Page 294 - parasitology for medical and clinical laboratoryprofessionals

274 CHAPTER 12

PROCEDURE 12-5

(continued)

Identification Procedure for Blood Parasites
Fresh anticoagulated blood is necessary for preserving the morphology of the ery-

throcytes and to stain the parasites themselves. For microfilaria, stains other than

Wrights-Giemsa are used but it is possible to detect the presence of microfilaria from
a slide stained by this stain. This stain gives better morphology with details of para-
sites that also exhibit more color intensity than slides stained with only Wrights stain.
For suspected cases of malaria and other blood parasites, based on travel, clinical

signs and symptoms, both a thick and thin blood film should be made. Methanol is

used to fix the RBCs on the thin film. RBCs are lysed on the thick smear with water
before staining. The thick smear is made to afford a higher concentration of malaria
parasites and the thin film provides better morphology for identifying parasites by

species.

Identification Procedure for Malaria

Malaria is identified by the fever cycle for the various species and history of travel,
although infection with more than one Plasmodium species may cause some confusion.
The morphology of the trophozoites and gametes as well as the presence of a few well-
known phenomena is helpful. The four species of malaria that infect humans have vary-
ing geographic distribution but some zones overlap. A number of other species other
than these four exist but only certain animals and birds are affected. Of the four species
that infect humans, the “malignant” strain is P. falciparum (see Figure 12-8). The char-
acteristic morphology of these four species may be used from microscopic examina-

tion to identify the species. Fresh blood from a fingerstick is the best specimen but a
tube with the anticoagulant EDTA is adequate if stained within 1 hour of collection.
With Giemsa stain, the parasite’s cytoplasm is stained bluish, with red to purple-red
chromatin. Malarial stippling, if present, will stain as faint, discreet pink-red dots in
the cytoplasm.
For malarial determinations, the best time to make blood smears for identifi-

cation of the parasite is halfway between paroxysms (see Table 12-3). In malaria,
samples are examined every 6 to 8 hours for up to 3 days to allow for the differ-
ences the reproduction of the four species of malaria that affect humans. Usually
during a parox ysm (a severe attack or increase in violence of a disease that