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Endoparasites - Fasciola

Fasciola hepatica: Common liver fluke

Fasciola gigantica: Giant liver fluke

General Description: Fasciola hepatica adults are greyish-brown, leaf-shaped flukes up to 30mm by 13mm in size. Fasciola gigantica are 25 to 75mm by 5 to 12mm.

Life Cycle: Adult flukes in liver lay eggs in bile which carries them into the intestine. Eggs leave the host in faeces and development of larval stages begins. First, a ciliated larva called the miracidium develops inside the egg and hatches in about 2 weeks. Once free, it must enter an amphibious snail of the Lymnaea genus within 24 hours to continue developing. As with Fasciola hepatica, the indirect life cycle of Fasciola gigantica uses a snail, Lymnaea, as the intermediate host. However, these snails live in permanent rather than temporary bodies of water, preferring slow-moving, clear water that supports plant life. In the snail, polyembryony stages of sporocysts, rediae and cercariae occur. This greatly increases the possible number of adult flukes produced from one egg. Cercariae leave the snail about 5 to 6 weeks after the miracidium entered. Free on pasture, cercariae swim until reaching grass where they develop into dormant cyst forms known as metacercariae. When eaten by sheep, these are released from the cyst as immature flukes which penetrate the intestinal wall, and enter the liver. The immature flukes migrate through liver tissues for 5 to 6 weeks before settling in bile ducts where flukes mature in about 4 weeks. Total prepatent period is approximately 8 to 12 weeks.

Location: Bile ducts of liver.

Geographical Distribution: Fasciola hepatica may be found worldwide in wet areas where Lymnaea snails exist.

Significance: Common liver flukes are a worldwide problem in sheep raised in wet areas. They cause decreased growth, weight loss, and organ condemnation at slaughter.

Effects on Host: Migration of flukes in the liver damages tissue and causes scar tissue formation (fibrosis), disrupting normal liver function. Tissue destruction and bile duct irritation by the fluke's spines cause erosion of cells, which must be replaced by the host. Inability of cells to mature before they are eroded results in leakage of albumin into the bile system and intestine. Flukes ingest red blood cells. Demands on the host to replace lost albumin and blood cells result in decreased gain, anaemia and weight loss.

Diagnostic Information: Fasciola eggs are similar to those of strongyles but are larger and are capped on one end (operculum). The presence of such eggs in faeces indicates a fluke infection. Finding flukes in the liver at slaughter is diagnostic.

Control: Aquatic and amphibious snails must be eliminated by draining wet areas and applying copper sulfate, which is toxic to snails. Infected sheep may be treated with specific anthelmintics.

   
Liver showing damage caused by migrating fluke larvae   Bile duct opened to show numerous adult liver fluke   Liver fluke being removed from bile duct
         
   
Common liver fluke   Liver fluke eggs   Snails which serve as intermediate hosts for liver flukes
         
 
Typical land on which intermediate host snails are found   Metacercariae attached to grass
         
 
                    Liver fluke eggs showing miracidium ready to hatch   Close up of a single metacercaria

           

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