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Laboratory Procedures for Identifying Parasitic Organisms and Their Ova 273

PROCEDURE 12-5

THICK AND THIN SLIDES FOR IDENTIFYING BLOOD PARASITES
Clinical Rationale
Blood and Tissue Parasites
Blood, which is also considered a tissue, is an excellent medium for a number of
blood parasites. The major so-called blood parasites are from the genera Plasmodium

(malaria), Babesia (babesiosis), Trypanosoma (trypanosomiasis), and microfilaria from
a variety of organisms. Malaria outnumbers all of the others listed here combined
and it is estimated that up to one-half billion people in the world may be afflicted by a

strain of malaria, and several million people die each year from the infection. Tissue
parasites are obtained by surgical biopsy and include Trichinella spiralis, Leishmania
spp., and Toxoplasma gondii. Biopsy specimens are needed to test for Trichinella
spiralis encysted in muscle tissue, and amastigote stage of Leishmania spp. because

these organisms are intracellular. Toxoplasma gondii is also identified from a pre-
pared slide from a tissue biopsy. Depending on the species of certain organisms,
amoebic meningoencephalitis may be caused by Naegleria fowleri, and the spinal

fluid must be cultured to identify the pathogen.

Some bacteria are not classified as parasites but infect either/or red and white
blood cells, which they require in order to survive and to reproduce. But the most
common of these tissue parasites are those that infect the blood. Blood is stained by
either Giemsa or Wright stain but the combination Wright/Giemsa stain is the most
effective for observing both blood cell morphology and inclusions in the red blood

cells. With a properly prepared blood film, and staining by a Wrights-Giemsa stain,
most if not all parasites typically found in the blood may be detected by microscopic
examination. The most common blood parasite occurring on a worldwide basis is that
of four species of malaria, including Plasmodium falciparum, P. ovale, P. vivax, and
P. malariae. But the laboratory professional must be alert for other possibilities, as
travel throughout the world and to endemic areas for a large variety of parasitic
infections is not uncommon.
Blood smears are necessary to determine the presence of malaria (Plasmodium),

Babesia, Trypanosoma, and some species of microfilaria of Wuchereria bancrofti
that circulate in the blood during the night. There they are ingested by mosquitoes,
where they continue their development, becoming infective larvae in two weeks,
and then are reinjected into other humans. Motile organisms such as Trypanosoma
and microfilariae can be seen on direct wet mounts of fresh blood specimens under

both low- and high-power magnification. But definitive identification is made from a

permanently prepared and stained slide where morphological characteristics can be
readily observed for specific identification.

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