Liver fluke

Fasciola hepatica

ORDER Echinostomida

FAMILY Fasciolidae

TAXONOMY

Fasciola hepatica Linnaeus, 1758, "in aquis dulcibus ad radices lapidum, inque hepate pecorum. Diss. de Ovibus;" Europe.

OTHER COMMON NAMES

English: Sheep liver fluke; French: Grande douve du foie, douve du foie de mouton; German: Großer Leberegel.

PHYSICAL CHARACTERISTICS

Adult liver flukes may reach 1.7-2.2 in (4-5 cm) in length and 0.6 in (1.5 cm) wide. They are typically about 1.3 in (3 cm) long, 0.4 in (1 cm) wide, and have a spiny tegument. They taper toward the rear. The front end bears an oral sucker and a cone-shaped tip. The sucker on the fluke's ventral (lower) surface is larger than the oral sucker. The ventral sucker is about a third of the body length behind the oral sucker. The branched ovary is situated behind and to the side of the ventral sucker about a third of the way back in the body. The testes are also branched and extend throughout the body behind the ovary.

DISTRIBUTION

Worldwide, but found most often in Europe and Latin America in habitats congenial to their freshwater snail and definitive hosts.

HABITAT

Liver flukes are found in swampy, generally wet freshwater areas inhabited by snails, especially of the species Lymnaea truncatula, Stagnicola bulimoides, and Fossaria modicella. Snails are their sole intermediate host. The definitive hosts of this fluke include grazing herbivores (in the bile ducts) primarily, including sheep and cattle, but also dogs, cats, rabbits, and humans.

BEHAVIOR

The eggs, deposited in the environment in the definitive host's feces, hatch in freshwater areas, usually within about 10 days, longer if temperatures are cool. They have been known to survive in particularly cold water for several years. The embryos develop into miracidia, which quickly swim to and penetrate the soft tissue of snails. Miracidia can survive only 24 hours in the free-living state. Sporocysts form and produce first-generation rediae, which in turn produce second-generation rediae and eventually numerous cercariae. The cercariae live in the snail for 4-8 weeks, then exit and swim to vegetation lying just below the water line. There, they drop their tails and encyst. Passing herbivores become infected when they eat the vegetation, often grass. Humans typically become infected by drinking water containing flukes or by eating vegetation such as watercress. The flukes travel to the abdominal cavity in the first 24 hours, then to the liver over the next few days. Research indicates that immature flukes are able to orient during their migration from the duodenum (the first section of the small intestine) toward the liver. Within six to eight weeks, they reach the bile ducts, sometimes spreading to the lungs, where they mature and lay eggs. The eggs are then carried to the duodenum and pass into the feces.

FEEDING ECOLOGY AND DIET

This parasitic digenetic fluke has two hosts: Lymnaea species as the intermediate host, and wild or domesticated ruminants as the definitive host. Humans may become secondary hosts. The fluke feeds on bile duct lining, causing calcification of the duct.

REPRODUCTIVE BIOLOGY

The tan or yellow eggs are about 0.0048-0.006 in (120-150 pm) long and 0.0025-0.0035 in (65-90 pm) wide. In warm water (78.8°F or 26°C), they develop into miracidia in less than two weeks. The miracidia are ciliated, and somewhat triangular in shape with the front end being broader than the rear. The front end also has a noticeable slender outgrowth with two eyespots behind it. The cercariae, which range from 0.0098-0.013 in (250-350 pm) long, resemble tadpoles in shape with a bulbous anterior end and long tail making up about two-thirds of the overall length. In artificial laboratory conditions, adult flukes have been known to survive as long as 11 years.

CONSERVATION STATUS Not listed by IUCN.

SIGNIFICANCE TO HUMANS

Infected humans may develop symptoms ranging from skin inflammation to pneumonia. Fluke infection can result in massive hemorrhages in horses, a reduction of milk production in dairy cattle, and mortality in sheep. Sheep mortality is often caused by bacterium Clostridium novyi, which thrives on the infected livers of sheep. ♦

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