Special Educational Materials
College of Public Health · Global Health

 
Abbreviations
 
  bt: button-like thickening
  cg: cephalic gland
  cgd: cephalic gland duct
  de: developing embryo
  e: embryophore
  gc: germ cell
  h: hooklet
  he: hexacanth embryo
  is: inner shell
  ls: lateral spine
  m: miracidium
  mc: mammillated coat (cortex)
  ns: nervous system
  op: operculum
  ops: opercular shoulders
  os: outer shell
  pf: polar filaments
  pg: primitive gut
  pm: primary membrane
  pp: polar plugs
  rss: radially striated shell
  s: shell
  tae: thickened abopercular end
  ths: thick hyaline shell
  ue: unsegmented embryo
  vm: vitelline membrane
 
Entamoeba histolytica Giardia lamblia Schistosoma mansoni Hymenolepis nana Ascaris lumbricoides Trichuris trichuria Hookworm Strongiloides spp. Heterophyes hetereophyes

Diagnosis | Structure | Introduction | Key | Plate 1 | Plate 2 | Footnotes

 Diagnosis of Helminth Infections

Helminths are cosmopolitan and play a significant role in the morbidity and mortality of people in many parts of the world, especially among the poorer classes in third world countries. Most prevalent are the intestinal helminths and infection with these helminths are most often diagnosed by finding and identifying eggs and/or juvenile nematodes when examining fecal specimens. Either fresh or preserved specimens may be used for diagnosing an infection. Although the utilization of certain specific products, procedures, and techniques for collecting, preserving, and examining fecal specimens may be given emphasis, most of the material that relates to parasites applies regardless of the collection method, specific product, or procedure employed. Identifying an egg found but not readily recognized often presents a problem for the laboratory worker and may require looking in several reference sources before identification can be made. Size, shape, structure, and stage can each impact on identification. The relative size of some eggs seen in a fecal specimen is shown in Plate 1. If an appropriate reference source is not readily available, identification may take several days or sometimes longer.

 The Structure of Helminth Eggs

Plate 2 shows drawings of representative eggs and their structures from each group, trematodes, cestodes, and nematodes. These structures are also used in the Key to Helminth Eggs Found in Feces and most are defined in the glossary. The eggs drawn in the plate are representative of those with more simple structure (e.g. hookworm) and others with more complicated structure (e.g., Hymenolepis diminuta), and some just beginning development (Ascaris lumbricoides) and other with complete embryos (Schistosoma mansoni). All of the structures identified in the plate may not be evident in eggs found in a fecal preparation. Usually, in specimens examined or preserved immediately after passage there are sufficient features for the observer to make an identification. In some cases, closely related species have eggs that appear identical (e.g., Taenia solium and T. saginata). Since the eggs of these two species cannot be readily differentiated, characteristics of the gravid proglottid or scolex are used to identify the species present. Other members of the family Taeniidae produce similar eggs (members of the genera Echinococcus and Multiceps) but man is not infected by the adult stage and therefore eggs are not found in human feces.

 INTRODUCTION TO THE KEY TO HELMINTH EGGS FOUND IN FECES

In developing the key, the author has relied on many reference sources for basic information. Those eggs that are most commonly seen in fecal specimens have been included. Some species, such as Capillaria philippinensis and Ternidens deminutus, seen only in limited geographic areas, have been included because of the ease of travel from those areas. Other species, such as Dicrocoelium dendriticum and the free-living Heterodera sp., seen as spurious infections, have been included also. Others that are rarely seen, for example Capillaria hepatica and Schistosoma mekongi, have been omitted.

It was necessary to rely on various literature sources to arrive at the outer extremes of the sizes of eggs used in the Key. Although some of these have been confirmed using specimens available, confirming all size ranges was not possible. In a few instances, sizes of eggs consistently exceeded reports in the literature and the size range was changed to reflect these differences. The author would welcome comments based on actual measurements of eggs from properly identified species that vary from those given in the Key.

In addition to those species of helminths infecting man, eggs of mites, insects, helminths of animals, and free-living helminths may be accidentally ingested, move through the gastrointestinal tract, and pass with the feces. The presence of such unusual eggs in fecal specimens adds to difficulty in making accurate and positive identifications. Eggs in spurious infections are usually present only for 1 or at the most 3 days and would not be found in subsequent fecal specimens, which further substantiates the importance of collecting several specimens on alternate days from each patient being examined. Obviously, it is impractical to include in the Key all eggs that might be found in fecal specimens.

