That intermittent fever was an infectious disease, although not one which was transmitted direct from man to man, had been assumed for a long time. Therefore it was natural, at a time when bacteriology was triumphing, to look for a living agent causing infection in malaria, which search was, seemingly, successful (Klebs, Tomasi-Crudeli, 1879). Hence it was not surprising that the discovery of the real malarial parasites in November, 1880, by the military doctor A. Laveran195 in Constantine (Algeria), at first met with violent opposition, even after Richard (1882) had confirmed it and Marchiafava, Celli, Grassi and others, had further extended it. Not that the existence of structures found in the blood of malaria patients by Laveran and Richard was denied; on the contrary, the investigations of the opponents furnished many valuable discoveries, but the organisms were differently interpreted and considered to be degeneration products of red blood corpuscles. Only when Marchiafava and Celli (1885) saw movements in the parasites, which Laveran called Oscillaria malariæ and later Hæmatozoön malariæ, was their animal nature admitted and the parasites were named Plasmodium malariæ. Shortly before this, Gerhardt (1884) had stated that the disease could be transmitted by the injection of the blood of a malarial patient to a healthy person.
This supplied the starting point for further investigations, which were made not exclusively, but principally, by Italian investigators (Golgi, Marchiafava and Celli, Bignami and Bastianelli, Grassi and Feletti, Mannaberg, Romanowsky, Osier, Thayer and others). In 1885 Golgi described the asexual cycle in the blood, in the case of the quartan parasite. These investigations, after attention had been drawn by Danilewsky (1890) to the occurrence of similar endoglobular parasites in birds, were extended to the latter (Grassi and Feletti, Celli and Sanfelice, Kruse, Labbé and others).
The result was as follows: Malaria in man (and birds) is the result of peculiar parasites included in the Sporozoa by Metchnikoff, which parasites live in the erythrocytes, grow in size and finally “sporulate,” that is, separate into a number of “spores” which leave the erythrocytes and infect other blood corpuscles. Morphologically and biologically several species (and respectively several varieties) of malarial parasites may be distinguished, on which the different intermittent forms depend. Transmission of the blood of patients to healthy people produces a malarial affection which corresponds in character to the fever of the patient from whom the inoculation was made. The combined types of fever (tertiana duplex, quartana duplex or triplex) are explained by the fact that the patient harbours two or three groups of parasites which differ in their development by about twenty-four hours, whilst the irregular fevers depend on deviation from the typical course of development of the parasites. In addition to stages of the parasites which could easily be arranged in a developmental series concurrent with the course of the disease, other phases of the parasites also became known, such as spheres, crescents, polymitus forms, which seemed not to be included in the series and, therefore, were very differently interpreted.
The decision reached at the beginning of the last decade of the last century, which found expression in comprehensive statements (Mannaberg, Ziemann and others), only concerned a part of the complete development of the malarial parasites. No one could with any degree of certainty demonstrate how man became infected, nor were there reliable hypotheses based on analogy with other parasites concerning the exit of the excitants of malaria from the infected person and their further behaviour. Numerous hypotheses had been advanced, but none was able to elucidate the various observations made from time to time in dealing with malaria. One hypothesis only seemed to have a better foundation. Manson (1894), who knew from his own experience the part played by mosquitoes in the development of Filaria from the blood of man, applied this also to the malarial parasites living in the blood, whereby at least the way was indicated by which the Hæmosporidia could leave man. The parasites were said finally to get into water through mosquitoes which had sucked the blood of malarial patients, and the germ spread thence to men who drank the water. In some cases the parasites were supposed to reach man by the inhaling of the dust of dried marshes. On the other hand, Bignami believed that the mosquitoes were infected in the open air by malarial parasites which occurred there in an unknown stage and the insects transmitted the germs to man when biting. R. Koch combined both hypotheses, without, however, producing positive proof. R. Ross, then (1897–8) an English military doctor in India, was the first to succeed in this. He had been encouraged by Manson to study the fate of malarial Plasmodia which had entered the intestine of mosquitoes with malaria-infected blood, especially in the case of the Plasmodium (Proteosoma) living in the blood of birds. He showed that the Proteosoma penetrate the intestinal wall of the mosquitoes, grow and develop into large cysts which produce innumerable rod-like germs, which burst into the body cavity and penetrate the salivary glands. Ross allowed mosquitoes to suck the blood of birds affected by malaria, and some nine days later, let the infected mosquitoes which had been isolated suck healthy birds. After five to nine days Proteosoma were found to occur in the blood of the birds used. The Proteosoma and Halteridium of birds were also further investigated by MacCallum (1897–8), Koch and others, and important results followed.
