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

162 Manual of basic techniques for a health laboratoryFig. 4.115 Place the blade above thepoint of the needleThe specimen should be about this size: ● (2–3mm in diameter).It should remain attached to the tip of the needle. The specimen shouldnot be bloodstained; the biopsy must be bloodless.8. Place the fragment of skin in the drop of sodium chloride solutionon the slide (using the scalpel or razor blade if necessary). Do notflatten the piece of skin; if only one microfilaria is present, it mightbe damaged.9. Cover with a coverslip. If any part of the specimen is not in contactwith the liquid, add more solution, injecting it under the coverslipwith a Pasteur pipette, until the whole area underneath the coverslipis wet.10. Wait 2–3 minutes. Meanwhile, clean the spot from which the specimenwas taken with ethanol. Apply an adhesive dressing.Fig. 4.116 Collecting a slit skin specimenCollection of specimens in the fieldIf no microscope is available, or during mass epidemiological surveys:1. Place the piece of skin in a small bottle containing 2ml of sodiumchloride solution.2. Wait 15 minutes for the microfilariae to leave the skin.3. Fix the specimen by adding 2ml of 10% formaldehyde solution(reagent no. 28). Mix and replace the cap on the bottle.4. When you return to the laboratory, shake the bottle well.Centrifuge the liquid (after removing the piece of skin) at mediumspeed (2000g) for 5 minutes.5. Transfer the deposit from the centrifuge tube to a slide and cover itwith a coverslip.Fig. 4.117 Onchocerca volvulusmicrofilariae in wetpreparationMicroscopic examinationExamine the wet preparation under the microscope using the ¥ 10 objective.Microfilariae are highly motile. If any are present, they will beseen moving towards the sodium chloride solution (Fig. 4.117).Microfilariae of Onchocerca volvulus have the following features:Length: 200–315mm.Width: 5–9mm (the same as an erythrocyte).Curvature of the body: angular.Front end: slightly broader than an erythrocyte.Tail: curved and tapered.If no microfilariae emerge, wait for 10 minutes and look at the centre of the piece ofskin; you may see one or two microfilariae moving. If you are in any doubt, take afresh blood specimen from the patient’s finger and prepare a smear on a slide.Cover it with a coverslip and examine it under the microscope. If you see anymicrofilariae, prepare a stained skin smear (see below) and a stained thick bloodfilm (see page 170) to identify the species.If microfilariae are present, they will be clearly visible. Staining is not necessary, asthe microfilariae can be identified by their characteristic angular curves.