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

4. Parasitology 185Fig. 4.144 Twist the needle in both directionsFig. 4.145 Collecting a sample of fluid from thelymph node6. Attach the needle to the syringe, with the piston pulled back (Fig. 4.145). Pushthe piston gently halfway down the barrel to discharge a drop of the glandularfluid in the needle on to a slide.Microscopic examinationExamine the slide first using the ¥ 10 objective, then change to the¥ 40 objective to examine the parasites in greater detail. Close thecondenser iris diaphragm sufficiently to give a sharp image.Wait until the convection currents stop. It is impossible to see themovement of trypanosomes among moving cells.Begin by examining the periphery of the preparation, near the edgesof the coverslip, as shown in Fig. 4.146, as the trypanosomes tendto make their way to the edges. Then examine the rest of the preparation.The preparation will contain erythrocytes and leukocytes.Trypanosomes are about 20mm long and are often hidden by cellularelements which are disturbed by the flagella of thetrypanosomes as they move (Fig. 4.147).Fig. 4.146 Examining a lymph node sampleunder the microscopeExamination of blood films for Trypanosoma bruceigambiense and T.b. rhodesiensePrincipleTrypanosomes belonging to the species Trypanosoma brucei are detectedin the blood:— in wet preparations— in thick films after staining— following concentration by repeated centrifuging— in serological tests.Important: In African trypanosomiasis trypanosomes appear in the blood at intervalsfor a period of a few days, mainly during the first 3 months of the disease andespecially during bouts of fever.Fig. 4.147 Appearance of trypanosomes ina lymph node preparationMicroscopic examination of capillary bloodMaterials and reagents● Microscope●Microscope slides