In any case Ross (1898) had clearly established the importance of mosquitoes in the spread of malaria among birds. It was now only a question of proving whether, and how far, mosquitoes were concerned with human malaria. Ross himself worked to this end. Here the experiments of Italian investigators (Bignami, Bastianelli, Grassi)196 were of importance. These investigators studied the fate of malarial parasites in man, produced malaria in men experimentally by the bites of infected mosquitoes, and established that only mosquitoes belonging to the genus Anopheles were concerned, and not species of Culex. These latter are only able to transmit Proteosoma to birds. It is true that Culex can ingest the human malarial parasites, but the latter do not develop in them. Development only occurs in species of Anopheles. In Anopheles (and similarly for Proteosoma in Culex) sexual reproduction takes place; crescents, spheres and polymitus forms are necessary stages of development in the mosquito.
With these discoveries the campaign against malaria became more definite. It was directed partly against the transmitters, whose biology and life-cycle were more thoroughly investigated, instead of merely against the infection of the adult Anopheles. The latter do not, as was believed for some time, transmit the malarial germs to their offspring. They always infect themselves from human beings, whereby the relapses appearing in early summer, and the latent infection, especially of children of natives, play a principal part (Stephens and Christophers, Koch). Further, the crusade was directed against the infection of man by the bites of Anopheles. Important results have been obtained in these directions. Low and Sambon in 1900 lived in a mosquito-screened hut in a malarial part of the Roman Campagna for three of the most malarious months and did not contract the disease. In the same year Dr. P. T. Manson was infected with malaria by infected mosquitoes sent from Italy. The rôle of mosquitoes having been proved, it may be hoped that ultimately the eradication of malaria, or at least a considerable restriction of it, will be achieved.
It is of importance to record that, although malarial parasites occur in mammals (monkeys, bats, etc.) the human ones are not transmissible to mammals, not even to monkeys. The species, therefore, are specific to the different hosts (Dionisi, Kossel, Ziemann, Vassall).
An important work dealing with the modern applications of the mosquito-malaria theory in all parts of the Tropics was published by Sir Ronald Ross in 1911. It is entitled “The Prevention of Malaria” (John Murray, London, 21s.).
DEVELOPMENT OF THE MALARIAL PARASITES OF MAN.
The commencement of the developmental cycle and of the infection of man, is the sporozoites (fig. 80, 1) which are passed into the blood of a person by the bite of an infected mosquito. Prior to this the parasites collect in the excretory ducts of the salivary glands (fig. 80, 27) of the Anopheles. The sporozoites are elongate and spindle-shaped, 10 µ to 20 µ long and 1 µ to 2 µ broad, with an oval nucleus situated in the middle. They are able to glide, perform peristaltic contractions, or curve laterally. Schaudinn has studied the penetration of the red blood corpuscles (fig. 80, 2) by the sporozoites in the case of the living tertian parasite. The process takes forty to sixty minutes in drawn blood. After its entrance the parasite, which is now called a trophozoite, contracts, and becomes an active amœbula (fig. 80, 3). It develops a food vacuole and grows at the expense of the invaded blood corpuscle (fig. 80, 4), which is shown by the appearance of pigment granules (transformed hæmoglobin) in it. When the maximum size is attained, multiplication by schizogony (fig. 80, 5-7) begins with a division of the nucleus, which is followed by further divisions of the daughter nuclei, the number of which varies up to 16 or even 32, depending on the species of the parasite. Then the cytoplasm divides into as many portions as there are nuclei, the result being a structure suggestive of the spokes of a wheel or of a daisy, the centre of the resulting rosette being occupied by dark pigment. Finally, the parts separate from one another, leaving behind a residual body containing the pigment, and the daughter forms issue into the blood plasma as merozoites (fig. 80, 7). They are actively amœboid (fig. 80, 8) and soon begin to enter other blood corpuscles of their host, for the entry into which thirty to sixty minutes are necessary, according to Schaudinn’s observations.197
Here they behave like sporozoites which previously entered and again produce merozoites. This process is repeated until the number of parasites is so large that, at the next migration of the merozoites, the body of the person infected reacts with an attack of fever,198 which is repeated with the occurrence of the next or following generations.
