It may possibly appear superfluous, after our complete description of all the forms and phases of malarial parasites, to speak again of their diagnosis, yet our excuse lies in the fact that it is not at all rare, even at this late date, for objects in the blood to be regarded as malarial parasites which are not, and for the actual parasites to escape the observation of the inexperienced investigator.
Malarial parasites can naturally be confused only with those bodies which occur in human blood. These bodies are red blood corpuscles, white blood corpuscles, blood platelets, and products of coagulation.
There is absolutely no excuse for mistaking a normal red blood corpuscle for a parasite, and scarcely more a shadow corpuscle, since there is not the slightest resemblance between them. Still, a thorn apple form red blood corpuscle may be mistaken for a brassy corpuscle, or oscillating fragments of red blood corpuscles occurring in the form of small balls, hyaline filaments, or long threads with sinuous movements may be confused with flagella. The greatest danger for the inexperienced, however, lies in the confounding of the "vacuoles" of the red blood corpuscles with non pigmented young parasites.
The thornapple forms are differentiated from the brassy corpuscles in that they contain no parasites. In the latter the parasites are always seen as small rings, or as hyaline, round or oval, slightly or not at all pigmented spots. Moreover, the thornapple forms are covered with sharp teeth, while the brassy corpuscles have a rumpled appearance (Plate VI, Fig. 20).
The salutatory fragments of red blood corpuscles are always more or less the color of hemoglobin, while the oscillating flagella are entirely colorless and show sometimes one or several very fine pigment granules. The spheric fragments and hyaline strings are, on account of their large form, scarcely similar to flagella.
The "vacuoles" of the red blood corpuscles are in some ways very similar to the young, non pigmented parasites. As is well known, these vacuoles are the result of mechanical injury to the preparation, and the more carefully the drop is spread out, etc., the less frequently are they seen. Vacuoles also appear when a drop of immersion oil touches the object, and after the preparation has become several hours old.
These "vacuoles" are not what their name describes them to be. They are not holes in the blood corpuscle, but are places from which the hemoglobin has retracted, perhaps in a zooidal way, so that only the delicate, colorless stroma remains, looking like a clear spot or small ring. The "vacuoles" have the faculty of changing their form, so as more or less to simulate the ameboid movements of parasites.
Plate IV, Fig. D, shows a "vacuole" in the act of changing its shape. This is seen on the warm stage quite frequently.
The form and size of the "vacuoles" vary greatly, from the smallest point to large figures, taking up two thirds of the blood corpuscle. Moreover, it is not rare to see in one blood corpuscle many punctate vacuoles.
The most important difference between "vacuoles" and parasites is that the former show no structure, while, as living organisms, this is a characteristic of the latter.
In the unstained preparations "vacuoles " are differentiated from parasites by their markedly sharp contour; the ameboid organisms before they come to rest show an extremely delicate border, which shades off into the substance of the blood corpuscle. The "vacuoles" further show a luster not seen in the parasites. It is difficult to express in words the details of the differentiation, yet but little experience is necessary to make it very evident in the great majority of cases.
We may add that when the ring form predominates, it speaks for parasites, but when large areas of spotted red blood corpuscles are seen, while other parts of the preparation show none at all, the diagnosis of "vacuoles," caused probably by some local injury to the preparation (pressure, oil), is certain. It is naturally never possible to confuse " vacuoles" with pigmented parasites, since the former never contain pigment.
Among the white blood corpuscles the only ones that can be confused with parasites are the melaniferous leukocytes so common to malarial blood. The differentiation here lies in the fact that in the white blood corpuscles there are always one or several large, compact nuclei to be seen. In unstained preparation the parasites may occasionally show a nucleus, yet this is always vesicular and contains a dark round spot-the nucleolus.
The ameboid movement of the leukocytes should never give rise to error, because the adult parasites-the only ones that might manifest a similarity-are immotile.
Leukocytes devoid of pigment show no resemblance to parasites, if for no other reason than because parasites of this size are invariably pigmented, omitting entirely the previously mentioned difference in relation to the nuclei. It is impossible to understand how Laurie could confuse leukocytes with parasites, and this even in our day.
The blood platelets, when isolated and round, may be confused with free spores; when they lie together in heaps, as occurs so frequently, they may be confused with sporulation forms.
In regard to the isolated blood platelets, it should be made a rule never to diagnosticate a free spore in an unstained preparation. This diagnosis is, in the great majority of cases, impossible, for free spores possess absolutely nothing by which they can be characterized, except those of the quartan parasite, which are recognized by their nucleolus. In addition, a free spore may be confused with a large coccus or an yeast cell in the same way as with a round blood platelet.
It is, therefore, clear that the bodies found free in the plasma are, so far as diagnosis is concerned, best left out of consideration. In the case of stained preparations the matter is different. Here we may differentiate a blood platelet from a spore with certainty (Plate VII, Figs. 1 and 2; 27-29) by the former staining diffusely and showing no structure.