There are some places in the Key where one may have some difficulty in making a decision. For example, in deciding if a small operculate egg, 35 m or less, has a miracidium with symmetrical or asymmetrical organs, particularly cephalic (cytolytic) glands may prove problematic. Anyone, even an experienced parasitologist, may have some trouble with this decision. It will be necessary to use high magnification (1000x), and even turn the egg around to make the determination.

The most recent treatment is the same for many of the trematodes. Those producing small operculate eggs, e.g., Clonorchis sinensis, Opisthorchis viverinni, infecting the liver and Heterophyes heterophyes infecting the intestine can be treated similarly. The pathologic effects on the host are different for different species, so a specific diagnosis should be made when possible.

Regardless of some difficulties, the Key does provide a practical, effective approach to the identification of helminth eggs that are found in fecal specimens but not readily recognized, particularly when supported by photomicrographs and drawings. Those who have used the Helminth Egg Key have found it helpful.

Footnotes are indicated by a number in brackets at the appropriate place in the Key.

 KEY TO HELMINTH EGGS FOUND IN FECES

1. a. Egg with operculum, sometimes inconspicious 2
  b. Egg without operculum 13
2. a. Small egg 35 m or less; containing a larva when passed 3
  b. Larger egg, greater than 35 m; with or without developed larva 7
3. a. Eggs with opercular shoulders; when passed, organs of larva are not obviously symmetrical but are asymmetrical 4
  b. Eggs without opercular shoulders; when passed, organs of larva are, particular cephalic glands, are obviously symmetrical 6
4. a. Egg ovoidal (light-bulb-shaped), with pronounced shoulders: narrow, raised operculum; abopercular end with a button-hook projection, sometimes incomplete; 27 to 35 x 12 to 20 m; in feces or duodenal drainage = Clonorchis sinensis
  b. Egg with pronounced shoulders; abopercular end with or without raise papillae or knob 5
5. a. Egg ovoidal (light-bulb-shaped); with a slightly raised, relatively broad operculum; abopecular end with or without prominent papilla; 23 to 33 x 12 to 20 m; from feces or duodenal drainage = Opisthorchis viverrini
  b. Egg more slender and tapering from center to both ends; papillae on abopecular end, if present, small; average size 30 x 20 m; in feces or duodenal drainage = Opisthorchis felineus
6. a. Egg without distinct shoulders; broadest below the middle; slight or no thickening at abopercular end; 24 to 28 x 115 to 17 m; in
feces
= Metagonimus yokogawai
  b. Egg without distinct shoulders; broadest at middle; slight or no thickening at abopecular end; 26 to 30 x 15 to 17 m; in feces = Heterophyes heterophyes
7. a. Egg very large; over 130 m in length 8
  b. Egg less than 125 m in length 10
8. a. Egg oval in shape 9
  b. Egg rhomboid, widest in the middle and tapering toward both ends; 150 to 170 x 60 to 70 m; in feces = Gastrodiscoides hominis
9. a. Egg 130 to 145 x 60 to 75 m; with yoke granules evenly distributed throughout yoke cells; in feces [1] = Fasciolopsis buski
  b. Egg 130 to 155 x 65 to 90 m; with yoke granules concentrated around nuclei of yoke cells; in feces or from duodenal drainage [1] = Fasciola hepatica
10. a. Egg less than 50 m and containing a larva when passed; with a thick shell usually more rounded on one side; 35 to 45 x 22 to 30 m; in feces or from duodenal drainage; usually seen as a spurious infection = Dicrocoelium dendriticum
  b. Egg over 50 um and not containing a larva when passed 11
11. a. Egg with flattened operculum and pronounced shoulders; abopercular end of shell thickened; 75 to 120 x 45 to 65 m; in sputum; or in feces in 1/3 to of the cases = Paragonimus westermani
  b. Egg without shoulders, operculum rounded and often indistinct 12
12. a. Egg relatively thin-shelled; oval in shape with a narrow operculum, sometimes difficult to see; 85 to 115 x 45 to 65 m; in feces = Echinostoma ilocanum
  b. Egg relatively thick-shelled; broadly barrel-shaped with a relatively
broad operculum often difficult to see; abopercular end of shell with a
button-like thickening slightly off center; 55 to 75 x 38 to 55 m; in feces
= Diphyllobothrium latum
