Popular Science Monthly/Volume 69/November 1906/Pathogenic Protozoa


By Professor GARY N. CALKINS


AT the present time the importance of protozoa-study is recognized in all branches of biological science where, as single-celled organisms, they illustrate the manifold principles of living things. Thus it is in physiology, in cellular biology, in psychology and in general biology. There is one field, however, a field that is daily growing more extensive, in which the importance of protozoa has only recently been recognized, and this is the field of pathology. To the medical world for the most part, the group of protozoa consists of the few types of parasitic forms that cause human disease, and in this world any one who has a knowledge of Trypanosoma, or Amæba, or Plasmodium, is a student of the protozoa, while a deeper knowledge makes him a biologist. At the present time there are many students of the group in this sense, and the relations of protozoa to human welfare bid fair to be the most popular aspect of protozoan study, while in the public mind already the term protozoa is apparently the synonym of some new and fearsome thing. Commissions for the study of protozoan diseases have been appointed in many countries and chairs for the study of protozoology have been established in the universities of Cambridge and London mainly for the study of the pathogenic forms of these unicellular animals. The present paper deals with a few aspects of this more recent field of protozoa work.


Few pathologists in good standing gave a thought to protozoa until after the malarial organisms had been worked out and the life history completely known through the researches of biologists and surgeons. Thanks to the work of Laveran, Ross, Grassi and Schaudinn, there is no longer a phase in this disease that is unknown, and the relations of the various symptoms of the malady to stages in the life history of the organisms are perfectly established. I do not need to go into the malaria problem, for the life history of the organisms, their relations to the mosquito Anopheles, the coincidence of merozoite 'spore' formation and pyrexial attacks of the disease, are familiar to all who have followed, even remotely, the progress of medical science. I will pass on rather to less familiar problems that are to-day puzzling the medical world.

Amoebic Dysentery

Since 1860 various forms of Amœba have been found in the human intestine. In 1876 Loesch first claimed these rhizopods to be the cause of dysentery, and since that time students of the subject have been about equally divided into supporters of Loesch and his opponents. In recent times, however, the belief is widespread that two forms of the disease occur, one of which, tropical, pernicious, or amoebic dysentery, is invariably accompanied by the rhizopod Entamœba histolytica, which Schaudinn distinguishes from the ordinary harmless intestinal amœba A. coli. This form becomes a tissue and cell-infecting parasite and to this characteristic it owes its malignancy. Musgrave in the government laboratory at Manila has worked out the organism most carefully and has been successful in producing the disease in monkeys and other animals through cultures free from the ordinary forms of bacteria. The life history of the Amœba has been followed by Schaudinn, and in the present place this is perhaps more interesting than the pathological details.

The Amœba has little structural detail, a nucleated cell with minute form changes, a slight differentiation into ectoplasm and entoplasm, a great power of reproduction, by division and by budding, leading to masses of the organisms in the intestine and attached organs, where intestinal lesions, liver abscesses and the like, become the characteristic features of the disease. One interesting phenomenon worked out by Schaudinn is the preliminary nuclear metamorphosis before spore formation. In the majority of rhizopods in which the life history is known the formation of conjugating gametes is preceded by fragmentation of the nucleus either by division, or by a kind of nuclear secretion of chromatin. A condition of the cell, known as that of the 'distributed nucleus' results from this fragmentation and each fragment of chromatin becomes the nucleus of one of the gametes. Unfortunately, Schaudinn gives no figures in his preliminary publication on Entamœba, but his description tallies exactly with analogous processes in other rhizopods, notably in Arcella, Centropyxis, Difflugia and others.

In Entamœba as in these free-living rhizopods the spore-like bodies resulting from this distributed condition of the nucleus conjugate and so bring about a renewal of vitality of the parasite, favorable to the infection of new hosts. Musgrave has shown that these parasites may live a free life in ordinary drinking water and that infection takes place presumably in this way, and his observations indicate that Entamoeba is a facultative rather than an obligatory parasite of man. Schaudinn regards Entamœba coli as a harmless commensal, but Entamœba histolytica as a definite tissue-destroying parasite, a faculty which gives to this form of tropical dysentery its pernicious and often fatal characteristic.

Rabies Furiosa

From dysentery to hydrophobia seems a long jump and yet we may find that these two diseases and with them others apparently as diverse, as small-pox, sleeping sickness, etc., owe their malignancy to the destructive action of protozoa on different tissues of the body. I take up rabies next because the organism of hydrophobia agrees in many of its characteristics with that of dysentery, and belongs, as I believe, on the present evidence, in the group of rhizopods.

