3. Blood-plaques.—These are not colored by Ehrlich's stain, nor by eosin and methylene-blue. With Wright's stain they appear as spheric or ovoid, reddish to violet, granular bodies, 2 to 4 µ in diameter. When well stained, a delicate hyaline peripheral zone can be distinguished. In ordinary blood-smears they are usually clumped in masses. A single platelet lying upon a red corpuscle may easily be mistaken for a malarial parasite ([Plate VI]).
Blood-platelets are being much studied at present, but, aside from the facts mentioned under their enumeration ([p. 165]), little of clinical value has been learned. They have been variously regarded as very young red corpuscles (the "hematoblasts" of Hayem), as disintegration products of leukocytes, as remnants of extruded nuclei of erythrocytes, and as independent nucleated bodies. The most probable explanation of their origin seems to be that of J. H. Wright, who, from his recent studies, regards them as detached portions of the cytoplasm of certain giant-cells of the bone-marrow and spleen.
VII. BLOOD PARASITES
A study of blood bacteriology is useful, but is hardly practicable for the practitioner. Most bacteria can be detected only by culture methods. The spirillum of relapsing fever can be identified by the method for the malarial parasite in fresh blood. The blood must be taken during a paroxysm. The organism is an actively motile spiral thread, about four times the diameter of a red corpuscle in length. The movements which its active motion causes among the corpuscles render it conspicuous. It can also be seen in stained preparations (Fig. 78). The disease has rarely been seen in the United States.
| FIG. 78.—Spirillum of relapsing fever (Karg and Schmorl). |
Of the numerous animal parasites which have been found in the blood, three are especially interesting clinically: Plasmodium malariæ, Filaria sanguinis hominis, and Trypanosoma hominis.
1. Plasmodium Malariæ.—This organism is one of a large group, the hemosporidia ([p. 247]), many of which live within and destroy the red corpuscles of various animals. Three varieties are associated with malarial fever in man—the tertian, quartan, and estivo-autumnal malarial parasites.
(1) Life Histories.—There are two cycles of development: one, the asexual, in the blood of man; and the other, the sexual, in the intestinal tract of a particular variety of mosquito.
(a) Asexual Cycle.—The young organism enters the blood through the bite of the mosquito. It makes its way into a red corpuscle, where it appears as a small, pale "hyaline" body. This body exhibits ameboid movement and increases in size. Soon, dark-brown granules derived from the hemoglobin of the corpuscle make their appearance within it. When it has reached its full size—filling and distending the corpuscle in the case of the tertian parasite, smaller in the others—the pigment-granules gather at the center or at one side; the organism divides into a number of small hyaline bodies, the spores or merozoites; and the red corpuscle bursts, setting spores and pigment free in the blood-plasma. This is called segmentation. It coincides with, and by liberation of toxins causes, the paroxysm of the disease. A considerable number of the spores are destroyed by leukocytes or other agencies; the remainder enter other corpuscles and repeat the cycle. Many of the pigment-granules are taken up by leukocytes. In estivo-autumnal fever segmentation occurs in the internal organs and the segmenting and larger pigmented forms are not seen in the peripheral blood.
The asexual cycle of the tertian organism occupies forty-eight hours; of the quartan, seventy-two hours; of the estivo-autumnal, an indefinite time—usually twenty-four to forty-eight hours.