Before Laveran’s discovery, Lankester had described a parasitic organism living in the blood-cells of a frog, and within the last twenty years numerous other organisms have been discovered and described by various investigators living in the blood-corpuscles of reptiles, birds, monkeys, and bats. There are at least three species of Hæmatozoa, as they are called, which live in the blood of man, and these three correspond to the three kinds of malaria—the tertian, the quartan, and the æstivo-autumnal, or, as it is often termed, the irregular type of malarial fever, which occurs so frequently in the late summer and autumn in Italy and elsewhere. The hæmatozoön causing the last-named fever has been especially studied by the Italian observers, and it differs more markedly from those causing the tertian and quartan fevers than the latter do inter se. It is not universally conceded that the differences between these three forms of organism are such as to establish a difference of species, but the weight of opinion is in favour of this view. Ross even places the parasite of the æstivo-autumnal fever in a separate genus, and we have throughout this article adopted his nomenclature. Zoologically he groups all the three species infesting man in Wassielevski’s family Hæmamœbidæ, which, besides the human parasites, includes a species found in monkeys, three species in bats, and two in birds. The species causing tertian and quartan fevers are grouped by Ross in the genus Hæmamœba, the former being called Hæmamœba vivax, the latter Hæmamœba malariæ. The parasite causing the æstivo-autumnal fever is called Hæmomenas præcox.
With the exception of a few details the life-history of all these forms is practically identical, although the time which is occupied by different phases of their life-cycle varies in the different species. The account given here applies in the main to them all.
The organism which Laveran saw living in the blood-corpuscles of his malarious patient was a minute cell of irregular shape whose nucleus can be demonstrated by the use of appropriate reagents. The cell constantly but slowly changes its outline, pushing out and withdrawing blunt rounded processes; in fact, the cell resembles the lobate forms of one of the simplest microscopic animals we know, the Amœba ([Fig. 1]). The movements and change of shape consequent on them are termed amœboid, and the organism in this stage is known as an amœbula. These amœbulæ whilst in the blood-cell grow rapidly, and in some way they collect the hæmoglobin, or colouring matter of the red corpuscle, within their own bodies, and convert it into a number of dark brown or black pigment granules, which crowd around the nucleus of the parasite. This pigment, the so-called malarial pigment or melanin, had been recognized by Virchow and others about the middle of the nineteenth century as a characteristic product in the blood of malarial patients. The amœbulæ continue to grow rapidly, at the expense of their cell-host, until, after a definite period, which varies from one to several days, they become mature, and by this time they have completely filled up the red corpuscle, whose scanty remains form a tight skin round the fully-grown parasite ([Fig. 1], 1-8). When mature, one of two things happens—either they become (1) gametocytes, whose meaning and fate we will consider later, or they become (2) sporocytes. In the latter case the nucleus of the amœbula breaks up into a number of small nuclei, and each surrounds itself by a small mass of protoplasm and forms a spore (Fig. 1, 5-8). The result of this process of division may be roughly realized if we imagine an orange with
but one pip in each quarter. Then the skin of the orange will represent what is left of the red blood-corpuscle, the flesh will represent the divided sporocyte, each quarter will represent a spore, and the pip will represent its nucleus.
At this stage the skin to which the red corpuscle has been reduced breaks, and the spores fall into the liquid part of the blood ([Fig. 1], 9). The pigment granules which escape at the same time also pass into the liquid of the blood, and are eaten up and removed by those scavengers of the vascular system, the white corpuscles. Each of the spores, after remaining a short time in the fluid of the blood, attaches itself to a new red corpuscle, penetrates its body, and becomes a small amœbula, which repeats the life-history described above. In this way a few organisms will soon produce enough spores to infect a very large number of blood-corpuscles; as many as 60 per cent. are in some cases infected. The severity of the attack naturally depends in a great degree on the number of corpuscles infected. Laveran not only first recognized and described the organism[4] we are dealing with, but he definitely connected its presence with malaria; but it was not until some time later, in 1885, that Golgi described the sporulation of the sporocyte and pointed out that the moment of the escape of the spores from the red corpuscle coincides with the paroxysm of the fever. Since all the amœbulæ of one crop are at about the same stage of growth in any one host, millions of spores in a well-infected patient are thrown into the liquid of the blood at about the same time; and it is clear that this must be accompanied by a profound disturbance of the system. This disturbance manifests itself in a feverish attack. The period when the spores have left the corpuscles and are free in the liquid of the blood is also the time at which the administration of quinine is said to be most effective. Further, it is only at this stage that the disease can be artificially transferred from one man to another. All efforts to transmit the gametocytes have ended in failure.