Peculiar structures have been known for several years in the brain and nerve cells of animals with rabies. Negri, however, in 1904 was the first to suggest that these are as distinctive of hydrophobia as Plasmodium is of malaria, and he and his assistants succeeded in establishing the conviction that these 'Negri' bodies are sufficiently characteristic to afford a quick and safe diagnosis of rabies. Beyond the suggestion that these bodies are organisms comparable to the smallpox organism as described by Councilman and his collaborators, Negri did not attempt to outline their systematic position. From the apparent absence of nuclei and other structures in the Negri bodies, pathologists have been inclined to interpret them as secretions, or degenerations of a specific type, and very few have been hardy enough to regard them as protozoa. Among these few, however, is Dr. Anna W. Williams, of New York, who spent a summer at Woods Hole three years ago, working on protozoa, and who has done a great deal on pathogenic protozoa; and to her belongs the credit of establishing the protozoon nature of these enigmatical structures.

One of the curious features of pathogenic protozoa is that they are very refractory to the ordinary cytological stains, and do not behave like other cells under hematoxylin or methylin blue, etc. This is why the organism of syphilis was overlooked until last year, and this is why Negri failed to get any evidence of cell structure in his rabies organism. In all of his preparations and figures the bodies appear as highly vacuolated structures with no sign of nucleus or differentiated cell body. Dr. Williams, using the Giemsa stain, a method that has been singularly successful in staining delicate pathogenic protozoa, showed that not only do these Negri bodies have a definite structure with nucleus and cytoplasm, but also that a series of nuclear metamorphoses occur which are identical in their consecutive phases with the typical freeliving rhizopods. Nuclear fragmentation takes place until as in Entamœba or Centropyxis or Amœba, etc., the cell is filled with chromatin particles. Dr. Williams also showed that reproduction by division and by budding takes place, and she failed only to make out the conjugation processes. From my own study of Dr. Williams's preparations, I have no doubt at all that she is right about the organism, and believe that, under her name Neurorcytes, it should be grouped with the rhizopod-like protozoa.

This work on Neurorcytes is also very interesting from the side light it throws on the small-pox problem. Here again, in certain phases of variola, we find curious intra-cellular and intra-nuclear bodies having a striking resemblance to the ordinary unstained forms of the Negri bodies. These small-pox bodies were early recognized as characteristic of this disease, and Guarnieri in 1892, believing they were protozoa of a specific kind, named the organism Cytorcytes variolæ. Perhaps the majority of pathologists to-day and many biologists, are opposed to this interpretation,' and these bodies, like the Negri bodies, are more commonly regarded as specific secretions or degenerations than as protozoa. I have no doubt myself, from long study of these small-pox organisms, that they are protozoa, and believe that with fresh material and by using the stain which Dr. Williams has so successfully used for Neurorcytes, the last doubter will be convinced.

At the risk of going somewhat far afield in pathological speculation, let me briefly call attention to one other possibility of amœboid organisms and disease, viz., cancer. For years it has been known that vegetable cells in ordinary edible forms, like the cabbage or turnip, etc., may be stimulated to abnormal multiplication by rhizopod parasites. Such a parasite—Plasmodiophora brassicæ—enters the young root cells of a cabbage, stimulates those cells to an unwonted degree of multiplication until great tumors are formed giving rise to the vegetable disease known as 'club-root.' The plant cells become storage reservoirs of the spores of Plasmodiophora, and when the plant dies down the spores are liberated in the soil. Now it has been argued that if vegetable cells can be stimulated to abnormal activity, there is no real biological objection to animal cells being similarly stimulated to division by animal parasites, and some pathologists have gone so far even as to see in certain cell inclusions of cancer peculiar bodies which they compared with the Plasmodiophora spores. The comparison, however, can not be sustained, and without going into the subject extensively I may say in short that nothing has ever been seen in cancer cells that can be interpreted as a protozoon parasite. This, however, does not weigh against the parasite theory of cancer—a theory which I personally, believe to be the only one that satisfactorily explains the disease. The organism of yellow fever has never been seen, but no one doubts the parasitic nature of that disease and the fact that the virus or the germs of yellow fever pass through the finest Berkefeldt and Chamberland filters indicates that we have to do in this disease with organisms too minute to be seen. Ultra-microscopic protozoa are not the only ones which will do this, for small amœbas may, under pressure, be forced through some of these niters, while in a few organisms, notably the flagellates, there are some phases in the life history when the individuals become so small that they are no longer discernible with the highest powers of the microscope. This is the case in certain of the trypanosomes and spirochetes, which are now known to cause some of the most malignant of human diseases.

