Summary
Taxonomy
Phylum: Platyhelminthes
Class: Trematoda
Subclass: Digenea
Order: Echinostomatiformes
Family: Fasciolidae
Among flukes of veterinary importance the closest relatives of F. magna are Fasciola hepatica of domestic and free-ranging ruminants and other mammals in many parts of the world, including a few locations in Canada, Fasciola gigantica of cattle and sheep in several parts of world (but not Canada), and Fasciolosis buski of people and pigs in Asia. The other liver fluke of ruminants in North America, and other parts of the world, is Dicrocoelium dendriticum.
Morphology
Eggs of F. magna are oval, measure approximately 140 µ by 85 µ, and have a thin, smooth shell, an operculum (lid) at one end, and often a small "pimple" at the opposite end. When passed, each egg contains a clump of cells. It is fairly difficult to distinguish the eggs of F. magna from those of F. hepatica, but both are very different from those of D. dendriticum.
Host range and geographic distribution
Fascioloides magna occurs primarily in free-ranging ruminants in North America and, following elk importation in the 19 century, in areas of Europe. Many elk in Banff National Park are infected. The parasite has been reported not uncommonly from cattle and sheep in Canada that have shared habitat with the wildlife hosts.
A recently published review suggests that the mammalian hosts for F. magna can be differentiated into: a) definitive hosts, in which the adult flukes live in thin-walled hepatic cysts and establish patent infections (primarily New World cervids, e.g., elk); b) aberrant hosts, in which the flukes do not complete development but damage the liver to such an extent that the hosts may die (primarily domestic sheep and their relatives); and c) dead-end hosts, in which flukes reach the liver and may produce a few eggs, which rarely leave the host (bovids, suids (domestic pigs and their relatives), llamas and Old World cervids).
Life cycle - indirect
Adult F. magna live in cyst-like structures in the liver parenchyma. Eggs are passed in the feces, at least in some hosts (see Host Range and Geographic Distribution section). In an aquatic or semi-aquatic environment a miracidium (first larval stage) develops in each egg, which hatches. The miracidium penetrates a suitable snail intermediate host, in which a sporocyst, then rediae, then cercariae develop. These leave the snail and encyst as metacercariae on vegetation. Development in the snail takes from six to eight weeks. Fascioloides magna, like F. hepatica, can increase its numbers in the definitive and intermediate hosts.
Infection of the definitive host is by ingestion of vegetation with metacercariae attached. The immature flukes penetrate the intestinal wall and cross the peritoneal cavity to the liver, enter the liver and migrate through the parenchyma to the bile ducts. The pre-patent period of F. magna in its "normal" hosts (New World cervids) is approximately seven months. Rarely the flukes are located in unusual sites, particularly the lungs.
Life Cycle: Fascioloides magna
Epidemiology
Pathology and clinical signs
The pathology and any clinical signs associated with F. magna depend in part on the suitability of the host species and on the number of flukes. In the "normal" definitive hosts, the adult flukes live singly or in pairs in thin-walled cysts in the liver parenchyma and stimulate a localized inflammatory response, sometimes complicated by pressure atrophy of the surrounding tissues. Often the drainage from these cysts is obstructed by the inflammation, and they come to contain dark-coloured fluid in which flukes and eggs are found. Chronic infections may result in hepatic fibrosis.
In "aberrant" hosts, for example sheep, the hepatic pathology results from the wanderings of the immature flukes. In sheep in Canada, F. magna infection is usually an incidental finding at post mortem, but very heavy infections in any host can be associated with clinical signs and are occasionally fatal.
In "dead-end" hosts, for example cattle, the parasites are found in thick-walled cysts in the hepatic parenchyma, together with black, viscous fluid. These cysts rarely communicate with the biliary system and eggs are rarely found in the feces.
Diagnosis
Treatment and control
Two drugs, albendazole (VALBAZEN) and triclabendazole (FASINEX) have been used extralabel in Canada for Fascioloides magna in wildlife hosts. In Canada albendazole is approved for use in cattle for other parasites, including Fasciola hepatica, and triclabendazole can be obtained from the Veterinary Drugs Directorate of Health Canada using an Emergency Drug Release. Producers in Canada can also purchase a combination of ivermectin and clorsulon (IVOMEC PLUS) in the United States for extra label use on their own cattle.
There are no products approved in Canada for F. magna in sheep, and essentially no demand for treatment.
Effective control of F. magna becomes a concern in relation to translocations of wildlife (that is how the parasite established in Europe!), possible health impacts for free-ranging or farmed wildlife hosts, and possible health impacts for domestic animals. Effective control is usually difficult or impossible because of the problems associated with making a definitive diagnosis in infected animals, with control of the parasite in the intermediate hosts, and with preventing contact betweeen mammalian and molluscan hosts, and between wildlife and domestic species.