Diagnosing the Trematodes

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The page below is a sample from the LabCE course Intestinal Worms. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Diagnosing the Trematodes

None of the fluke infections are endemic in the US except for Nanophyetus salmoncola, which is occasionally transmitted by salmonid fish in the Pacific Northwest. However, part of a patient's history may indicate residence or travel in one of the endemic countries. Thus, it is possible to diagnose fluke infections in the US and the ability to identify their eggs is necessary for a definitive diagnosis. Keep in mind that although not all of the trematodes covered in this course live in the intestines, they can all be diagnosed by finding their eggs in human stool specimens. One thing noticeable on many fluke eggs that are not seen in other types of worm eggs is an operculum, which is a little "lid" at the one end of the worm. Occasionally it can be flipped up or missing.
Table 2 shows diagnostic features of the flukes that live in the intestinal tract and biliary system, along with their typical geographic locations and means of how they are acquired. The following page will have charts of the flukes living in the lungs and veins.
Table 2. Flukes Living in the Intestinal Tract and Liver/Biliary System.
Species and DescriptionImageGeographic LocationMeans of Infection
Fasciolopsis buski
"Giant intestinal fluke"
130-150 x 60-90 µm
It is morphologically indistinguishable from Fasciola hepatica.
South and Southeastern AsiaConsuming metacercariae on uncooked acquatic vegetation
Heterophyes and Metagonimus
Similar to Clonorchis eggs; mostly indistinguishable from them Egypt, Middle East and Far East, Siberia, Balkan states, Israel, SpainConsuming metacercariae on undercooked fish
Nanophyetus salmoncola
87-97 x 38-55 x µm
Similar to F. hepatica and F. buski, but smaller.
(9) Pacific Northwest USConsuming metacercariae on undercooked salmon
Fasciola hepatica
Indistinguishable from Fasciolopsis eggs
130-150 x 60-90 µm.
This image has an open operculum, but that is not necessary for identification.
(10) Sheep and cattle raising areas of more than 70 countries, including parts of Europe, Latin America, Asia, AfricaConsuming metacercariae on vegetation
Clonorchis and Opistorchis
~27-35 x 11-20 µm
Note opercular shoulders and a slight knob at the opposite end.
(11) East Asia into far eastern RussiaConsuming metacercariae on undercooked freshwater fish
8. DPDx. "Fasciolopsiasis - Figure A: Egg of F. buski in an unstained wet mount."CDC.gov, 8 Dec 2017, https://www.cdc.gov/dpdx/fasciolopsiasis/index.html
9. DPDx. "Stool Specimens - Intestinal Parasites: Comparative Morphology Tables - Figure 5." CDC.gov, 3 May 2016, https://www.cdc.gov/dpdx/diagnosticprocedures/stool/morphcomp.html
10. DPDx. "Fascioliasis - Figure A: Egg of F. hepatica in an unstained wet mount, taken at 400x magnification."CDC.gov, 2 May 2019, https://www.cdc.gov/dpdx/fascioliasis/index.html
11. DPDx. "Clonorchiasis - Figure B: C. sinensis egg. Note the operculum resting on "shoulders;" image taken at 400× magnification."CDC.gov, 6 Jun 2019, https://www.cdc.gov/dpdx/clonorchiasis/index.html