Other names: Fasciolodiasis, Giant liver fluke, Large American liver fluke
Fascioloides magna is a parasitic flatworm found within the livers of infected deer and other ruminants. Adult liver flukes are fat, oval, purple-gray in color, and up to 8 cm (3.15 in) long and 3 cm (1.2 in) wide.
Deer liver flukes rarely cause clinical illness in free-ranging deer and do not seem to have a major impact on wild deer populations. These flukes present more of a problem when they infect domestic cattle and sheep. Infected cattle do not often show clinical signs, but may have decreased milk production or produce underdeveloped offspring, and infection may result in liver condemnation at the slaughterhouse. Sheep are much more severely affected by deer liver flukes and often die from extensive liver damage. This parasite does not infect humans and meat from infected deer is safe for consumption.
In North America the primary hosts for F. magna are white-tailed deer and caribou. These flukes can also infect mule deer, elk, moose, bison, and other wild ruminants. Domestic cattle, sheep, pigs, horses, and llamas can also become infected with deer liver flukes.
The deer liver fluke is found primarily in 5 regions in North America: the Great Lakes region; the Gulf coast, lower Mississippi, and southern Atlantic seaboard; the northern Pacific coast; the Rocky Mountain trench; and northern Quebec and Labrador. F. magna was introduced into Italy in 1865, and has since been reported in Germany, Poland, the Czech Republic, Croatia, Austria, and Hungary.
Adult flukes live in fibrous capsules within the liver and release eggs that are swept into the intestines and shed in the feces. The eggs hatch in water where a free-swimming immature form of the liver fluke known as a miracidium emerges. The miracidium then penetrates snails (intermediate hosts) where they continue to develop and multiply. Several immature, free-swimming cercariae (the larval stage of the liver fluke) emerge from each infected snail and cling to vegetation in a protective form called a cyst. Encysted cercariae, known as metacercariae, infect new hosts when they are ingested with vegetation. They then migrate through the intestinal wall to the liver, where they become encased in fibrous cysts and develop into adults. The entire life cycle can be completed within 5 months under favorable conditions. There are three different categories of mammalian hosts for deer liver flukes: definitive hosts, dead-end hosts, and aberrant hosts. The flukes can complete their life cycle within definitive hosts, which include white-tailed deer, elk, caribou, mule deer, and fallow deer. In dead-end hosts immature flukes reach the liver but rarely mature and produce eggs. Dead-end hosts include moose, Sika deer, cattle, bison, horses, llamas, and pigs. Flukes cannot complete their migration within aberrant hosts, which include bighorn sheep, roe deer, and domestic sheep and goats.
Most deer infected with liver flukes do not show clinical signs. In definitive and dead-end hosts the flukes become encapsulated within the liver, which prevents further fluke migration and liver damage. When definitive and dead-end hosts are parasitized by a large number of flukes, they may become emaciated, suffer severe liver damage, and sometimes die. In aberrant hosts, such as bighorn sheep and domestic sheep, immature flukes continue to wander throughout the liver, which can cause extensive liver damage. Clinical signs in aberrant hosts include lethargy, loss of appetite, depression, weight loss, and death. In aberrant hosts that have died of deer liver fluke parasitism, necropsy may reveal a swollen, necrotic, hemorrhagic liver with many fluke migration pathways.
A diagnosis can be made at necropsy by identifying F. magna within the liver of an infected animal. Deer liver fluke infections can also be diagnosed by identifying fluke eggs within the feces.
Drugs are available for the treatment of captive and domestic animals, but they are rarely used in wild free-ranging animals. However, medicated baits have been distributed in a wildlife refuge in Texas to decrease the prevalence of liver flukes in wild white-tailed deer.
F. magna can be introduced to new areas when infected animals are relocated, so it is important to treat deer with anti-fluke medications before translocation. Susceptible domestic animals should not be permitted to graze where infected snails are common, and they should not share pastures with potentially infected deer.