13. a. Egg contains a ciliated larva; has a conspicuous spine or minute knob, the knob often difficult to see 14
  b. Egg does not have a ciliated larva; without a spine or minute knob 17
14. a. Egg with a conspicuous spine 15
  b. Egg broadly oval, with a minute knob on one side near base of egg; often with a loose coating covering the shell;
70 to 108 x 55 to 80 m; in feces
= Schistosoma japonicum
15. a. Egg with a terminal spine 16
  b. Egg large, elongated with a lateral spine; 110 to 180 x 45 to 75 m; in feces, rarely urine = Schistosoma mansoni
16. a. Egg with terminal spine; 140 to 220 x 50 to 90 m; tapering slightly toward both ends and thickest in the center; in feces. [2] = Schistosoma intercalatum
  b. Egg with terminal spine; 100 to 170 x 50 to 80 m; narrowly ovoidal; usually in urine, rarely in feces [2] = Schistosoma haematobium
17. a. Egg fully embryonated; containing an embryo without cilia and with three pair of hooklets, sometimes difficult to see 18
  b. Egg either fully embryonated or not fully embryonated but never with hooklets 21
18. a. Egg with a single, thick, dark, radially pitted shell; spherical, 30 to 60 m; subspherical, 30 to 40 x 20 to 30; in feces = Taenia sp.
  b. Egg shell moderately thin without radial pitting; embryophore encasing hexacanth embryo separated from shell by a relatively large space 19
19. a. Egg single; embryophore with polar thickenings 20
  b. Eggs usually in packets of 10 to 25; single eggs without thickenings or filaments on the embryophore; shell thin and nearly transparent; spherical, 30 to 60 m; in feces = Dipylidium caninum
20. a. Egg oval with thin shell composed of two layers; embryophore with polar thickenings from which filaments extend into space beneath inner shell; diameter 30 to 60 m; in feces [3] = Hymenolepis nana
  b. Egg round to slightly oval; outer shell thicker and dark; embryophore occupies about one-third of space within shell, with polar thickenings but without polar filaments; diameter 65 to 85 m; in feces = Hymenolepis diminuta
21. a. Egg with a thick, dark shell 22
  b. Egg with a clear, transparent shell 26
22. a. Egg barrel-shaped; with Mucoid plug at each pole; with smooth shell 23
  b. Egg round to oval, without Mucoid plugs at the poles; shell covered with a rough mammillated coating 25
23. a. Egg small with flattened sides and blunt ends; shell with radial
striations, 36 to 45 x 19 to 22 m; in feces
= Capillaria philippinensis
  b. Egg tapering to both poles; shell without striations 24
24. a. Egg tapering from center toward both poles; 50 to 65 x 22 to 30 m; in feces = Trichuris trichiura
  b. Egg larger, 70 to 88 x 25 to 30 m, often slightly flattened at middle and tapering to both poles; in feces = Trichuris vulpis
25. a. Egg round to broadly oval; 45 to 75 x 35 to 50 m; in feces (normal fertile egg) = Ascaris lumbricoides
  b. Egg elongate, oval or rhomboidal; no organized embryo present; 88 to 95 x 40 to 45 m; in feces [4] = Ascaris (unfertilized)
26. a. Egg shell very thick; often seen in older preserved specimens or those where fixation has been delayed 27
  b. Egg shell thin 28
27. a. Egg contents not segmented beyond four cell stage;
43 to 68 x 33 to 48 m; in feces
= Ascaris (decorticated)
  b. Egg with larval stage inside; 48 to 52 x 32 to 36 m; in feces = Physaloptera caucasica
28. a. Egg flattened on one side 29
  b. Egg not flattened on one side 30
29. a. Egg contains a partially developed, rhabditoid juvenile; 50 to 65 x 20 to 30 m; relatively rare in feces; or with a fully developed juvenile, found in folds of anus = Enterobius vermicularis
  b. Egg bean-shaped with air spaces at poles; larva developed to morula
stage; 80 to 120 x 20 to 45 m; in feces
= Heterodera sp.
30. a. Egg with both poles rounded 31
  b. Egg with one pole more pointed than the other; embryo usually developed to the morula stage; 70 to 100 x 24 to 45 m; in feces = Trichostrongylus sp.
31. a. Egg developed to the four to eight cell stage; 56 to 70 x 35 to 50 m; in feces = Hookworm sp.
  b. Egg with embryo developed beyond eight cell stage 32
32. a. Egg with embryo from 16 cells to rhabditoid juvenile stage; 56 to 70 x 35 to 50 m; in feces [5] = Hookworm sp.
  b. Egg with embryo developed to eight cells or greater but not to juvenile stage; 75 to 85 x 46 to 55 m; in feces [6] = Ternidens deminutus




