Parasitology 2nd exam

• Facultative or obligate parasites of molluscs, fish, turtles
• Longitudinal septum – divides body into dorsal and ventral compartments
• Opisthaptor - ventral sucker
• Platyhelminthes


  • multicellular acoelomates
  • bilateral symmetry
  • flattened form to maximize S/V ratio
  • both free-living and parasitic forms
  • 1-opening digestive system, usually highly branched
  • no circulatory/respiratory system; direct diffusion of gases
  • nervous system with some cephalization: anterior ganglia, 2 or more nerve cords, often lateral, sensory structures: chemoreceptors, tactile/pressure receptors, visual receptors – eyecups, eyespots
  • excretory system – flame cells (protonephridia) and ducts
  • reproductive system: monoecious – hermaphrodites, dioecious – separate sexes
• Types of adult flukes

  • monostome
  • amphistome
  • distome
  • echinostome
  • strigeoid
  • schistosome
• Flukes of interest as human (and other) parasites

  • Liver flukes – Clonorchis sinensis, Opistorchis viverrini, Fasciola hepatica
  • Lung flukes – Paragonimus westermani
  • Intestinal flukes – Heterophyes heterophyes, Fasciolopsis buski, Gasterodiscoides hominis, Metagonimus sp.
  • Others – Dicrocoelium dendriticum, echinostome flukes, Alaria sp.
  • Blood flukes – Schistosoma mansoni, S. haematobium, S. japonicum
• General life cycle of Digenea flukes

  . polyembryony at each stage in 1st intermediate host

  3. daughter sporocyst and daughter redia stages may also be present

  4. redia stage or sporocyst stage sometimes absent

  5. definitive host takes in cercaria or metacercaria stage
• Cause inflammation of bile ducts, pipestem fibrosis, liver tissue damage; eggs in liver become granulomas
• Females can produce up to 4000 eggs/day, live 25-30 years
• Very widespread in the Far East; 15 million reported infected as of 2020
• May stimulate formation of gallstones, implicated in liver cancer
• Sheep/deer/beef liver flukes

  • Fasciola, Fascioloides
• Other liver flukes

  • Opisthorchis viverrini, O. felineus, Fasciola, Fascioloides
• Regions of liver flukes

  • Southeast Asian liver fluke - Opisthorchis viverrini
  • Europe and Asia liver fluke - O. felineus
  • Sheep/deer/beef liver flukes - Fasciola, Fascioloides
• Fasciola hepatica


  • Large, ~ 30 mm long, has "shoulders" behind a distinct oral cone
  • Large acetabulum about at shoulders
  • In bile ducts, feed on the lining
  • Cause inflammation, pipestem fibrosis with cirrhosis, sometimes abscesses, jaundice
• Lung fluke life cycle

  1. Occur in pairs in the lungs, usually become encapsulated; can live up to 20 years

  2. Eggs are released and are coughed up or swallowed

  3. Crustaceans are 2nd intermediate hosts

  4. Crustacean-eating mammals and birds may be paratenic hosts
• Liver cancer implicated parasites

  • Opisthorchis viverrini
  • O. felineus
  • Fasciola
  • Fascioloides
  • Fasciolopsis buski
  • Gasterodiscoides hominis
  • Metagonimus yokogawai
• Fasciola hepatica is known since the 14th century, very common in livestock, also infects humans
• Lung flukes

  • Paragonimus westermani
• Intestinal flukes

  • Heterophyes
  • Fasciolopsis buski
  • Gasterodiscoides hominis
  • Metagonimus yokogawai
• Heterophyes life cycle

  1. Adults in small intestine, between villi

  2. Heavy infections damage intestinal lining, cause diarrhea, can perforate mucosa

  3. Cercariae with eyespots, metacercariae in fish (2nd intermediate hosts)

  4. Fish-eating mammals may serve as reservoirs
• Fasciolopsis buski


  • Up to 75 mm long, oval, no "shoulders"
  • Common in the Far East, India, southeast Asia
  • Adults can produce up to 25,000 eggs/day
  • Cause mucus diarrhea, can block intestine and/or damage intestinal lining
• Gasterodiscoides hominis

  • Amphistome fluke with round posterior section; up to ~15 mm long
  • Adults attached to the cecum and colon; heavy infections may cause diarrhea, pain, reactions to eggs in other organs
  • Humans are accidental hosts; pigs and other mammals are definitive hosts
• Metagonimus yokogawai

  • Similar to Heterophyes, up to 2.5 mm long, in small intestine
  • Fish-eating mammals are also reservoirs
  • Heavy infection causes diarrhea, pain
• Parts of the Mediterranean basin

  • Heterophyes, C. sinensis, Dicrocoelium dentriticum, Echinostoma spp., Echinoparyphium recurvatum, Alaria americana, Schistosoma (S. mansoni, S. haematobium, S. japonicum)
• Heterophyes

  • Up to 2.5 mm long, found in the small intestine, life cycle similar to C. sinensis with fish as 2nd intermediate hosts, eggs may migrate causing inflammation, heavy infection causes diarrhea and pain
• Other flukes

  • Humans are incidental hosts for Dicrocoelium dentriticum in livestock, Echinostoma spp. in waterfowl and semi-aquatic vertebrates, echinostomes are common human parasites in the Far East
• Treatment for flukes includes Praziquantel as the drug of choice, Mebendazole, Triclabendazole for fascioliasis, Albendazole, surgical removal under certain circumstances
• Detection of eggs for diagnosis can be done in stool samples for various flukes including Schistosoma, Clonorchis, Opisthorchis, Heterophyes, Metagonimus, Fasciola, Fasciolopsis, Paragonimus, in sputum for Paragonimus, and in urine samples for Schistosoma haematobium
• Monogenean flukes

  • External parasites of fish, found on skin, gills, or in bladder, have prohaptor and haptor with hooks, suckers, and/or clamps, feed on host skin/epidermis and blood, dioecious, oviparous, some viviparous
• Monogeneans

  • Cleidodiscus opisthaptor
• Monogeneans have a direct life cycle and can be found on the gills of fish
• Liver flukes

  Clonorchis sinensis, Opistorchis viverrini, Fasciola hepatica
• Taenia solium vs Taenia saginata

  • solium has rostellum with hooks, saginata has neither
  • saginata can grow up to 75' with ~1000 proglottids, solium averages 10' with ~900 proglottids
  • saginata has a higher branched uterus, solium has a less branched uterus
  • Saginata= beef , Solium= pork
• General life cycle of Digenea flukes


  1. egg -> miricidium -> sporocyst -> redia -> cercaria -> (metacercaria) -> adult