Individual details of structure, and complications in the mode of development, will receive attention later. It is only necessary here to take a very general view, and to emphasize the two main stages in the life-cycle the " active " form and the cyst. When these are understood, the details are easily learned : but failure to understand these simple generalities has led, unfortunately, to many errors in the past, and for this reason it seems necessary to stress these elementary points. When they are clearly and generally comprehended it will become impossible for certain current but inaccurate expressions to survive. It will no longer be possible for a writer to describe a patient as " infected with cysts," or to speak of "cyst-carriers," or to ask for methods of medication which will " kill the cysts " in preference to the active forms all which expressions, and others akin to them, are obvious absurdities.

It will be noted that the life-cycle as a whole requires but two environments the human bowel, and some suitable resting place outside it. No secondary or intermediate host is necessary for the completion of the developmental cycle of any of the intestinal protozoa of man. In this connexion, however, it must be noted that other animals may assist in the dispersal of the cysts, and thus aid in spreading infections : and this leads us to consider the usual modes of dissemination of the intestinal protozoa of man in nature.

Dissemination. The cysts of all the intestinal protozoa of man

8 THE INTESTINAL PROTOZOA OF MAN

are comparatively delicate structures, and their contents are incapable of withstanding desiccation. In damp faeces, however, or in water, the cysts can usually survive and remain infective for several weeks. It thus seems probable that, in nature, water plays an important part in their dissemination : and it may be assumed that the swallowing of water, or damp uncooked foodstuffs, accidentally contaminated with faecal matter containing cysts, is the usual means whereby infections spread from man to man. All unhygienic conditions which favour con- veyance in this manner must, accordingly, be regarded as contributing to the dissemination of infections.

Food and drink may, of course, become contaminated with faeces in innumerable ways. It is impossible to discuss them all here, but we must mention one of them which is of special interest namely, con- tamination by flies. It has been demonstrated by Wenyon and O'Connor (1916, 1917), Flu (1916), Buxton (1920), and others, that house-flies are able to spread the cysts of the common species of in- testinal protozoa. Wenyon and O'Connor have shown that a fly, when it feeds upon human faeces containing cysts, does not digest them, but passes them alive and unchanged through its alimentary canal, and voids them again still living with its own faeces. The time taken in passing through the fly is, sometimes, astonishingly short cysts taken in at the fly's mouth being redeposited within as little as 5 to 30 minutes. A large number of flies, after feeding upon a stool containing numerous cysts, might therefore disseminate them over a comparatively wide area in a short space of time. Each speck of such fly faeces, if swallowed with food or drink before it has time to undergo complete desiccation, is capable of infecting a human being. It is thus clear that the part played by flies in the spread of infections is not negli- gible, and may be of prime importance : and it is also evident that the destruction of flies, as a prophylactic measure against the spread of infections, merits serious attention.

It has recently been urged by Roubaud (1918) that the fly may, in reality, do more good than harm in this respect. He argues that, since the minute quantity of faecal matter deposited by a fly readily dries up, and any protozoal cysts which it may contain are thus killed, the fly may, in reality, contribute to the destruction rather than to the dispersal of cysts in nature. Infective faeces, when devoured by flies, is reduced to a fine state of division ; and the prompt desiccation which results renders the contained cysts non-infective in a very short time.

INTRODUCTION 9

This consideration is, no doubt, of importance when we are con- sidering what happens under hot and dry atmospheric condition-,. When the air is humid, however, or when there are opportunities for the flies' faeces to be deposited in or on damp comestibles intended for human consumption, it is clear that we cannot— without further evidence regard the activities of the fly as beneficial, or even as harm- less. Recently Woodcock (1918) has attempted to show that the part played by flies in the dissemination of amoebic cysts is compara- tively unimportant. He considers the humidity of the atmosphere to be the factor of primary importance determining the survival and dispersal of cysts. It is evident, however, that dampness of the air is the very factor which would prevent the faeces of flies from drying too rapidly ; and consequently, even if it were proved that humidity is of great importance, it would in no way invalidate the conclusion that flies play a most important part in the dissemination of the intestinal protozoa of man. Both factors are, doubtless, intimately connected, and deserve careful consideration.

