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

4. Parasitology 127●●●The size can be assessed in relation to the microscope field:— if a ¥10 objective is used, the egg takes up about one-tenth of the field— if a ¥40 objective is used, the egg takes up about one-third of the field.The egg can be measured by inserting a micrometer scale slide in the eyepieceof the microscope. One division of the scale using the ¥ 10 objective and the ¥10eyepiece = 1mm.Another method of measuring is to compare the egg with one of another speciescommon in the locality whose size under the microscope is known (e.g. Ascarislumbricoides).How to recognize eggsThe method recommended is:● Establish the probable identity of the egg from its general appearance.● Make a systematic study of all the characteristics of the egg to confirm itsidentity. In order to gain experience (if possible, under the guidance of aninstructor):— study the different eggs found in your locality;— identify, one by one, all the characteristics of each egg as described in thismanual.Table 4.5 lists the helminth species whose eggs are found in stools.The terms used for the identification of helminth eggs and a key to their identificationare given in Figs. 4.39 and 4.40, respectively. Fig. 4.41 shows the relative sizesof helminth eggs.Ancylostoma duodenaleSize: 50–80mm.Shape: oval with rounded slightly flattened poles (one pole often more flattenedthan the other).Shell: very thin; appears as a black line.Content: varies according to the degree of maturity.Colour: pale grey; dark brown after staining with iodine solution.Type A (in fresh stools) (Fig. 4.42)Four, eight or 16 grey granular cells, clear but not shiny (blastomeres).Type B (in stools a few hours old) (Fig. 4.43)A uniform mass of many small grey granular cells.Type C (in stools 12–48 hours old) (Fig. 4.44)The whole of the egg is filled by a small larva (the future worm), wrapped arounditself. The egg is “embryonate”.Ascaris lumbricoidesThere are four types of Ascaris egg:●●A: fertilized egg with double shell.B: unfertilized egg with double shell.