Fig. 80.—Life-cycle of the tertian parasite (Plasmodium vivax). Figs. 1 to 17, × 1,200; figs. 18 to 27, × 600. (After Lühe, based on figures by Schaudinn and Grassi.) 1, sporozoite; 2, entrance of the sporozoite into a red blood corpuscle; 3, 4, growth of the parasite, now sometimes called a trophozoite; 5, 6, nuclear division in schizont; 7, free merozoites; 8, the merozoites which have developed making their way into blood corpuscles, (arrow pointing to the left) and increase by schizogony (3–7); after some duration of disease the sexual individuals appear; 9a-12a, macrogametocytes; 9b-12b, microgametocytes, both still in the blood-vessels of man. If macrogametocytes (12a) do not get into the intestine of Anopheles they may perhaps increase parthenogenetically according to Schaudinn (12a; 13c-17c). The merozoites which have arisen (17c) become schizonts 3–7. The phases shown underneath the dotted line (13–17) proceed in the stomach of Anopheles. 13b and 14b, formation of microgametes; 13a and 14a, maturation of the macrogametes; 15b, microgamete; 16, fertilization; 17, oökinete; 18, oökinete in the walls of the stomach; 19, penetration of the epithelium of the stomach; 20–25, stages of sporogony on the outer surface of the intestinal wall; 26, migration of the sporozoites to the salivary gland; 27, salivary gland with sporozoites.
The growth and schizogony last different times, according to the species of the parasite, about forty-eight hours in the case of the parasite of febris tertiana or tropica, and seventy-two hours for the quartan parasite. The various intermittent forms produced by them depend on this specific difference in the malarial parasites.
The schizogony can, however, only be repeated a certain number of times, supposing that the disease has not been checked prematurely by the administration of quinine, which is able to kill the parasites. It appears that after a number of attacks of fever the conditions of existence in man are unfavourable for the malarial parasites, and this brings about the production of other forms which have long been known, but also long misunderstood (spheres, crescents, polymitus). The merozoites in this case no longer grow into schizonts, or at least not all of them, but become sexual individuals called gametocytes (fig. 80, 9-12), which only start their further development when they have reached the intestine of Anopheles. This does not take place in every case, nor with all the gametocytes which exist in the blood of patients with intermittent fever. Of those parasites which remain in the human blood the male ones (microgametocytes) soon perish, the females (macrogametocytes) persist for some long time, and perhaps at last acquire the capacity of increasing by schizogony. They might thus form merozoites which behave in the body as if they had proceeded from ordinary schizonts (fig. 80, 13c-17c). If their number increases sufficiently, in course of time the patient, who was apparently recovering, has a new series of fever attacks, or relapses, without there having been a new infection. This is the view of Schaudinn, who from researches of his own concluded that relapses were brought about by a sort of parthenogenetic reproduction of macrogametocytes. R. Ross, on the contrary, believes that in the relatively healthy periods the number of parasites in the blood falls below that necessary to provoke febrile symptoms; relapses then result merely from increase in the numbers of the parasites present in the individual. Ross’s view is now generally accepted.
Fig. 81.—Stages of development of pernicious or malignant tertian parasites in the intestine of Anopheles macultpennis. (After Grassi.) a, macrogametocyte (crescent) still attached to human blood corpuscles; b, macrogametocyte (sphere) half an hour after ingestion by the mosquito; c, microgametocyte (crescent) attached to the blood corpuscle; d, microgametocyte (sphere) half an hour after ingestion; the nucleus has divided several times; e, microgametes attached to the residual body (polymitus stage).
Fig. 82.—Oökinete of the malignant tertian parasite in the stomach of Anopheles maculipennis, thirty-two hours after ingestion of blood. (After Grassi.)
If the gametocytes, which are globular, or in the pernicious or malignant tertian parasite crescentic (fig. 81), gain access to the intestine of an Anopheline,199 they mature. The macrogametocytes extrude a part of their nuclear substance (fig. 80, 13a, 14a) and thereby become females or macrogametes. The microgametocytes, on the other hand, undergo repeated nuclear division, preparation for this being made apparently whilst in the blood of man. This results in the formation of threadlike bodies which move like flagella and finally detach themselves from the residual body (fig. 80, 13b, 14b). These are the males or microgametes200 (fig. 80, 15b).
