The adult worm may reach 30 X 13 mm. It is a leaf shape with a cone-shaped projection at the anterior end. Living specimens are grayish-brown. (See picture at left).
Infects swine, sheep, goats, cattle, other ruminants, horses, humans, and other mammals that eat the vegetation on which the metacercaria encysts.
The eggs leave the host in the feces. If the egg gets into fresh water the embryo will develop into a miracidium. It takes 2 to 4 weeks for the miracidium to develop. Then, the operculum of the egg is pushed off and the miracidium uses its cilia to swim out of the egg. The miracidium finds a snail (Lymnaea truncatula) and bores into it. (If the miracidium fails to find a snail within 24 hours, it runs out of energy and dies.) In the snail, the trematode takes up residence in the digestive gland and develops into a sporocyst. Germinal cells within the sporocyst develop into rediae. When the rediae are fully developed they burst out of the sporocyst. Each redia has a mouth and digestive system and feeds on the snail's tissue. Within the redia, germinal cells develop into a second generation of rediae (daughter rediae). The germinal cells within the daughter redia develop into cercariae. The time from sporocyst to cercaria is about 1 to 2 months depending on the temperature. The cercaria burrows out of the snail and swimw around until it finds a plant. Next, it climbs a short distance out of the water and encysts on the plant as a metacercaria. When the plant and metacercaria is eaten by a mammal, the larval trematode emerges from the metacercarial cyst, burrows through the intestinal wall, and migrates to the liver, and burrows into the liver. After a few weeks of migrating through the liver parenchyma (growing all the time) the young fluke penetrates into a bile duct where it matures to the adult stage. Adult liver flukes feed on the epithelium of the bile duct. Eggs are laid in the bile duct and carried out into the intestine by the bile. The complete life cycle may be completed in 3 to 5 months; the pre-patent period is 10 to 12 weeks.
Site where adult parasite is found in host:
Bile ducts and occasionally the gall bladder.
Egg - Measures 130 - 150 by 63 - 90 µm. It has an operculum at one end.
Common Diagnostic Test:
- Fecal flotation: may distort the egg.
- Sedimentation: recovers eggs undistorted.
The clinical signs depend on the host.
Acute disease in sheep (due to a large initial infection) appears while the young flukes are migrating through the liver. The bleeding into the damaged liver may result in weakness, pale mucous membranes, and hepatomegaly. Chronic disease results in anemia and hypoalbuminaemia as blood is lost into the bile ducts. The sheep show a loss of condition and sometimes edema.
In cattle, the chronic form of the disease is more common and there is the added feature of calcification of the bile ducts over time.
- Sheep: 7.5 milligrams per kilogram of body weight (3.4 milligrams per pound). Administer as a single oral dose using dosing gun or dosing syringe.
- Cattle - 4.54 milligrams per pound of body weight (10 milligrams per kilogram). Administer as a single oral dose using dosing gun or dosing syringe.
- Withdrawal times: Cattle: 27 days before slaughter, Sheep: 7 days before slaughter.
- Cattle: 7 milligrams per kilogram or 3.2 milligrams per pound of body weight.
- Withdrawal time: 8 days before slaughter.
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