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

126 Manual of basic techniques for a health laboratory4.4.1 Identification of eggsThe characteristics used to identify eggs of helminth species are as follows:SizeThe length and width are measured and are generally within a specific range.ShapeEach species has its own particular shape.Stage of development when passedThe eggs of some species consist of a single cell, some eggs have several cells, andsome eggs are usually embryonated, i.e. they contain a larva.Occasionally, if stool specimens are 1–2 days old, eggs may develop to moreadvanced stages. Ascaris lumbricoides (roundworm) eggs usually have only one cellwhen passed in the faeces; however, the single cell may divide and, in specimensover 12 hours old, eggs with two or four cells may be seen.Ancylostoma duodenale or Necator americanus (hookworm) eggs present in specimensthat are several hours old may contain 16, 32 or more cells. After 12–24hours, the eggs may be embryonated and later still the larvae may hatch.When observing the stage of development of helminth eggs, be sure that the stoolspecimen is freshly passed. If it is several hours or a day old, expect to see changesin the stage of development of some species. Ideally only fresh samples should beaccepted for diagnosis.Thickness of the egg shellThe eggs of some species such as Ascaris lumbricoides have thick shells, whereasothers such as Ancylostoma duodenale or Necator americanus have thin shells.ColourThe eggs of some species such as Ancylostoma duodenale, Necator americanus andEnterobius vermicularis are colourless, whereas others such as Ascaris lumbricoidesand Trichuris trichiura are yellow or brown.Other characteristicsThe presence of characteristics such as opercula (lids), spines, plugs, hooklets ormammilated outer coats can also be aids to identification.If an egg or an object that looks like an egg is found, the above-mentioned characteristicsshould be carefully observed in order to make a specific identification.Occasionally, atypical or distorted eggs are seen. In such cases, it is necessary tolook for more typical forms in order to make a reliable diagnosis. Remember thatmore than one species of helminth may be present in a patient.Measurement of eggs● 1 micrometre (1mm) = 0.001mm.The size in mm given in this manual is that of the long side of the egg.The size can be estimated by comparison with that of an erythrocyte, whichmeasures 7.5–8mm.