Fig. 83.—Section of the stomach of an Anopheles, with cysts (oöcysts) of the malignant tertian parasite. (After Grassi).
Copulation takes place in the stomach of the Anopheline (fig. 80, 16). A microgamete penetrates a macrogamete and coalesces with it. The fertilized females elongate very soon and are called oökinetes or “vermicules” (figs. 80, 17; 82). They penetrate the walls of the stomach, pierce the epithelium (fig. 80, 18, 19), and remain lying between it and the superficial stratum (tunica elastico-muscularis). Then they become rounded and gradually develop into cysts which grow larger and are finally visible to the naked eye, being called oöcysts (figs. 80, 20-24; 83). Their size at the beginning is about 5 µ, the maximum that they attain is 60 µ, only exceptionally are they larger.
The sporulation (figs. 80, 21-25; 84), which now follows, begins with repeated multiple fission of the nucleus. Long before the definitive number of nuclei, which varies with the individual, is attained the protoplasm, according to Grassi, begins to segment around the individual large nuclei but without separating completely into cell areas. According to Schaudinn, however, there is a condensation of the outstanding protoplasmic strands. It is certain that the number of nuclei increases with simultaneous decrease in size. They soon appear on the surface of the strands or sporoblasts, surround themselves with some cytoplasm and then elongate (fig. 84). In this manner the sporozoites are formed and break away from the unused remains of the cytoplasmic strands of the sporoblasts (fig. 80, 26). The number of the sporozoites in an oöcyst varies from several hundreds to ten thousand.
Fig. 84.—Four different sporulation stages of malarial parasites from Anopheles maculipennis, much magnified. a-c, of the malignant tertian parasite; a, four to four and a half days after sucking; b and c, five to six days after sucking; d, of the tertian parasite, eight days after sucking. (After Grassi.)
The sporulation is influenced in its duration by the external temperature (Grassi, Jansci, Schoo). In the tertian parasite it takes place quickest at a temperature of 25° to 30° C. and takes eight to nine days. A temperature a few degrees lower has a retarding effect (eighteen to nineteen days at 18° to 20° C). A still lower one has a restraining or even destructive effect. Temperatures over 35° C. also exercise a harmful effect. The malignant tertian parasite seems to need a somewhat higher temperature and the quartan parasite a lower one.
The sporozoites of the various malarial parasites show no specific differences. They were stated by Schaudinn to occur in three forms, and these were described as indifferent (neuter), female and male. There is, however, little or no evidence for this hypothetical differentiation. The last were said to perish prematurely, that is, in the oöcyst. The others after the rupture of the oöcysts enter the body cavity of the Anophelines, whence they are carried along in the course of the blood. Finally they penetrate the salivary glands (fig. 80, 27) probably by their own activity, break through their epithelia and accumulate in the salivary duct (fig. 80, 27). At the next bite by the mosquito they are transmitted to the blood-vessels of man.
The Species of Malarial Parasites of Man.
In view of the differences in opinion regarding “species” and “varieties,” the dispute whether the malarial parasites of man represent one species with several varieties, or several species is almost superfluous. If necessary two genera may be distinguished.
The parasites of the tertian and quartan fever are alike in that their gametocytes have a rounded shape (figs. 80, 12, 13), whilst the corresponding stages of the pernicious or malignant tertian parasites are crescentic (figs. 81, 88). These differences are used by some writers as the distinguishing characteristic of two genera: Plasmodium, Marchiafava and Celli, 1885, for the first mentioned species; Laverania, Grassi and Feletti, 1889, for the pernicious or malignant tertian parasite. Whether there is a genuine quotidian fever and accordingly a special quotidian parasite is still disputed.
These parasites are treated in practical detail in Stephens and Christophers’ “Practical Study of Malaria,” 3rd edition, 1908.
Plasmodium vivax, Grassi and Feletti, 1890.
Syn.: Hæmamœba vivax, Grassi and Feletti, 1890; Plasmodium malariæ var. tertianæ, Celli and Sanfelice, 1891; Hæmamœba laverani var. tertiana, Labbé, 1894; Hæmosporidium tertianum, Lewkowitz, 1897; Plasmodium malariæ tertianum, Labbé, 1899: Hæmamœba malariæ var. magna, Laveran, 1900, p.p.; Hæmamœba malariæ var. tertianæ, Laveran, 1901.