Trypanosomiasis and the 'Sleeping Sickness'

In this country, and indeed in temperate zones generally, there is no dread of trypanosomes, and 'sleeping sickness' is more often the subject of thoughtless jest than of intelligent consideration. In England, where African interests are more keenly followed, a deep interest is taken in this matter, and the Liverpool School of Tropical Medicine deals largely with trypanosomiasis. In his presidential address before the British Association for the Advancement of Science, meeting in South Africa last summer, Colonel Bruce said:

Trypanosomiasis in man, the 'sleeping sickness,' which occurs on the west coast of Africa, particularly in the basin of the Congo, has within the last few years spread eastward into Uganda, has already swept off some hundreds of thousands of victims, is spreading down the Nile, has spread all round the shores of Lake Victoria and is still spreading southward round lakes Albert and Albert Edward, and now threatens the Transvaal and Zululand.[2]

Many different species of trypanosomes are known, and the normal habitat is the blood. No form of vertebrate is exempt, the blood of fish and amphibia, reptiles, birds and mammals forming suitable culture media for their growth and reproduction. In some stages of their life history they apparently become intra-cellular parasites, lose their flagella and membranes and assume the gregarine-like form. In man they may be either comparatively harmless flagellates, swimming about in the blood plasm, or, by bursting of the capillaries, they may penetrate the membranes of the brain and spinal cord and give rise to the invariably fatal disease of man—'sleeping sickness.' The presence of Trypanosoma gambiense in the human blood gives rise to the 'trypanosome fever' of Africa, not much worse apparently than is malaria in this country, but when the parasites enter the nervous system and congregate in the cerebro-spinal fluid, or in the ventricles of the brain, the result is invariably fatal. The result of this nerve-invasion is the appearance of various nervous symptoms like apathy, lassitude, trembling, and, finally, somnolence, increasing to a phase of intense coma and death in from three to twelve months. Death usually is due to the disorganization of the nervous system, but in chronic cases it may follow from weakness and emaciation, or, in acute cases, it may result from the blocking of the capillaries by the parasites.

The full life-history of the organism of sleeping sickness is not yet known. In all blood-dwelling protozoa, however, the infection is carried from individual to individual by some intermediate host, usually a blood-sucking invertebrate. Trypanosoma gambiense is thus transmitted from man to man by the tsetse fly, Glossina palpalis; Trypanosoma brucei, the 'tsetse fly disease' of horses, by Glossina morsitans; Trypanosoma lewisi is carried from rat to rat by the louse, Hæmatopinus; T. ziemanni from owl to owl by the mosquito, Culex pipiens. Nor are the intermediate hosts limited to the insects. Trypanoplasma borreli, for example, is transmitted from carp to carp by the fish leech, Piscicola geometra.

The history of the trypanosomes in the digestive tracts of these various carriers has been made out in several cases, although by no means in all. In the gut of the leech, of the louse, Hæmatopinus, and of the mosquito, three different species of Trypanosoma have been worked out by different and competent observers, and in all cases it has been found that this environment is the scene of the most important phases in the life history of the parasite, viz., the conjugation stages, which lead to renewal of vitality. The flagellate parasites thus agree in the main features of their life cycles with the malaria organisms in man and in the insect host Anopheles. Indeed, so widespread is this phenomenon that we are justified in assuming, where actual evidence is not forthcoming, that similar important processes take place in all intermediate hosts, and that we must look for conjugation phases of the parasite of Texas fever (cattle) in the tick (Boophilus bovis), and of sleeping sickness in the fly (Glossina palpalis). On the other hand, we are justified in assuming a protozoon parasite, even though the parasite is not known, in cases where its existence has been proved in an intermediate host. This is the case, for example, in yellow fever, where a definite incubation period of the parasite of about twelve days in the mosquito Stegomyia fasciata is known, and where it is fully established that, apart from this mosquito, no other means of transmission of the disease exists.

