The malarial parasites are unicellular organisms whose cycle of development from beginning to end takes place within the reel blood corpuscles. They are, therefore, parasites (cytozoa) of the erythrocytes, and on this account occupy a special position in pathology.
The young forms show a more or less ameboid movement, while the adult organisms change their form but little, and then not so much by the protrusion and drawing in of pseudopodia as by the contraction of certain layers, which results in a very gradual change of shape.
On account of the alterations in form produced by the ameboid movement we can describe no typical shape for the young parasites. As a matter of fact, they change constantly under the eye of the observer. Only after exposure to external influences and their consequent death do we see anything like a constant form. This is usually that of a ring, occasionally a disk.
Their growth within the flat red blood corpuscle makes it easy to understand why the young and half grown parasites show usually a flat disk form; otherwise the parasite appears, depending on whether it is at rest or in ameboid movement, round, oval, or with regular or irregular contour. The adult parasite, especially when it has escaped from the red corpuscle, is usually spheric, yet there are stages in its development that show more or less constant crescent, spindle, and oval forms. The parasite measures, according to its age, 1 to 10 micro.
The parasites usually occur isolated in single blood corpuscles, yet it frequently happens in the case of certain varieties that several (two to six and over) are found in the same corpuscle.
The body of the parasite is colorless, and in the living condition shows no structure, appearing as a homogeneous, hyaline mass. A nucleus or granulations are only occasionally observed, and then only in particular varieties (for instance, the quartan parasite) or in full grown forms.
The young parasite appears like a small speck in the red blood corpuscle, and depending on whether the parasite is deep in the corpuscle or near its surface, the speck stands out more or less clearly. It is diagnosed by its frequent change of shape, the result of its ameboid movement; though it must not be forgotten that this movement may cease temporarily or even entirely. On a warm stage it is more lively than on a cold one.
At this time, before the pigment granules appear, the parasite is very difficult to see and to recognize. Still in severe malarial regions a close acquaintance with these small, non pigmented or very slightly pigmented organisms is very important, since the parasites of estivoautumnal fever occur in the peripheral blood only in this stage, and later, when they develop pigment, they are found almost exclusively in the capillaries of the internal organs. Parasites of the quartan variety are, even in the second half of their existence, usually found in the peripheral blood.
In addition to an increase in size and a gradual slowing of movement the quartan parasites are characterized by their pigmentation.
This pigment (malarial pigment, melanin) is nothing else than the product of digestion of the hemoglobin, at the expense of which the parasite is nourished. It appears in the form of very fine, dust like particles, in the form of larger granules, lines, needles, grains, or clumps. The longest needles or lines measure about 1 micro. By the running together of numerous pigment granules larger clumps, in the shape of glandular masses, are formed. The color of the pigment is, in the last mentioned rough masses, black, while the fine needles, granules, and dust are usually reddish brown. Laveran described the color as a dark, fiery red ("rouge feu tres fonce")- Very rarely the pigment is light blue or greenish.
Concentrated mineral acids have no effect on the pigment, but it is cleared by weak alkalis, after the action of which it appears reddish brown or yellowish. The pigment is dissolved by ammonium sulphid (Kiener). The Berlin blue reaction cannot be obtained from the pigment.
According to Kelsch and Kiener, melanin shows a similarity in its microchemic behavior only with the pigment of melanotic tumors. It is entirely different from the iron containing or iron free transformation products of hemoglobin found in hemorrhagic foci. The amount and appearance of this pigment vary in the different varieties of parasites, but about this we shall speak later on.
The pigment granules participate in the ameboid movements of the parasites in that they move in and out with the pseudopodia. Yet in addition the pigment shows a second communicated movement, which is most marked in the adult sexual forms of the parasites. This consists in a more or less lively, to and fro wavering of the pigment elements. When slight, the pigment granules move sluggishly, scarcely changing their place, but when marked, they whirl back and forth like a swarm of gnats. Laveran has appropriately compared the appearance in this last case with the bubbling of boiling water.
In my opinion this movement of the pigment is produced by the to and fro motion of the plasma, which, confined or relaxed by different circumstances, forces the pigment granules more or less rapidly back and forth.
The duration of this motion varies, although in every case it continues much longer than any of the other movements of the parasite. I have observed it in the moist oxygen chamber for from twenty four to forty eight hours. According to Ziemann, it continues in the parasites in the cadaver.
Besides the pigment, we may sometimes find in the parasites small granules of undigested hemoglobin, recognizable by its color; again, non contractile vacuoles, occasionally in large numbers, particularly in adult forms.