TrematodaBy: Raja Sinha
Outline• Evolution and Systematics• Types of Trematodes• Behavior
– Metabolism, reproduction, life cycle• Geographic Distribution• Effect on Humans
– Diseases caused and economic harm• Summary
Evolution and Systematics• 2 subclasses
– Aspidogastrea and Digenea• 6000-18000 species
– Monogenea was considered a subclass• Paraphyletically grouped with mollusks and
annelids
• Trace Fossils
Aspidogastrea• 80 species• 1mm to several cm• Mostly parasitize mollusk, turtles, and
cartilaginous fishes• Less complicated life cycle
Fig. 1. Shared characteristics of the group are a large ventral disc with a largenumber of small alveoli (Rhodes 1972).
Fig. 2. Growth of ventral disc of Multicotyle purvisi (Rhodes1972).
Digenea• 4000-5000 species• Most are monoecious• Complex reproductive system• Very complicated life cycle
– Terminology
Fig. 3. A picture of a digenetic trematode.
Ecological Distribution• Found globally• Host species is the limiting factor• Prevalent in Asia and Africa
– Dung et al. 2007
Terminology• Miracidium - the free-swimming ciliated first larva of a
digenetic trematode that seeks out and penetrates a suitablesnail
• Sporocysts – A saclike larval stage in many trematodeworms.
• Rediae - A larva of certain trematodes that is producedwithin the sporocyst
• Metacercariae - The encysted maturing stage of a trematodein its intermediate host prior to transfer to the definitive host
• Cercariae - The parasitic larva of a trematode worm, havinga tail that disappears in the adult stage
Types of Trematodes
Fig. 4. List of Trematodes species. (Mertz 2005).
Fasciola hepatica
Fig. 5. The general life cycle of the Fasciola species (USAF 2004).
Human Blood Fluke(Schistosoma mansoni)
Fig. 6. The general life cycle of Schistosoma species (USAF 2004).
Schistosomiasis• Description• Causes• Symptoms• Diagnosis• Treatments• Prevention
Experiments• Bet-Hedging Hypothesis (Thomas and
Poulin 2003)– species reproducing mainly by self-
fertilization should produce smaller eggsthan those species that regularly practicecross-fertilization
– Inconclusive results
Changing environment canmake sex beneficial• Lively, C. 1992
– Potamopyrgus antipodarum– Trematodes castrate snails– Variations in populations leads to
resistance to infection– Selection favors trematodes that
reproduce sexually
References• Dung, D.T.; N. De; J, Waikagul; A. Dalsgaard; J. Chai; W. Sohn;
and K. Murrell. 2007. Fishborne Zoonotic Intestinal Trematodes,Vietnam. Emerging infectious disease. 13: 1828-1833.
• Freeman S. and J.C. Herron. 2007. Mechanism of EvloutionaryChange. Evolutionary Analysis. 4: 310-313.
• Mertz, L. 2005. Trematodes. Lower Metazoans and LesserDeuterostomes. 2: 197-211.
• Rhodes, K. 1972. Aspidogastrea, especially Multicoytle purivisi.Advances in parasitology. 10: 77-151.
• Thomas F and R. Poulin. 2003. Egg size variability in trematodes: testof the bet-hedging hypothesis. Journal of Parasitiology. 89: 1159-1162.
• USAF. 2004. Chapter 6: The Trematodes. Accessed online athttp://www.phsource.us/PH/PARA/Chapter_6.htm
