Biology 625
Fall semester lecture note outline

Updated: 09 August 2001

The text below simply represents a crude lecture outline of one of the topics covered in class. It is not meant to substitute for attending lectures or ignoring the textbook. Additional material, including line drawings, kodachromes, and more extensive information on life-cycles and basic biology, will be supplied in the lectures.

TOPIC #7: Order: Echinostomatiformes

  1. infect all classes of vertebrate
  2. many species have no second intermediate host and metacercariae encyst on vegetation
  3. adults often fail to resemble one another. However, developmental biology clearly reveals common ancestry
  4. adults often, but not always, with spined or scaled tegument (especially anteriorly)
  5. acetabulum usually very anterior in location
Fascioloides magna (family: Fasciolidae)
  1. Common names are Giant liver fluke; American liver fluke; Deer fluke
  2. Adults large, up to 8 cm long
  3. Cephalic cone absent in adults
  4. Large eggs passed in feces unembryonated, measuring 114-168 x 94-96 micrometers
  5. Life-cycle
    1. Adults occur in pairs or small groups within fibrous capsules in liver parenchyma
    2. Adults may release 4,000 eggs per day, released into bile ducts
    3. Once passed in the feces, eggs generally take about 30-35 days to embryonate in summer, although temperature greatly affects embryonation
    4. Ciliated miricidia hatch and are viable for 1-2 days
      1. positive phototaxis
      2. strong chemotaxis for gastropod mucus
    5. Penetrate suitable snails, especially Lymnaea spp., each forming a sporocyst within the pulmonary sac
    6. Each sporocyst gives rise up to 14 rediae
    7. Rediae migrate to liver of snail and produce up to 9 daughter rediae
    8. Each redia gives rise to 9-10 cercariae
    9. Thus, reproductive potential approximately 1:1,000 over a period of 40-60 days, temperature dependent. Development may be delayed and asexual stages can overwinter in snails
    10. Cercariae exit snail and encyst on underwater vegetation
    11. Once ingested, metacercariae become active, penetrate intestinal wall, migrate along ventral surface of peritoneal wall, and eventually reaching and penetrating liver
    12. Migrate through liver tissue and search for other flukes
    13. Encapsulate in groups of 2-more in hepatic parenchyma and mature; maturation usually does not occur until another fluke is found and takes 3-7 months
    14. Capsules composed primarily of collagen and of host origin, apparently an attempt by the host to limit migration of the flukes
    15. Adults may live 5 or more years and produce 4,000 eggs per day
  6. Hosts and Epidemiology
    1. Occur in numerous wild and domestic ruminants, especially in North America. Has been accidentally introduced into spotty populations in Europe with introduction of elk and deer from North America
    2. Origin in North America. In 5 major areas in North America where abundance is highly varied:
      1. Great Lakes area, in both the U.S. and Canada
      2. Pacific Northwest
      3. Northern Quebec and Labrador
      4. Rocky Mountain trench (Montana, B.C. and Saskatchewan)
      5. Numerous spotty areas in Southeast U.S.
    3. Natural infections occur in cervids and bovids. However, the parasite develops well only in select hosts
      1. Definitive hosts (Adults mature, live in thin-walled fibrous capsules within the liver, and eggs easily liberated in to the bile ducts). Primarily new world cervids (Elk, caribou, Mule deer, White-tail deer, Black-tail deer, Red deer, Fallow deer). Experimental infections suggest that some white-tailed deer, perhaps 20-25%, my be refractory to infection. Migration through liver limited, and fibrous capsules containing worms are thin-walled
      2. Aberrant hosts (Parasites cannot successfully establish successful infections). Includes sheep, goats, roe deer; bighorn sheep easily killed. Hosts often die of acute tissue damage due to excessive migration of flukes. Damage usually focused on liver, although perforation of hepatic capsule and even other organs may occur
      3. Dead-end hosts (Flukes successfully reach liver but only rarely mature. If they mature, few eggs are produced and they often do not reach the intestine). Includes moose, cattle, bison, yak, horse, pigs, peccary, llama, Sika deer. Excessive fibrosis and thick-capsule walls, resulting in the eggs not exiting. Flukes often fail to pair and undergo extensive migration throughout liver.
    4. Prevalence of flukes increases with host age; young are rarely infected.
      1. Most hosts have few worms. Mean intensity of infection is usually less than 10 worms
      2. Infections of 20-125 flukes sometimes seen
      3. A few elk have been found with over 500 adult flukes
      4. Older animals tend to have a few more worms in each capsule, suggesting new flukes may be able to invade previously established capsules
    5. Spotty distribution of the parasite in cervids is thought to be due to the widespread decline of white-tailed deer since the 16th century, followed by reduced numbers of other cervids. This may have reduced the geographic distribution to refuge populations
    6. Original host thought to be white-tailed deer, where only 60-70% of a population usually becomes infected with only 5-10 flukes per host. The fibrous capsules are thin-walled, and eggs are easily released. Intensity and prevalences tend to be higher in elk, more eggs are produced, and more pathology is seen
    7. Translocation of the parasite to other geographic localites in other countries tends to be through elk where infections are more intense and more eggs are produced
    8. Domestic animals probably cannot maintain populations of the fluke in the absence of cervids since few or no eggs are liberated into the environment
  7. Pathology
    1. Most infections subclinical
    2. In aberrant hosts, symptoms include weight loss, lethargy, anorexia, and depression
    3. Infected livers tend to be enlarged due to migrating immature worms and may hypertrophy to 4-5 times normal size. Tracks of tissue damage and hemorrhage may be present, and fibrosis of livers
    4. White fibrous capsules containing worms are clearly evident
    5. Capsules compress adjacent parenchyma and result in hepatic pressure necrosis, atropy of bile ducts,
    6. High numbers of flukes may cause rupture of hepatic capsule and hepatic portal vein, resulting in death
    7. Heavy concurrent infections with high numbers of metacercariae, i.e. 2000 metacercariae, may die of peritonitis because of rupture of jejunal wall
  8. Diagnosis, Treatment and Management
    1. Large, unembryonated eggs in feces
    2. Triclabendazole has broad-spectrum activity against both adults and larval stages. Treated bait is effective in some areas to control flukes in wild populations
    3. Rafoxanide kills immature flukes
    4. Oxyclozanide kills adults fluke
    5. Separation of cervids and livestock to prevent domestic animals from being heavily infected. Most infections in domestic animals occur as animals graze in contaminated wetlands
    6. Removal of snails from buffer zones between cervids and livestock, which may include use of copper sulphate to kill snails or draining of some non-essential wetlands
    7. Translocation of infected cervids from endemic areas into non-endemic areas should not be allowed

