The Animal Parasites of Man

by Harold Benjamin Fantham; Max Braun; J. W. W. Stephens; Fred. V. Theobald

All the afore-mentioned infections are termed acute. They are distinguished from the very different chronic malarial infection by the frequent occurrence of relapses, which finally lead to changes of some organs and particularly of the blood. The relapses are then generally marked by an irregular course of fever.

The term masked malaria is used when any disturbance of the state of health of a periodic character shows itself and disappears after treatment with quinine.[194] Generally it is a question of neuralgia.

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. Laveran[195] 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).