This species, P. vivax,201 is the causal agent of the simple or spring tertian fever and is, therefore, named directly the tertian or benign tertian parasite (figs. 80, 3-8; 85). During the afebrile period in the patient, the young trophozoites or amœbulæ appear on or in the red blood corpuscles as pale bodies of 1·5 µ to 2 µ diameter which at first show only slow amœboid movements. Their nucleus is difficult to recognize in the early stage. Soon the food vacuole is formed and this grows concomitantly with the trophozoite and the parasite has a ring-like appearance. Afterwards the vacuole diminishes, and at this period the first brownish melanin granule appears. From this time the activity and number of the pigment granules increase with continuous growth. When the parasite has grown to about one-third the diameter of the erythrocyte the latter shows characteristic red Schüffner’s dots or “fine stippling,” after staining with Romanowsky’s solution. Later, after about twenty-four hours, the blood corpuscles begin to grow pale, then to increase in size, and after thirty-six hours, that is, about twelve hours before the next attack of fever, schizogony of the parasite is initiated by the division of the nucleus. The parasite at this time occupies half to two-thirds of the enlarged blood corpuscle. The daughter nuclei continue dividing until sixteen, and occasionally twenty-four, daughter nuclei are produced. The pigment which, up till now lies nearer the periphery, moves to the middle, while the nuclei lie nearer the surface.
Fig. 85.—Development of the tertian parasite in the red blood corpuscles of man; on the right a “Polymitus.” (After Mannaberg.) See also fig. 80, 3—7.
Around each nucleus a portion of cytoplasm collects and thus young merozoites are produced. These separate from each other and from the little residual masses202 which contain the melanin and pass from the blood corpuscles, which now can hardly be recognized, to the blood plasma, where they soon attack new erythrocytes.
The migration of the merozoites initiates a new attack of fever and two groups of tertian parasites in the blood, differing in development by about twenty-four hours, are the conditions for febris tertiana duplex.
After a lengthy duration of fever the gametocytes (figs. 80, 9—12) appear. They are uninucleate. The microgametocytes are about the size of fully developed schizonts, the macrogametocytes are somewhat larger. Their further development takes place in Anophelines.
The chief distinctive characteristics of the simple tertian parasite, as seen in infected blood, are:—(1) The infected red-cell is usually enlarged; (2) the presence of fine red granules known as Schüffner’s dots in the red blood corpuscles, after Romanowsky staining; (3) the fragile appearance of the parasite compared with other species. Large forms are pigmented, irregular and “flimsy-looking,” sometimes appearing to consist of separate parts. Irregularity of contour is common.
Ahmed Emin203 (1914) has described a small variety of P. vivax.
Plasmodium malariæ, Laveran.
Syn.: Oscillaria malariæ, Laveran, p.p., 1883; Hæmamœba malariæ, Gr. et Fel., 1890; Plasmodium malariæ var. quartanæ, Celli et Sanfel., 1891; Hæmamœba laverani var. quartana, Labbé, 1894; Hæmosporidium quartanæ, Lewkowitz, 1897; Plasmodium malariæ quartanum, Labbé, 1899; Plasmodium golgii, Sambon, 1902; Laverania malariæ, Jancso, 1905 nec Grassi et Fel. 1890; Hæmomœba malariæ var. quartanæ; Lav., 1901.
Plasmodium malariæ is the parasite of quartan malaria (fig. 86). The trophozoites of the quartan parasite differ from the corresponding stages of the tertian parasite in that their motility is less and soon ceases. They differ also in their slower growth, by the early disappearance of the food vacuole, by the more marked formation of the dark brown pigment, and by the fact that the red blood corpuscles attacked are not altered either in colour or size.
Fig. 86.—Development of the quartan parasite in the red corpuscles of man—asexual stages. (After Manson.)
When the parasites have grown almost to the size of the erythrocytes schizogony occurs. The pigment granules arrange themselves in lines radiating towards the centre and the merozoites are also radially disposed in groups of 6, 8, 10 or even 12, but are often arranged less regularly. The whole development, growth and schizogony, occupies seventy-two hours.
The appearance of quartana duplex or triplex is conditional on the presence in the blood of the patient of two or three groups of Plasmodia differing in their development by twenty-four hours.
The chief distinctive characters of the quartan parasite are: (1) The erythrocyte is unchanged in size; (2) the rings are compact and show pigment early; in the larger forms the chromatin is dense and relatively plentiful; (3) the pigment, which is relatively well-marked, may be arranged at the periphery.