Fasciola hepatica (family: Fasciolidae)

  1. large, leaf-shaped, with cephalic cone
  2. mainly in herbivores
  3. intestinal cecae, testes, and ovary dendritic
  4. life-cycle
    1. adults in gall bladder, bile ducts
    2. eggs out with feces, unembryonated
    3. develop in 9-10 days
    4. hatch
    5. miricidia penetrate several species of snails
    6. sporocyst; two redial generations
    7. cercariae emerge 5-7 wk post-infection
    8. encyst as metacercariae on underwater vegetation
    9. ingested; cross gut and penetrate liver (glycocholic acid in bile the stimulus; migration cue)
    10. feed in liver for 2 months, then enter bile ducts
    11. after another month, mature and produce eggs
  5. pathology
    1. inflammation and erosion of bile ducts
    2. tissue destruction of liver
    3. fibrosis of liver and bile duct walls
    4. back pressure in liver, leading to cirrhosis and jaundice
    5. blockage of bile ducts
    6. abscesses in liver
    7. migrating juveniles may cause ectopic abscesses in lungs, brain, skin, eye
    8. halzoun - adults attach in nasopharynx after eating raw liver (middle east)
  6. Fascioliasis (Fasciola hepatica and Facioloides magna) in Kansas (1995, Vet Parasitol. 56: 281-291)
  7. related species
    1. Fasciola gigantica (found in a variety of Artiodactylids in Africa, India, portions of Europe, Indonesia, Asia, and Hawaii. The most common infections occur in cattle, sheep, and goats. Patent infections in humans also occur). Hybrids between this species and Fasciola hepatica have been reported.
    2. Fasciola jacksoni (nasty pathology in Asian elephants)
    3. Fasciolopsis buski (swine and humans in Asia)

Echinostoma spp. (family: Echinostomatidae)

  1. Tend to be relatively non-host specific in semi-aquatic vertebrates (multiple species in humans too)
  2. Many species erroneous/confused with one another in the literature
  3. review: 1990, Adv Parasitol 29: 215-269; 1996, Adv Parasitol 38: 311-368.
  4. Many species elongate; anterior sucker and large acetabulum anterior
  5. 27-51 circumoral collar of spines, depending upon species
  6. life-cycle
    1. adults in gut
    2. eggs passed in feces
    3. hatch; miricidia penetrate snails
    4. for instance, sporocyst; two redia
    5. cercariae
    6. metacercariae in molluscs, planaria, fish, tadpoles, etc.
    7. eaten by definitive host
  7. typical species
    1. Echinostoma caproni (37-collar spined; in Biomphalaria and Bulinus snails; both mammalian and avian hosts; Africa)
    2. Echinostoma hortense (27-28-collar spined)
    3. Echinostoma malayanum (43-collar spined)
    4. Echinostoma paraensei (37-collar spined)
    5. Echinostoma trivolvus (37-collar spined; in Helisoma trivolvis snail host; both mammalian and avian hosts; North America)
    6. Echinostoma revolutum (37 collar spined; in lymnaeid snails; mammals and birds; Europe and Asia)
    7. Euparyphium (Echinostoma) ilocanum (49-51 collar spined)

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