Liver Flukes of Cattle and Sheep

EMMETT W. PRICE.

MANY species of flukes infest the livers of warm-blooded animals.

All liver flukes are hermaphroditic; that is, the male and female reproductive organs occur in one individual. Usually mating is unnecessary to insure their perpetuation.

Because all liver flukes of domestic animals must have one or more intermediate hosts for completion of their life cycles, it is hard to develop control measures that apply to all species.

Four well-known species attack cattle, sheep, and other domestic livestock. They are the common liver fluke, the giant liver fluke, the large American fluke, and the lancet fluke.

The liver flukes cause extensive losses. The Department of Agriculture estimated that the annual loss was 3.5 million dollars in cattle and 4.5 million dollars in sheep between 1942 and 1952. The figures represented mostly condemnations of livers at slaughter and did not include unthriftiness, lack of condition, the poor use of feed, and special care, which are hard to measure.

THE COMMON LIVER FLUKE, Fasciola hepatica, is widely distributed.

The adult fluke in the bile ducts is flat and leaflike. The anterior end is conical and is set off from the rest of the body. The parasite is about an inch long in sheep and a little larger in cattle. It apparently is long lived. Viable, egg-producing worms were recovered at the Agricultural Research Center from a sheep that had been experimentally infected and kept for 11 years under conditions that precluded reinfection.

F. hepatica occurs most abundantly in the United States in Florida, Louisiana, Texas, California, Oregon, Washington, Nevada, Idaho, Utah, and Montana. It has also been found in Arizona, New Mexico, Colorado, Arkansas, Wyoming, Michigan, Wisconsin, Alabama, and Missouri. We do not know why it has not become established in the Eastern States, other than Florida, because conditions seem favorable there for their propagation.

The principal hosts for the common liver fluke are cattle, sheep, and goats. It has been found also in swine and deer. Infected rabbits and hares are encountered often in some parts of the country, particularly in the gulf coast region of Texas, and complicate attempts to control the parasite.

The life history of F. hepatica has been known since 1882, when Rudolph Leuckart in Germany and A. P. W. Thomas in England independently discovered that the snail Lymnaea truncatula was the intermediate host in those countries. Since that time about 14 lymnaeid snails have been shown to be vectors of the parasite throughout the world.

In the United States the snails serving as intermediate hosts are Fossaria bulimoides, F. cubensis, F. ferruginea, F. modicella, Pseudosuccinea columella, Lymnaea traskii, and Stagnicola bulimoides techella. Some of those snails have somewhat restricted ranges, but their combined distribution makes it possible for the liver fluke to become established in every state.

The common liver fluke deposits its eggs in the bile ducts in enormous numbers. They pass out of the host in the droppings. The eggs embryonate when they reach water and hatch in 2 to 6 weeks, depending on temperature.

The liberated larva, or miracidium, is ciliated and swims about in the water until it comes in contact with a suitable snail, into which it bores. Then it changes into a form known as a sporocyst. From then on its form changes a number of times, finally becoming what is known as a redia. Large numbers of minute stages, shaped like minute tadpoles and known as cercariae, are produced in the redia. These eventually escape from the snail, swim about, and become encysted on grass or other vegetation or under the surface of the water. Development within the snail takes 50 to 80 days.

When the encysted cercariae, or metacercariae, are taken into the body of cattle, sheep, or other hosts, they encyst in the small intestine, penetrate its walls, and reach the abdominal cavity. There they wander about, penetrate the capsule of the liver, and eventually reach the bile ducts, where they mature in about 3 months.

The most obvious injury to cattle is to the liver, where the parasites cause irritation of the bile ducts, enlargement and thickening of the walls, and fibrosis of the liver tissue. The lumen of the bile ducts is heavily encrusted with deposits of calcium phosphate. Such livers are unfit for human use. Systemic disturbances resulting in loss of condition, digestive disorders, anemia, and other symptoms of parasitism are reported in cattle, particularly in the Gulf Coast States. Whether this condition is due entirely to the flukes is questionable, as similar effects are reported to be rare in cattle in Oregon and other Northwestern States.

Sheep are particularly susceptible. Sheep cannot be raised profitably in many places without a regular system of medication. The livers of infested sheep have thickened bile ducts, but the encrustations with calcium, characteristic in cattle livers, are absent. Sheep heavily infested with young flukes may die without showing symptoms. Sheep with chronic infestations tire easily and appear dull, weak, and anemic.