Laverania malariæ, Grassi and Feletti, 1890 = Plasmodium falciparum, Welch, 1897.
Syn.: Plasmodium malariæ var. quotidianæ, Celli et Sanf., 1891; Hæmamœba malariæ præcox, Gr. et Fel., 1892 (nec H. præcox, Gr. et Fel., 1890); Hæmamœba laverani, Labbé, 1894; Hæmatozoön falciparum, Welch, 1897; Hæmosporidium undecimanæ and H. sedecimanæ and H. vigesimo-tertianæ, Lewkowitz, 1897; Hæmamœba malariæ parva, Lav., 1900; Plasmodium præcox, Dofl., 1901; Plasmodium immaculatum, Schaud., 1902; Plasmodium falciparum, Blanch., 1905.
The names most commonly used for the parasite of malignant tertian malaria are Plasmodium falciparum and Laverania malariæ.
The summer and autumn fever (febris æstivo-autumnalis), also called malignant tertian or sub-tertian, is caused by a malarial parasite which is distinguished by the small size of its schizont, while the gametocytes are crescentic (figs. 81, 88).
Most authors identify this kind of fever or the parasites which cause it (Laverania malariæ) with the pernicious malaria of the tropics. Ziemann, however, repeatedly has drawn attention to certain small but definite differences between the usual malignant tertian or pernicious parasites which occur in the tropics and the tropical parasites of some malarial districts, particularly of West Africa, and insists that at least two varieties or sub-species occur. Other investigators distinguish from this or these forms a quotidian parasite. On the other hand, the assertion is made that there are no specific differences, but that the malignant or pernicious tertian parasite which normally needs forty-eight hours for its development in the blood of man, can also develop in twenty-four hours. The establishment of the duration of the development is a matter of especial difficulty, because the stages of schizogony are far less numerous in the peripheral blood than in that of the internal organs. It is also stated that the tropical parasite very seldom forms crescentic but rather rounded gametocytes. According to such an observation the organism would belong to Plasmodium and not to Laverania. The question whether the tropical fevers are caused by two different parasites does not seem to be definitely settled.
The young trophozoite of the malignant, pernicious tertian, or sub-tertian parasite (fig. 87) are but slightly active and are very small, even after the formation of the comparatively large food vacuole, which makes the body appear annular (“signet ring” stage). Often two and even more parasites are found in one blood corpuscle.
Fig. 87.—The pernicious malignant or sub-tertian parasite in the red corpuscles of man, asexual stages. (After Manson.)
Fully grown they only attain two-thirds or less of the diameter of the erythrocytes, which display an inclination to shrink and then appear darker than the normal (brass-coloured). In the early stage dots or stippling—sometimes called Maurer’s dots—appear on the blood corpuscles as in those attacked by the ordinary tertian parasite (Plasmodium vivax), but the Maurer’s dots are relatively coarse and few, and are not easily stained. These dots were first described by Stephens and Christophers in 1900, and subsequently by Maurer in 1902.
About thirty hours after the entrance into the blood corpuscles, the parasites are rarely found in the peripheral blood, but they are present in the internal organs, and especially in the spleen. The schizogony, which now begins in the internal organs, proceeds on the same lines as that of the quartan parasite, that is, usually with the merozoites radially arranged around a central agglomeration of dark brown pigment.
The number of merozoites formed is quoted differently, e.g., 8 to 24, on an average 12 to 16. However, according to the recent cultural researches of J. G. and D. Thomson204 (1913) the number of merozoites of P. falciparum is 32. D. Thomson, from examination of spleen smears at autopsy, also concludes that the number of merozoites may reach 32. During their formation the blood corpuscle which is attacked gets paler and disintegrates.
Fig. 88.—The crescents of the malignant tertian parasite. (After Mannaberg.) See also fig. 81.
The gametocytes which finally appear are attenuated, curved bodies, rounded at each end and known as crescents (figs. 81, 88), and are provided with a nucleus and with coarse pigment masses. In the males the pigment is more scattered than in the females, where it is around the nucleus. Their length is 9 µ to 14 µ, and their breadth is 2 µ to 3 µ. At first they are still in the pale blood corpuscles, later they free themselves and are found in numbers in the peripheral blood in cases of pernicious malaria of Southern Europe and the tropics, while, on the other hand, they occur much more rarely in the peripheral blood in West African malignant tertian. Their further development takes place under the same conditions as in the other malarial parasites.
