Fascioloides magna
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| Fascioloides magna |
Fascioloides magna, also known as Large American liver fluke or Giant liver fluke, is a parasitic flatworm in the class Trematoda, Phylum Platyhelminthes, which grows to a maximum length of 100 millimeters, and is thick and oval in shape. It is distinguished from other members of the genus Fasciola by the lack of an anterior projecting cone. It occurs in domestic and wild ruminants, elk and deer being the normal hosts.
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[edit] History and Epizootiology
Fascioloides magna was once limited to North America, but a translocation of elk (Cervus canadensis) from North America to Italy in the 19th century led to establishing the parasite in Europe. Now found on the red deer (Cervus elaphus) is the natural reservoir for F. magna, although white tailed deer (Odoicoleus virginianus), mule deer (O. hemionus) and possibly caribou (Rangifer tarandus) can be competent hosts. The geographic distribution of the parasite depends on several factors, including the presence of an intermediate host, since they have indirect life cycles. Since the intermediate host is a snail (usually in the genus Lymnae), F. magna only occurs where the snail can exist (usually low, moist soils) or survive (irrigated pastures).
[edit] Life cycle
F. magna has an indirect life cycle. The natural host (elk) sheds parasite eggs in the feces. These eggs hatch into a miracidium, which in turn enters the foot of a Lymnae spp. snail. Cercaria hatch from the snail, and encyst into metacercariae that attach to vegetation where they are ingested by the herbivorous animal. The parasite migrates from the intestinal tract to the liver, where (in the natural host) it encysts in a fibrous capsule that communicates with the bile duct. Eggs are released through the bile duct and into the intestinal tract where the cycle can begin once again. In the natural host, pathogenicity is low, and limited primarily to the fluke's local damage to the animal's liver. The liver may be distorted and pitted with fibrous scars, but animals rarely die. The adult flukes live in the liver and can measure 5 cm in length.
In sheep and goats, which are aberrant hosts, the parasite does not encyst. A few wandering parasites can cause death due to extensive fluke migration through the liver parenchyma and abdominal cavity. In cattle and bison, the parasite never completes the cycle because they get encapsulated in a heavy fibrous capsule.
In these animals, the parasite never encapsulates and instead continues to migrate through the cattle, F. magna cause severe tissue reaction, resulting in thick-walled encapsulations that do not communicate with bile ducts. In sheep, encapsulations do not develop, and the parasites migrate in the liver and other organs, causing tremendous damage. Histologically, infected livers of cattle, sheep, and deer show black, tortuous tracts formed by migrations of young flukes.
[edit] Diagnosis and treatment
While the eggs of F. magna resemble those of F. hepatica, this similarity is of limited use; eggs usually are not passed in cattle and sheep. Recovery of the parasites at necropsy, as well as proper identification of F. hepatica or F. gigantica is necessary for definite diagnosis. When domestic ruminants and deer share the same grazing areas, the presence of disease due to F. magna should be kept in mind. Mixed infections with F. hepatica occur in cattle.
Oxyclozanide has been reported to be effective against F. magna in white-tailed deer, and rafoxanide has been used successfully against natural infections in cattle. Albendazole (7.5 mg/kg), clorsulon (15 mg/kg), and closantel (15 mg/kg) have shown efficacy against this fluke in sheep. Currently no products are approved for use against this fluke in the USA. Deer are required for completion of the life cycle; if they can be excluded from the areas grazed by cattle and sheep, then infection can be controlled. Control of the intermediate host (lymnaeid snails) may be possible once it has been identified in a region and the nature of its habitat examined.
