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Endoparasites - Fasciola

Fasciola hepatica: Common liver fluke.

General Description: Fasciola hepatica adults are greyish-brown, leaf-shaped flukes as large as 30 by 13mm. The anterior end has a cone-shaped projection, behind which the fluke´s broad "shoulders" begin.

Life Cycle: Adult flukes in the liver lay eggs in the bile, which carries them into the intestine. They leave in the host´s faeces. After hatching, the miracidium must penetrate a snail for the life cycle to continue. Multiplication occurs within the snail. After leaving the snail the cercariae encyst on grass, becoming metacercariae, as such they are eaten by cattle. The young parasites penetrate the gut and pass to the liver. The snail can live only in temporary pools of fresh water. The prepatent period is 14 weeks.

Location: Bile ducts of the liver, gallbladder.

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

Significance: Animals experience anaemia and decreased growth. Liver damage results in organ condemnation at slaughter. Fasciola hepatica is a moderately serious parasite of cattle in the Northern Hemisphere. In Australia it is found in cattle but is not as serious as it is in sheep.

Effect on Host: Migration of flukes in the liver damages the tissue, resulting in formation of scar tissue, disrupting normal liver function and thus decreasing production of albumin. Tissue destruction and bile duct irritation by the fluke´s spines necessitate considerable cell replacement. The host´s inability to achieve complete cell replacement allows leakage of blood proteins into the bile and intestine. Flukes also ingest red blood cells directly, causing iron-deficiency anaemia if the animal´s iron stores are exhausted by replacing lost cells. The combination of these conditions over a prolonged period causes decreased growth, weight loss, anaemia, and oedema.

Diagnostic Information: Fasciola eggs are similar to those of strongyles, but are larger (140 by 63 to 90 microns) and operculated. Presence of these eggs in faeces indicates a fluke infection. Finding adults or immature forms in the liver at slaughter is diagnostic.

Control: Snails must be eliminated from pasture by draining water and applying copper sulfate to the pasture. Infected animals are drenched with flukicide which will reduce pasture contamination. In Europe a fluke forecasting system proposes preventive control measures when persistent wet weather provides conditions conducive to snail proliferation and consequent Fasciola epidemics.


   
Adult flukes   Fasciola – egg   Eggs containing miracidia
   
Cercariae   Metacercaria   Slice through liver showing thickened bile ducts

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