Plate 1

Plate 1: Diagrams of trematode eggs. The identification of each trematode egg is given along with
its average size, length, or diameter, in m. 

Number
Name
Length (m)
1
Metagonimus yokogawai
26
2
Heterophyes heterophyes
28
3
Clonorchis sinensis
31
4
Opisthorchis viverrini
28
5
Opisthorchis filenius
30
6
Taenia spp.
35
7
Dicrocoelium dendriticum
40
8
Capillaria philippinensis
40
9
Hymenolepis nana
45
10
Enterobius vermicularis
57
11
Trichuris trichiura
60
12
Hookworm spp.
63
13
Ascaris lumbricoides (fert)
57
14
Diphyllobothrium latum
65
15
Hymenolepis diminuta
75
16
Trichostrongylus sp.
85
17
Ascaris lumbricoides (unfert)
91
18
Paragonimus westermani
97
19
Schistosoma japonicum
89
20
Schistosoma mansoni
145
21
Fasciola hepatica
140




Plate 2

 
  Trematodes 1. Paragonimus westermani 2. Schistosoma mansoni 3. Echinostoma ilocanum
 
  Cestodes 4. Hymenolepis nana 5. Taenia sp. 6. Diphyllobothrium latum
 
  Nematodes 7. Hookworm 8. Trichuris trichiura 9. Ascaris lumbricoides
 

Plate 2: The structures seen in eggs representing each Helminth group. 





FOOTNOTES

[1] It is often impossible to distinguish between the latter two species by the examination of the eggs. Collecting bile fluid from the opening of the common bile duct may be necessary to confirm or eliminate Fasciola hepatica. A few cases of human infection with F. gigantica have been reported. The eggs are extremely large, 150 to 196 x 90 to 100 m and should be distinguishable from F. hepatica or Fasciolopsis buski.

[2] S. intercalatum infection in man has been reported from seven countries in Africa. It has a larger size range than S. haematobium (140 to 220 m in length). Since the size ranges overlap, it may be necessary to use means other than size to differentiate between the two species. In South Africa, other species with terminal spines, S. matthei (180 to 232 m in length) and S. bovis (180 to 232 m in length), are rarely seen in human feces. Human infection with these latter species is accidental and some reports may reflect spurious infections. The eggs of S. rodhaini, a species infecting rodents, having a subterminal spine, and a rounded knob at the opposite end, have been recovered from the feces and from tissues of humans in Africa.

[3] The shells of Hymenolepis spp. eggs have two parts; an outer dark covering that may be lost in fixed specimens exposing an inner, almost clear shell wall.

[4] Either fertile or infertile eggs may lose their cortex in the specimen. In such cases, the very thick clear shell helps differentiate Ascaris eggs from those of similar size.

[5] Hookworm eggs in human feces may represent either Ancylostoma sp. or Necator americanus. Differentiation must be made from adult or Filariform juvenile stage. The Filariform stage is obtained for identification by culturing the eggs or juvenile worms (Melvin and Brooke, 1982; Ash and Orihel, 1987). When hookworm eggs are delayed in moving through the intestine, they may develop to the juvenile stage. Some juvenile worms may emerge in feces before it is passed so, in addition to eggs, occasionally juvenile nematodes of hookworms are seen in the fecal specimen. The juvenile stages of Strongyloides stercoralis also may be present in fecal specimens where hookworm eggs are present. In such cases, the juveniles must be positively identified.

[6] Hookworm eggs are differentiated from Ternidens deminutus eggs only on the basis of size.




Dr. Donald L. Price Center for Parasite Repository and Education
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This page last updated June 9, 2004
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