It may be noted here that Stiles (1913) had earlier suggested that the prevalence of intestinal protozoa in a community might be used as a criterion of the extent to which their food and drink are exposed to contamination with human faeces as a measure of the effectiveness of the sanitary arrangements within the community : and Stiles and Keister (1913) have already attempted to utilize this criterion in the special case of the carriage of Giardia cysts by house-flies.

Geographical Distribution. It is now certain that most of the intestinal protozoa of man are cosmopolitan in their distribution. They are not restricted, as is often assumed, to the tropics, but are known to occur in human beings in all parts of the world where search has been made for them. In all probability the gaps in our present knowledge, in this respect, make the distribution of some forms wrongly appear discontinuous ; though it is possible, or perhaps even probable, that future work will show that some species for example, the Coccidia are limited to certain geographical areas. On the other hand, although all races of man have not yet been examined with this object in view, it is reasonably certain that most races of man harbour Entamoeba coll and E. histolytica and probably the other intestinal amoebae and all the common species of flagellates. These are known to occur in such widely separated places that it can hardly be doubted that their real

10 THE INTESTINAL PROTOZOA OF MAN

distribution is world-wide. It is also reasonable to conclude that all the common intestinal protozoa of man have lived in man for ages. They are not recent intruders but age-long companions of the human species.

There is nothing very novel in this wide geographical distribution, though it has but recently become evident : for the cosmopolitan occurrence of the Protozoa generally has long been a commonplace observation to zoologists.

One of the most interesting of the facts which have emerged from the recent activity in the study of the intestinal protozoa of man, is the demonstration that all the commoner forms occur, apparently indi- genously, in the British Isles. Even Entamoeba histolytica previously assumed to be more or less restricted to the tropics has been shown to occur in no inconsiderable proportion of the inhabitants of these Islands. We owe the establishment of this fact chiefly to the work of Matthews and Malins Smith,* but their observations have been confirmed and extended by others. One of us has recently reviewed and sum- marized all the investigations undertaken to elucidate this problem, so that it will be unnecessary to deal with it in detail here.f It will suffice to note that most of the commoner species of known human intestinal protozoa occur at the present day in Britain the most noteworthy forms which have not yet been recorded being Balantidium and the Coccidia.

It is clear that Britain cannot occupy an isolated position in this respect, and that further investigations will show that all the intestinal protozoa of man are much more widely distributed than was generally supposed until quite recently. Hitherto the greatest attention has been paid to the distribution of E. histolytica, but there are already sufficient records available to show that most of the other intestinal protozoa are at least as widely dispersed.^ But we cannot discuss this subject in

* See especially Yorke, Carter, Mackinnon, Matthews, and Smith (1917), Matthews and Smith (1919, 1919a), Dobell (1921).

tSee Dobell (1921).

% Among more recent contributions to this subject the reader may be referred to the following : Galliard and Brumpt (1912), Paviot and Garin (1913), Landouzy and Debrd (1914), Bloch (1916) French cases; Kuenen (1918) Dutch cases; Fischer (1920) German cases; Yakimoff (1917)— Russian cases; Kofoid, Kornhauser, and Plate (1919), Cort and McDonald (1919) United States cases. There are also numerous other works dealing with the occurrence of intestinal protozoa in the inhabitants of temperate climates, but it would lead us too far to discuss or even to attempt to cite all of them. See also the papers on French cases of Balantidiosis cited on p. 119 infra.

INTRODUCTION I I

detail here. We must pass on to the consideration of another impor- tant and equally large subject.

Incidence of Infection. The recent Great War has fostered an immense amount of research upon the intestinal protozoa of man, and has led to the publication of a very large volume of records from all the chief theatres of military operations. It is impossible to attempt to summarize this work here, where we shall merely note what seem to be the most important general conclusions to be drawn from it.

The recorded findings as a whole after due allowance has been made for the very considerable but inevitable proportion of errors con- tained in them tend to show that intestinal protozoa are far commoner in man, in all parts of the world, than had previously been supposed. But at the same time they have revealed that these organisms are of less importance, from a medical standpoint, than was formerly believed. It has now become clear that the majority of the intestinal protozoa occur comparatively frequently in human beings everywhere ; but that very few species are responsible for the causing of human diseases, and that none give rise to epidemics of such diseases. In the War, the amount of disease due to intestinal protozoa was, in all probability, when the number of individuals involved is taken into account, almost negligible.

