Groups of blood platelets are differentiated from sporulation forms by the fact that they show no pigment, while no sporulation form, with one single exception, occurs without pigment; and this exception, namely, the sporulation form of the non pigmented quotidian parasite, is not likely to be confused, because it lies within a red blood corpuscle; moreover, these sporulation forms scarcely ever occur in the peripheral blood.
In stained preparations the differential characteristics which we have already described in relation to the isolated blood platelets obtain also for the groups.
Among the coagulation products of the blood only the rather uncommon accumulations come into consideration. These appear amorphous, shapeless, and lie free in the plasma. They are, therefore, harmless so far as a confusion with parasites is concerned. Besides these blood constituents, we must keep in mind foreign bodies, like epithelium, dust, and dirt particles, that in spite of the utmost care and cleanliness find their way into the preparation.
When seeking after pigment we frequently find indefinite small particles that may be confusing. Now, though malarial pigment is sometimes seen free in the plasma, especially at the time of sporulation before the phagocytes have been able to remove it, too much confidence should not be placed in such pictures, and from them alone a diagnosis should never be made.
It is of much more significance when, in unstained preparations, the pigment is found in leukocytes, even though there are only a few granules. Here they become an important diagnostic aid, especially when there is no suspicion of recurrent fever, for in this disease, too, pigment may occur in the blood.*
After recovery the pigment remains visible in the circulating blood only a very short time, being quickly deposited by the leukocytes in the well known deposit places. In fevers of the first group no melaniferous leukocytes are seen two or three days after the last fever paroxysm; in the case of fevers produced by crescents it is different, melaniferous leukocytes being encountered as long as the crescents are in the blood.
Finally, we must remember the possibility that pigment may occur in the blood in poisoning with carbon bisulphid and carbonyl sulphid, as C. Schwalbe has shown. I have repeatedly made the experiments on mice by injecting them subcutaneously with a few drops, and can confirm, in their entirety, Schwalbe's assertions. In such cases the anamnesis would assist possible doubts. Moreover, the appearances in the blood of the poisoned animals are very different from those produced by malarial parasites. It is scarcely necessary to go into details, especially since, so far as I know, these blood changes have never been observed in man.
In the blood of rabbits poisoned with dinitrobenzol, Huber has observed large numbers of vacuoles in the red blood corpuscles. The picture in connection with this would serve as an interesting comparison with young malarial parasites.
Placed, now, in the position where we can recognize malarial parasites, the question arises, Of what use is this knowledge? The answer to this is that the presence of one single malarial parasite in the blood establishes the diagnosis of malarial infection.
I will not go into details in regard to the assertions, made a few years ago, that "similar bodies" might be found in the blood in very different infectious diseases and cachexias. They detracted at the time from the pathognomonic and etiologic dignity of Laveran's malarial parasite, but the majority of the writers later confessed that they took objects-almost always vacuoles in the red blood corpuscles-for parasites which were evidently not parasites. From the numerous blood examinations undertaken in our day the conviction is certain that malarial parasites occur exclusively in the blood of malarial patients.
* The assertion has also been made that pigment occurs in the blood in Addison's disease; in two such cases that I have examined I have not been able to confirm this.
Moreover, it is not alone the presence of a malarial infection in general that may be diagnosticated, but the type of fever, and often even the severity of the attack, though naturally such detailed information is possible only to those who have had considerable experience and are acquainted with all the forms of the parasites.
It was Golgi's magnificent work in differentiating the various forms that brought this knowledge to a realization, and since that time his observations have been repeatedly confirmed. I can only add similar confirmation, in that, following in Golgi's footsteps, I have been able clearly to analyze many cases from the blood examinations alone.
The few contradictions that arose against Golgi's statements came from observers not in a position to pass a critical judgment on account of the small amount of material at their command. Moreover, the superstructure begun by Golgi has been added to especially by the labors of Marchiafava and Bignami.
I will omit a detailed explanation of the diagnosis of the type, since it would be necessary to repeat the greater part of what has already been said in relation to the special characteristics of the different parasites. For this, therefore, we refer to the proper sections, and here we will mention only a few points which come especially into consideration.
When we have before us a positive find, we must first decide whether the parasites belong to the first group (without crescents) or to the second group (with crescents), or whether both are present in the blood. This is evident to the expert in a very short time, in- that predominating large pigmented endoglobular forms draw his attention to the first group, and numerous small and slightly or not at all pigmented forms to the second group. If crescents or spheres are present, it is at once evident that we are dealing with the second group. These spheres of the second group may be differentiated with little practice from the spheric forms of the tertian and quartan parasites by their sharp, often double, contour, by the peculiar changes in the blood corpuscles, by the frequent, wreath like arrangement of the pigment, and by the marginal "knobs."