D. Thomson (1914),205 from studies of autopsy smears, has shown that crescents develop chiefly in the bone-marrow and spleen, and take about ten days to grow into the adult state in the internal organs. He believes that crescents are produced from ordinary asexual spores. Quinine, he states, has no direct destructive action on crescents, but it destroys the asexual source of supply.
The sporozoites of Laverania malariæ (P. falciparum) are represented in fig. 89.
Fig. 89.—Section through a tubule of the salivary gland of an Anopheles with sporozoites of the malignant tertian parasites; on the left at the top a single sporozoite greatly magnified. (After Grassi).
The principal distinctive characters of the malignant tertian parasite are: (1) The ring forms are very small, occasionally bacilliform, and may be marginal (“accolé” of Laveran); (2) the larger trophozoites are often ovoid, and about one-third or one-half of the erythrocyte in size; (3) the infected red cells sometimes show coarse stippling (Maurer’s dots); (4) the gametocytes, or sexual forms, are crescentic in shape.
J. W. W. Stephens (1914) has described a new malarial parasite of man; it is called Plasmodium tenue. It is very amœboid, with scanty cytoplasm and much chromatin, sometimes rod-like or irregular. The parasite was described from a blood-smear of an Indian child. The creation of a new species for this parasite has been criticized by Balfour and Wenyon, and by Craig.
Plasmodium relictum, Sergent, 1907.
Syn.: Plasmodium præcox, Grassi and Feletti, 1890; Plasmodium danilewskyi, Gr. et Fel., 1890; Hæmamœba relicta, Gr. et Fel., 1891; Proteosoma grassii, Labbé, 1894.
Hæmamœboid, pigment-producing, malarial parasites are often found in birds. Like the human malarial parasites they have been variously named. Labbé created the genus Proteosoma for them, and this name is still often used as a distinctive one unofficially. The correct name is stated to be either Plasmodium relictum or P. præcox, or possibly even P. danilewskyi, assuming that there is only one species. The nomenclature of the malarial parasites is most confused. The avian malarial parasites are transmitted by Culicine mosquitoes.
The organism was discovered by Grassi in the blood of birds in Italy, and causes a fatal disease in partridges in Hungary. Sparrows are affected in India, and it was this Plasmodium in which Ross first traced the development of a malarial parasite in a mosquito. The parasite may be transmitted from bird to bird by blood-inoculation, canaries being very susceptible.
The principal stages of the avian plasmodium closely resemble those of the malarial parasites of man. In its earliest stage P. relictum is unpigmented, but soon the trophozoite grows and becomes pigmented, meanwhile displacing the nucleus of the avian red-blood corpuscle, a characteristic feature, distinguishing it from Halteridium. Schizonts are formed, each of which gives rise to about nine merozoites in the circulating blood. Sexual forms or gametocytes also occur in the blood. These develop in Culex fatigans, C. pipiens and C. nemorosus. Oökinetes or vermicules are formed in twelve to fifteen hours in the stomach of the mosquito, and in one to two days well-developed round oöcysts may be seen. In three to four days sporoblasts have formed within the oöcysts and young sporozoites begin to develop. In nine to ten days the oöcysts are mature, being filled with sporozoites. The oöcysts then burst and the sporozoites travel through the thoracic muscles to the salivary glands of the Culicine.
Neumann, experimenting with canaries, found that Stegomyia fasciata could transmit the infection, but less efficiently than species of Culex.
The Cultivation of Malarial Parasites.
The successful cultivation of malarial parasites in vitro was first recorded by C. C. Bass and by Bass and Johns (1912).206 Since then, J. G. and D. Thomson,207 and McLellan (1912–13), Ziemann208 and others have repeated the experiments.