Nevertheless, the hundreds of thousands of cases of intestinal disease which occurred in the course of the War afforded great opportunities for studying intestinal organisms of all sorts ; and it is partly because so much importance was at first attached to the intestinal protozoa that the fact of their comparative unimportance has emerged. By far the most important intestinal protozoon, from the medical standpoint, is Entamoeba histolytica the organism which " causes " the disease known as amoebic dysentery, and other pathological conditions. Special attention has therefore been directed to this parasite, and as a result we now have fuller information about it than about most of the other intestinal protozoa. It is certain that this amoeba occurs in a very considerable percentage of persons all the world over, and it is probable that at least 10 per cent, of the entire population of the globe is infected. The number may, indeed, be much higher. The majority of the other intestinal amoebae, and most of the flagellates, occur with at least equal frequency : and in the case of some of them such as Entamoeba coli and Giardia there is evidence to show that they are even more com- monly present in mankind generally. These conclusions, by themselves,

12 THE INTESTINAL PROTOZOA OF MAN

indicate that intestinal protozoa must have relatively little pathological significance.

There is, however, some indication that all the intestinal protozoa of man occur with greater frequency in tropical aud subtropical countries than in temperate and cold ones. But it is still questionable whether this inequality of distribution has any direct relation to climate or tem- perature : it is probable that it depends primarily upon the more insanitary conditions and greater opportunities for the spread of infec- tion which are present in hotter countries generally.

We can say no more on this subject here, and will make no attempt to summarize the published records dealing with the incidence of intes- tinal protozoa in the various races of man, and in the various armies engaged in the War.- Our space is circumscribed, and we have yet to consider some other topics of importance from a more general standpoint.

The Relation of the Intestinal Protozoa to Man.— It is most important that all who begin the study of the intestinal protozoa of man should rid themselves of any prejudices that they may have against so-called " parasites." This term is loosely used, in common speech, for any organisms that live inside other organisms ; and preconceived notions derived from this reproachful name have been responsible for much misunderstanding and confusion in discussing the protozoa of man. A few general remarks on this subject will therefore be made here.

Animals which live inside other animals are called collectively Entozoa, and those which harbour them are called their Hosts ; and a moment's reflexion will show that such an association of two organisms may be of divers kinds. It is clear that such an association may be beneficial to both host and entozoon, or harmful to both : or it may be beneficial or harmful to one member of the pair, and indifferent to the other. Let us consider each of these possibilities in turn.

* Numerous references will be found in the Tropical Diseases Bulletin. The reader interested in this subject may be referred to the following recent works, which will also supply him with numerous further references to the immense literature dealing with the incidence of intestinal protozoal infections : Aubert (1917), Bahr and Young (1919), JBaylis (1920), Bentham (1920), Boney, Crossman, and Boulenger (1918), Brumpt (1918), Chatton (1918a), Derrieu (1920), Dobell (1917), Dobell, Gettings, Jepps, and Stephens (1918), Dobell (1921), Flu (1918a), Lebceuf and Braun (1916), MacAdam and Keelan (1917), Mackinnon (1918), Matthews and Smith (1919^), O'Connor (1919), Ravaut (1917), Smith and Matthews (1917, 1917a), Wenyon (1916), Wenyon and O'Connor (1917). Hundreds of additional papers could easily be cited.

INTRODUCTION

1 3

(1) When the association benefits both parties, the condition is one of Symbiosis a not very frequent state in nature. An example is afforded by some of the flagellates living in termites ("white ants"). In return for the food and lodging which the termite gives to the flagellate, the latter helps the former to digest its own food. No such symbiotic arrangement appears to exist between man and any of the protozoa which he harbours in his gut.

(2) When the entozoon lives at the expense of its host, the phenomenon is known as Parasitism. The entozoon is a Parasite in the biological sense and is always more or less harmful. When the harm done becomes manifest, the host is said to suffer from a Disease, of which the parasite is colloquially— and therefore inaccurately termed " the cause."

The intestinal protozoa of man furnish several instances of