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

4. Parasitology 153MethodPreparation of grease-free coverslips1. Mix in a cylinder: 10ml of 95% ethanol and 10ml of ether.2. Pour into a Petri dish and in it place 30 coverslips, one by one;shake and leave for 10 minutes.3. Take the coverslips out one by one and dry them with gauze.4. Keep them in a dry Petri dish.The above steps are summarized in Fig. 4.104.Fig. 4.104 Preparation of grease-free coverslipsConcentration of parasites1. Place approximately 0.5g of stool in a wide-mouth bottle. Fill the bottle to the2.5-ml mark with Willis solution.2. Using an applicator, crush the portion of stool and mix it well with the solution.Then fill the bottle to the top with Willis solution; the suspension should becompletely uniform.3. Place a coverslip carefully over the mouth of the bottle.4. Check that the coverslip is in contact with the liquid, with no air bubbles. Leavefor 10 minutes.5. Remove the coverslip with care; a drop of liquid should remain on it. Place thecoverslip on a slide and examine under the microscope (using the ¥10 objective)at once because the preparation dries very quickly. Otherwise seal the coverslipwith petroleum jelly and wax.Use the fine adjustment of the microscope to examine every object in the field(eggs tend to stick to the coverslip and are not immediately distinct).4.5.2 Formaldehyde–ether sedimentation technique(Allen & Ridley)PrincipleThe stool specimen is treated with formaldehyde, which preserves any parasitespresent. Lumpy residues are removed by filtration. Fatty elements of the faecalsuspension are separated by extraction with ether (or ethyl acetate), followed bycentrifugation, which sediments any parasites present.Materials and reagents● Microscope● Microscope slides● Coverslips● Centrifuge● Test-tubes● Test-tube rack● Centrifuge tubes● Wooden applicators● Brass wire filter, 40 mesh (425mm), 7.2cm diameter (nylon coffee strainersprovide an inexpensive alternative)● Small porcelain or stainless steel dish or beaker● Pasteur pipette