Associated with the Trypanosoma diseases, although not yet established, are those curious maladies of India and similar countries, known as dum-dum fever, kala-azar, splenomegaly, etc. The infection may be general or localized, and curious structures, known as the Donovan-Leischman-Wright bodies, have been observed in the spleen of dum-dum fever patients, in the lesions in 'tropical ulcer,' Delhi boil, 'oriental sore' and the like, and these bodies have been interpreted as stages in the history of some Trypanosoma. At the present time, however, it appears much more likely that they are more nearly related to the hæmosporidia than to the flagellates, and Laveran and Mesnil's view assigning them to the genus Piroplasma, the same genus as the parasite of Texas fever, may be accepted.

Diseases due to Spirochæta

Closely allied to Trypanosoma is the genus Spirochæta, a similar flagellate belonging to the order Monadida. In its general cork-screw shape it resembles Spirillum, one of the bacteria, but differs from this by reason of its plastic body and general mode of life. Many different species have been described, and considerable difference of opinion exists as to whether they should be classed as bacteria or as protozoa. One well-known form—Spirochæta obermeieri—has long been recognized as the cause of relapsing fever, and has been recently shown to be the cause of human tick fever in Africa, which is carried from man to man by the tick (Ornithodorus moubata). The organism of relapsing fever, however, does not seem to have a typical flagellate structure; there is unmistakable evidence of transverse rather than longitudinal division; its nucleus is distributed like that of a bacillus, and Novy has shown that it reacts like some bacteria during plasmolysis. The need of an intermediate host in the case of African tick fever seems to be the one distinctive protozoan characteristic.

While Spirochosta ooermeieri is perhaps a doubtful protozoon, there is less doubt in the case of other species, some of which have unquestioned flagellate characters, including typical nuclear structures, longitudinal division and the like, while in a number of species an undulating membrane analogous to that of a trypanosome can be made out. Some of the better known species are: Spirochæta dentium of the mouth; S. refringens of ulcerating tumors; S. gallinorum a blood parasite of fowls, and S. anodontæ, parasitic in the crystalline style of the mussel Anodonta mutabilis. A similar parasite, described first as S. balbiani, is found in the crystalline style of the oyster, but it has so many trypanosome characteristics that it is now called Trypanosoma balbiani. It serves to illustrate the close relationship between these two genera.

It is to this group of parasites that Spirochæta (Treponema) pallida, the cause of syphilis, belongs. The organism was discovered a year ago, and since that time has been submitted to the widest range of pathological research. A full life history has been published by Siedlecki and Krzyszakowicz, and some of the stages described by them are strikingly similar to those of Trypanosoma. Schaudinn regards it as sufficiently distinct from other Spirochæta species to justify a new generic name and calls it Treponema pallidum.

The organisms causing the several diseases that I have dealt with in the preceding account fall in three of the four great groups of protozoa, the Infusoria alone being unrepresented. The causes of tropical dysentery of rabies, and possibly of small-pox, are related to the ordinary rhizopod types; the causes of sleeping sickness, trypanosomiasis and syphilis belong to the animal flagellates, and here, possibly, belongs the organism of yellow fever; the causes of malaria, and, probably, of splenomegaly, dum-dum fever and the like belong to the sporozoa.

The effects of these different parasites upon their hosts differ in different cases. Sometimes they poison the host by the liberation of toxins, as in the case of malaria or yellow fever; sometimes by local lesions or general tissue disorganization, as in tropical ulcer, sleeping sickness, syphilis and rabies; or sometimes by the mere mechanical obstruction to normal physiological processes by the accumulation of parasites in capillaries and ducts, while in still other cases the parasites stimulate the latent dividing energy of tissue cells and lead to abnormal tumor-like growths. In many cases knowledge of the organism and of its mode of life has led to preventive measures and to the great saving of human life. In yellow fever, for example, the warfare on mosquitoes will stop entirely its epidemic nature and, thanks to Carroll's brilliant experiments, yellow fever to-day, like malaria or trichinosis, is an advertisement of ignorance or criminal negligence in communities where it exists. Preventive measures hold down the ravages of small-pox, anti-rabic serum lessens the malignancy of hydrophobia, while different specifics are fatal to other kinds of parasitic protozoa, quinine to Plasmodium malariæ, mercury to Treponema pallidum, and recently Koch's specific to Trypanosoma. With the knowledge of these other pathogenic protozoa and of their modes of life investigation will bring out the means of combating them and thus of reducing human suffering, and this prospect if for no other reason, is a full justification of the many commissions that have been appointed, and of the vast sums of money that have been spent, to further protozoan research.

  1. A lecture delivered at Woods Hole, July 10, 1906.
  2. Science, XXII., p. 298, 1905.