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

156 Manual of basic techniques for a health laboratoryFig. 4.108 Transferring the sediment to a slideformaldehyde–ether sedimentation technique. The formalin preserves the parasiteswithout distorting their morphology.4.5.4 Sedimentation technique for larvae of Strongyloidesstercoralis (Harada–Mori)PrincipleA strip of filter-paper is partially submerged in a test-tube containing water. Anylarvae of Strongyloides stercoralis present in the specimen migrate against the currentof water that rises by capillary action and accumulate at the bottom of thetube.Materials and reagents●●●●●●MicroscopeCellophane tapeTest-tubesTest-tube rackStrips of filter-paper (30mm ¥ 150mm)Spatula● Lugol iodine, 0.5% solution (reagent no. 37).Method1. Use the spatula to spread a small quantity of the faecal specimen along a strip offilter-paper (previously folded lengthwise to keep it straight), but leave the last 4or 5cm clean to be put into water.2. Put the strip of filter-paper, clean end first, into a test-tube containing filtered orboiled water 2.5–3.0cm deep; fold the strip at the top so that the bottom doesnot touch the bottom of the tube.3. Record the serial number or name of the patient indelibly on the tube.4. Plug the tube with cotton wool or, preferably, seal with cellophane tape and keepfor 7–8 days at room temperature.5. Look for the larvae at the bottom of the tube. Stain with iodine solution for 1minute and then examine under the microscope, using the ¥ 10 objective.The larvae usually seen in fresh stool specimens are the rhabditiform (first-stage)larvae of S. stercoralis. However, if the stool was passed more than 12 hours earlier,the larvae may have hatched into filariform (infective-stage) larvae. These must be