Liver Flukes

Fasciola hepatica in sheep liver

Liver flukes (Fasciola hepatica) are flat, leaf-shaped worms found in sheep, cattle, goats, and sometimes deer, elk, and other mammals that graze in wet or marshy pastures. In the early 1900s, a rapid spread of liver flukes in the U.S. from the West Coast and Rocky Mountain states toward the East and North caused disease in domestic livestock and significant losses for American cattle and sheep ranchers.

Some sheep and cattle died every year from liver fluke disease. Death was not the most common loss, however. Infected animals often lived with chronic disease, and were weak and unprofitable.

Snail (Linnaeus truncatulus)

How liver flukes spread

These parasitic worms begin their life cycle as eggs deposited in wet places such as slow-moving streams, canals, and ditches with grass or vegetation on their banks.  Hatching embryos must immediately find and invade the bodies of certain species of freshwater snails. The snails, as intermediate hosts, are essential for the developing fluke larvae. When they are ready to leave the snails, the fluke larvae form into cysts that attach to plants or float in water until consumed by a grazing animal. Immature flukes emerge from the cysts inside the animal host, and then migrate to the liver where they destroy organ tissue on their way to the bile ducts. Mature flukes living in the bile ducts lay eggs. The eggs are passed out of the animal in its feces, and the flukes’ life cycle begins again.

Ventral view of Fasciola hepatica (common liver fluke), 1898

1929: The Bureau of Animal Industry’s Zoological Division collaborated with the Meat Inspection Division to test the effect of temperature on liver fluke eggs. They demonstrated that fluke eggs in meat animal livers were killed by exposure to high (116 to 121 degrees Fahrenheit) and low (0 to 38 degrees Fahrenheit) temperatures.

1936: An allocation of emergency funds enabled the Zoological Division to start a large-scale campaign for liver fluke control in four western states: Washington, Oregon, Idaho, and Utah. The work consisted of draining swampy lands and cleaning streams to destroy the snails that served as intermediate hosts of the liver fluke Fasciola hepatica.  Transforming the environment in this way prevented snail propagation. Areas that could not be drained were treated with copper sulfate to poison the snails.

1938: Leonard Erwin Swanson of the Bureau of Animal Industry showed that liver fluke eggs required an abundance of oxygen to develop to the infective stage. Under muddy conditions, the eggs’ development was slowed.

1942-1943: Hexachloroethane suspended in a bentonite mixture developed by Bureau of Animal Industry was successfully used to remove mature liver flukes from cattle.