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Common Freshwater Fish Parasites Pictorial Guide: Digenean Trematodes

Deborah B. Pouder, Eric W. Curtis, and Roy P.E. Yanong

Introduction

This publication is one in a series of pictorial guides that is designed to assist in the identification of common freshwater fish parasites. The publications included in this series are:

  • Common Freshwater Fish Parasites Pictorial Guide: Sessile Ciliates
  • Common Freshwater Fish Parasites Pictorial Guide: Motile Ciliates
  • Common Freshwater Fish Parasites Pictorial Guide: Dinoflagellates, Coccidia, Microsporidians, & Myxozoans
  • Common Freshwater Fish Parasites Pictorial Guide: Monogeneans
  • Common Freshwater Fish Parasites Pictorial Guide: Digenean Trematodes
  • Common Freshwater Fish Parasites Pictorial Guide: Nematodes
  • Common Freshwater Fish Parasites Pictorial Guide: Acanthocephalans, Cestodes, Leeches, & Pentastomes
  • Common Freshwater Fish Parasites Pictorial Guide: Crustaceans

The information provided in this guide is not intended to be a complete, detailed description of each parasite or parasite group and its characteristics but rather is intended to assist in the visual identification of some of the most common species or groups of parasites seen in freshwater fish. For further information on each parasite, refer to publications in the "Recommended Reading" and "Reference" sections below.

Guide Information

  • Target Tissue: provides the location on/in the fish where the parasite is most commonly found.
  • Characteristic: provides a brief description about the appearance of the parasite.
  • Size: provides the size or size range of the parasite. (1 µm = 0.001 mm = 0.0001 cm) (µm = micron or micrometer; mm = millimeter; cm = centimeter)
  • Movement: provides the type of movement, if any, of the parasite.
  • Note: provides a brief comment of interest about the parasite.

Digeneans

("Digenes", "White Grub", "Yellow Grub", or "Black Grub")

Digenean metacercariae encysted on swordtail fin and skin

Digenean metacercariae encysted in gills

Digenean metacercariae encysted in liver

Digenean metacercariae encysted in mesentery near gall bladder

Adult digenean

Digenean removed from cyst in muscle

Target Tissues: Metacercariae (juvenile encysted stage) in any internal or external tissue (if fish are intermediate host); adults usually in gut lumen (if fish are final host)

Appearance: Metacercariae seen as nodules/masses approx. 1–4 mm in length, adults approx. 1–5 mm in length; adults and juveniles have visible oral and ventral suckers but no segments or chitinous hooks

Movement: Adults free-moving; may see movement inside metacercariae cyst

Note: Indirect life cycle often involving bird, snail, and fish; fish usually intermediate host; if in cyst, burst cyst to "free" individual for identification

Recommended Reading

SRAC Publication No. 410 Calculating Treatments for Ponds and Tanks. Southern Regional Aquaculture Center. https://fisheries.tamu.edu/files/2013/09/SRAC-Publication-No.-0410-Calculating-Treatments-for-Ponds-and-Tanks.pdf

SRAC Publication No. 475 Proliferative Gill Disease (Hamburger Gill Disease). Southern Regional Aquaculture Center. https://fisheries.tamu.edu/files/2019/01/SRAC_0475.pdf

SRAC Publication No. 4701 Protozoan Parasites. Southern Regional Aquaculture Center. https://agrilife.org/fisheries2/files/2013/09/SRAC-Publication-No.-4701-Protozoan-Parasites.pdf

UF/IFAS Circular 91 Nematode (Roundworm) Infections in Fish. https://edis.ifas.ufl.edu/FA091

UF/IFAS Circular 120 Fish Health Management Considerations in Recirculating Aquaculture Systems - Part 1: Introduction and General Principles. https://edis.ifas.ufl.edu/FA099

UF/IFAS Circular 121 Fish Health Management Considerations in Recirculating Aquaculture Systems - Part 2: Pathogens. https://edis.ifas.ufl.edu/FA100

UF/IFAS Circular 122 Fish Health Management Considerations in Recirculating Aquaculture Systems - Part 3: General Recommendations and Problem Solving Approaches. https://edis.ifas.ufl.edu/FA101

UF/IFAS Circular 716 Introduction to Freshwater Fish Parasites. https://edis.ifas.ufl.edu/FA041

UF/IFAS Circular 919 Stress--It's Role in Fish Disease. https://edis.ifas.ufl.edu/FA005

UF/IFAS Circular 920 Ichthyophthirius multifiliis (White Spot) Infections in Fish. https://edis.ifas.ufl.edu/FA006

UF/IFAS Circular 921 Introduction to Fish Health Management. https://edis.ifas.ufl.edu/FA004

UF/IFAS Fact Sheet FA-13 Use of Copper in Freshwater Aquaculture and Farm Ponds. https://edis.ifas.ufl.edu/FA008

