Manual of basic techniques for a health laboratory - libdoc.who.int

4. Parasitology 179Table 4.11 Comparison of infected erythrocytes in thin blood filmsP. falciparum P. malariae P. vivax P. ovaleSize of youngtrophozoite incomparisonwith diameterof an erythrocyte(at the samestage ofdevelopment) One-fifth to one- One-quarter to two- One-quarter to two- One-quarter to twothirdof diameter thirds of diameter thirds of diameter thirds of diameterAppearance ofinfectederythrocyteDots in theinfectederythrocyteRemains Remains unchanged or Enlarged and often Enlarged, oval, withunchanged becomes smaller and pale-staining torn jagged edgessometimes moredeeply colouredOften none a None Small pink dots Large red dots (James(Schüffner’s dots) dots) always presentStages found Trophozoites and/or All stages found in the All stages found in the All stages found in the(see Fig. 4.139) gametocytes; many same film same film same filmtrophozoites can befound in one cellaIn some erythrocytes infected with adult trophozoites of P. falciparum, a few large pink granules (“Maurer’s clefts”) can be found.dots (see Table 4.11). Thin films can be used to identify the species of malariaparasite (see Table 4.12).Note: In patients who have been suffering from malaria for a long time, monocytesmay be seen in the thin blood film; the cytoplasm often contains brown or greenishblackbodies (siderophils). In patients who have recently received an injection of anantimalarial drug, the parasites stain poorly and appear distorted and indistinct.Thick blood filmsIn thick blood films, the background should be clean and free from debris, as theinfected erythrocytes are lysed. The malaria parasites should have deep red chromatinand blue or pale purplish-blue cytoplasm. In thick films stained with Giemsa, thenuclei of leukocytes should be stained dark purple. Schüffner’s dots may be seenaround the malaria parasites.Thick blood films are used for estimating the parasite density, as described below.Parasite densityThe parasite density is the number of parasites counted in each microscope field. Itusually varies according to the species.Two methods can be used to count malaria parasites in thick blood films: determinationof the number of parasites per microlitre (ml) of blood, and the plus system.