A further complication to sheep raising in flukey areas in some Western States is "black disease" infectious necrotic hepatitis, a condition caused by the bacterium Clostridium novyi. The organism does not seem to cause disease in normal animals. In sheep, whose livers are extensively damaged by the wanderings of the young flukes, however, it becomes active and liberates toxins, which cause rapid death. The inside of the pelts of infected sheep is dark or black, and the carcass has a peculiar, sweetish odor.

THE GIANT LIVER FLUKE, Fasciola gigantica, is like F. hepatica in form and structure, but it is much longer and its ends are less pointed. It is the common liver fluke of India, other southern Asian countries, and Africa. It is also the common fluke of cattle in Hawaii. We have had some inconclusive reports of its occurrence in cattle in the gulf coast area.

The life cycle of Fasicola gigantica resembles that of F. hepatica. Its snail intermediate hosts are Lymnaea natalensis and Physopsis africana in Africa, L. acuminata in India, and Fossaria ollula in Hawaii.

The giant liver fluke also injures the livers of cattle and causes general systemic disturbance similar to those reported for flukey cattle in Texas and other Gulf Coast States.

THE LARGE AMERICAN FLUKE, Fascioloides magna, was first discovered in 1875 in deer in Italy, where it was believed to have been introduced through importations of American elk. Other ruminants in Europe are known to harbor the parasite, but its widespread distribution and variety of hosts on this continent leave little doubt as to its American origin. The parasite is fleshy and leaflike. It may be 3 inches long when it is fully extended.

This fluke occurs in Texas, Louisiana, Arkansas, Wisconsin, Michigan, Colorado, Illinois, Iowa, Idaho, Oklahoma, New York, Montana, and North Dakota. It has been found in British Columbia, Alberta, and Ontario.

Its definitive hosts in North America are cattle, sheep, goats, several species of deer, bison, and elk. It also has been found in horses.

Its life history resembles that of F. hepatica. The snail intermediate hosts all belong to the family Lymnaeidae and are Stagnicola bulimoides techella, Fossaria modicella, F. modicella rustica,and Pseudosuccinea columella in the United States, and S. palustris nuttalliana and F. parva in Canada.

The young flukes, which gain access to the liver in the same way as do those of F. hepatica, wander through the liver tissue and form extensive channels. Finally they come to rest as a result of the host's tissue reaction. They become surrounded by connective tissue and complete their sexual development within these cysts.

The cysts in cattle are thick-walled and have no access to the bile ducts. Therefore the eggs of the parasites cannot escape, and the life cycle is not completed. In deer and related mammals and in sheep and goats, the cyst wall is thin, the bile ducts are open, and the eggs and other material from the cysts flow freely into them and escape from the body in a normal manner. Consequently it is generally believed that cattle and closely related ruminants are not important in the spread of infection.

Infested cattle sometimes show symptoms of liver fluke disease in areas where F. magna is common. Sheep and goats are particularly susceptible to the effects of F. magna, and deaths are frequent. On postmortem examination, the livers show dark-red cysts, about the size of a walnut. If they are superficial, they are somewhat elevated above the surface of the liver. The liver substance, particularly in sheep and goats, often shows tracks or burrows filled with blood and pigment as a result of the migration of the flukes. Adhesions of the viscera, especially near the liver, are commonly seen; the liver, adjacent lymph nodes, and omentum show streaks and patches of black pigment, a condition that does not occur in other infestations of liver flukes.

THE LANCET FLUKE, Dicrocoelium dendriticum, is the smallest of the liver flukes that infest livestock. It rarely is more than one-half inch long. It has pointed ends and so has the shape of a lancet. The body is thin and semitransparent, except for the middle part of the posterior half, which contains the uterus filled with small, brownish eggs. Its small size and transparency make it difficult to detect on casual inspection of the opened bile ducts.

The lancet fluke is widely distributed in Europe, the Near East, and some other parts of the Eastern Hemisphere. It was first definitely established to be present in North America in 1930, when cases in sheep were found in Nova Scotia.

The infection was first found in the United States in 1940, when eight infested cattle from New York were found during the course of Federal meat inspection at Newark, N. J. The parasite since has been reported in sheep, cattle, goats, deer, woodchucks, and rabbits in 11 counties in central New York. Two cases have been found in Massachusetts and one in northern Pennsylvania. We have had unconfirmed reports of one case in North Dakota and another in Arkansas.

The lancet fluke requires two intermediate hosts, one a snail and the other an ant.

The first intermediate host is a land snail Zebrina detrita, Torquilla frumentum, Helicella ericetorum, and H. candidula in Europe, and Cionella lubrica in America. C. lubrica is minute and occurs widely in the United States, Canada, and parts of Mexico.