UF/IFAS Fact Sheet FA-23 The Use of Potassium Permanganate in Fish Ponds. https://edis.ifas.ufl.edu/FA032

UF/IFAS Fact Sheet FA-28 Monogenean Parasites of Fish. https://edis.ifas.ufl.edu/FA033

UF/IFAS Fact Sheet FA-37 Use of Potassium Permanganate to Control External Infections of Ornamental Fish. https://edis.ifas.ufl.edu/FA027

UF/IFAS Fact Sheet FA-55 Submission of Fish for Diagnostic Evaluation. https://edis.ifas.ufl.edu/FA055

UF/IFAS Fact Sheet FA-90 Pentastomid Infections in Fish. https://edis.ifas.ufl.edu/FA090

UF/IFAS Fact Sheet FA-107 Common Freshwater Fish Parasites Pictorial Guide: Sessile Ciliates. https://edis.ifas.ufl.edu/FA107

UF/IFAS Fact Sheet FA-108 Common Freshwater Fish Parasites Pictorial Guide: Motile Ciliates. https://edis.ifas.ufl.edu/FA108

UF/IFAS Fact Sheet FA-109 Common Freshwater Fish Parasites Pictorial Guide: Flagellates. https://edis.ifas.ufl.edu/FA109

UF/IFAS Fact Sheet FA-110 Common Freshwater Fish Parasites Pictorial Guide: Dinoflagellates, Coccidia, Microsporidians, and Myxozoans. https://edis.ifas.ufl.edu/FA110

UF/IFAS Fact Sheet FA-111 Common Freshwater Fish Parasites Pictorial Guide: Monogeneans. https://edis.ifas.ufl.edu/FA111

UF/IFAS Fact Sheet FA-113 Common Freshwater Fish Parasites Pictorial Guide: Nematodes. https://edis.ifas.ufl.edu/FA113

UF/IFAS Fact Sheet FA-114 Common Freshwater Fish Parasites Pictorial Guide: Acanthocephalans, Cestodes, Leeches, and Pentastomes. https://edis.ifas.ufl.edu/FA114

UF/IFAS Fact Sheet FA-115 Common Freshwater Fish Parasites Pictorial Guide: Crustaceans. https://edis.ifas.ufl.edu/FA115

UF/IFAS Fact Sheet VM-67 Management of Hexamita in Ornamental Cichlids. https://edis.ifas.ufl.edu/VM053

UF/IFAS Fact Sheet VM-77 Use of Formalin to Control Fish Parasites. https://edis.ifas.ufl.edu/VM061

UF/IFAS Fact Sheet VM-78 Bath Treatment for Sick Fish. https://edis.ifas.ufl.edu/VM037

UF/IFAS Fact Sheet VM-85 "Red Sore Disease" in Game Fish. https://edis.ifas.ufl.edu/VM059

UF/IFAS Fact Sheet VM-86 Use of Salt in Aquaculture. https://edis.ifas.ufl.edu/VM007

UF/IFAS Fact Sheet VM-87 Sanitation Practices for Aquaculture Facilities. https://edis.ifas.ufl.edu/AE081

UF/IFAS Fact Sheet VM-104 Cryptobia iubilans in Cichlids. https://edis.ifas.ufl.edu/VM077

References

Hoffman, G. L. 1999. Parasites of North American freshwater fishes. Cornell University Press, Ithaca, NY.

Longshaw, M. and S. W. Feist. 2001. Parasitic diseases. Pages 167–183 in W.H. Wildgoose, editor. BSAVA manual of ornamental fish, second edition. British Small Animal Veterinary Association, Gloucester, England.

Noga, E. J. 1996. Fish disease: diagnosis and treatment. Mosby-Yearbook, Inc., St. Louis, MO.

Stoskopf, M. K. 1993. Fish medicine. W.B. Saunders Company, Philadelphia, PA.

Woo, P. T. K., editor. 1995. Fish diseases and disorders, volume 1: protozoan and metazoan infections. CAB International, Wallingford, United Kingdom.

Publication #FA-112

Release Date:March 11, 2020

Reviewed At:February 12, 2024

Related Experts

Yanong, Roy P.

Specialist/SSA/RSA

University of Florida

Pouder, Deborah B

staff

University of Florida

Fact Sheet

About this Publication

This document is FA-112, one of a series of the School of Forest, Fisheries, and Geomatics Sciences, Program in Fisheries and Aquatic Sciences, UF/IFAS Extension. Original publication date July 2005. Visit the EDIS website at https://edis.ifas.ufl.edu for the currently supported version of this publication.

About the Authors

Deborah B. Pouder, coordinator of research programs and services; Eric W. Curtis, former biological scientist; and Roy P.E. Yanong, professor; Tropical Aquaculture Laboratory, School of Forest, Fisheries, and Geomatics Sciences, Program in Fisheries and Aquatic Sciences, UF/IFAS Extension, Gainesville, FL 32611.

Contacts

  • Deborah Pouder
  • Roy Yanong
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