DIFFERENTIAL CHARACTERS OF THE HUMAN MALARIAL PARASITES.
| Character | Plasmodium malariæ (Quartan) | Plasmodium vivax (Benign tertian) | Laverania malariæ = Plasmodium falciparum (Malignant tertian) |
|---|---|---|---|
| Schizogony | Complete in seventy-two hours | Complete in forty-eight hours | Complete in forty-eight hours or less |
| Trophozoite | Smaller than P. vivax, larger than L. malariæ | Young trophozoite large. | Young trophozoite small |
| Pseudopodia not marked or long | Long pseudopodia | ||
| Movements | Rather slow in immature forms | Active amœboid movements | Sometimes actively motile |
| Pigment | Coarse granules, peripherally arranged, little movement | Fine granules, with active movement | Granules fine and scanty, movement oscillatory |
| Schizont | Smaller than red corpuscle | Larger than red blood corpuscle | Smaller than red corpuscle |
| Merozoites | 6 to 12 forming rosette | 15 to 20 regularly arranged | 8 to 32 (according to different authors) arranged irregularly |
| Gametocytes | Spherical | Spherical | Crescentic |
| Distribution of parasites in vertebrate host | About equal number in peripheral and visceral blood | Larger numbers in visceral blood | Scanty in peripheral blood compared with the enormous numbers in the internal organs. The latter part of the cycle (schizogony) may occur in the internal organs only |
| Alterations in erythrocytes | Almost normal | Pale and hypertrophied. | Corpuscle may be shrunken and dark, or may be colourless. Maurer’s coarse dots sometimes seen |
| Schüffner’s dots seen in deeply stained specimens |
Essentially the method of cultivation, as used by Thomson, is as follows: 10 c.c. of infected blood are drawn from a vein and transferred to a sterile test tube, in which is a thick wire leading to the bottom of the tube. One-tenth of a cubic centimetre of a 50 per cent. aqueous solution of glucose or dextrose is placed in the test tube, preferably before adding the blood. The blood is defibrinated by stirring gently with the wire. When defibrination is complete the wire and the clot are removed, and the glucose-blood is transferred, in portions, to several smaller sterile tubes, each containing a column of blood about one inch in height. The tubes are plugged and capped and then transferred, standing upright, to an incubator kept at a temperature of 37° C. to 41° C. The blood corpuscles soon settle, leaving a column of serum at the top, to the extent of about half an inch in each tube. The leucocytes need not be removed by centrifugalization. J. G. Thomson (1913) and his collaborators did not find it necessary to destroy the complement in the serum, and they found that the malarial parasites developed at all levels in the column of corpuscles, and not merely on the surface layer of the corpuscles as first stated by Bass and Johns.
So far only the asexual generation of the malarial parasites has been grown in vitro. Thomson rarely observed hæmolysis in the cultures. Clumping of the malignant tertian parasites occurred. In cultures of the benign tertian parasite (Plasmodium vivax) clumping was not observed. J. G. and D. Thomson consider that this difference as regards clumping explains why only young forms of malignant tertian are found in peripheral blood, as the clumping tendency of the larger forms causes them to be arrested in the finer capillaries of the internal organs. It also explains the tendency to pernicious symptoms, such as coma, in malignant tertian malaria. Further it was found from cultures that P. falciparum was capable of producing thirty-two spores (merozoites) in maximum segmentation, while P. vivax produced sixteen spores (merozoites) as a rule, though the number might be greater than sixteen. (Quartan parasites produce eight spores or merozoites in schizogony.)
It may also be mentioned here that Babesia (Piroplasma) canis has been successfully cultivated in vitro by Bass’s method. This has been accomplished by Thomson and Fantham,209 Ziemann, and Toyoda in 1913. J. G. Thomson and Fantham used the simplified Bass technique recorded above, namely, infected blood and glucose, incubating at 37° C. In one of the B. canis cultures, starting with heart blood of a dog containing corpuscles infected with one, two, or, exceptionally, four piroplasmata, Thomson and Fantham succeeded in obtaining a maximum of thirty-two merozoites in a corpuscle. The cultures are infective to dogs and sub-cultures have been obtained.
Family. Piroplasmidæ, França.
The parasites included in this provisional family or group belong to the Hæmosporidia. They are minute organisms, sometimes amœboid, but usually possessing a definite form. They are endoglobular, being contained within mammalian red blood corpuscles, but they produce no pigment. The true Piroplasmata, belonging to the genus Babesia, destroy the host corpuscles, setting free the hæmoglobin, which is excreted by the kidneys of the cow, sheep, horse, dog, etc., acting as host. The disease produced, variously called piroplasmosis or babesiasis, is consequently characterized by a red coloration of the urine known as hæmoglobinuria, or popularly as “red-water.” One of the best known piroplasms is Piroplasma bigeminum or Babesia bovis (probably the latter name is correct), which is the causal agent of “Texas fever” or “red-water” in cattle and is spread by ticks.