Dicrocoelium Dendriticum

Also known as

Tthe lancet liver fluke or lanceolate fluke

Description

Dicrocoelium dendriticum is a trematode. These parasitic flatworms, known as flukes, are internal parasites of mollusks (which include snails) and vertebrate animals. Most trematodes have a complex life cycle with at least two hosts—the snail and a mammal.

This particular trematode is a liver fluke that lives in cattle or other grazing mammals. It is a common parasite of ruminants but humans can be accidental hosts.

Cattle, sheep, and goats don’t seem to develop any immunity to this particular parasite, and heavy infections may accumulate (up to 50,000 flukes in a mature sheep), yet the host has minimal pathologic changes or clinical signs. Cirrhosis (scarring and fibrosis) can develop within the liver, however, and bile ducts may be thickened and distended.

Economic loss is due primarily to condemnation of livers at slaughter. Clinical signs are generally not obvious unless the animal has a massive infection.

These parasites are believed to be endemic or potentially endemic (naturally occurring) in at least 30 countries. Dicrocoelium dendriticum is found throughout Europe, the Middle East, Asia (China, Japan, Vietnam), Africa, North and South America and Australia.

The parasite tends to be found most often in areas that favor the primary intermediate hosts, which are small, glossy air-breathing terrestrial snails that live in varied environments including mountainous terrain and valley meadows.

These snails hide in grass and duff, under rotten wood and in the forest leaf litter. They feed mainly on dead plant matter, microbial fungi and debris, but sometimes eats fresh plant parts.

The lancet liver fluke is unique in that it passes through two intermediate hosts—the land snails and certain types of ants—before being eaten by a grazing animal. Most infections in ruminants involve low numbers of flukes and are not associated with noticeable signs.

In more intensive infections, signs may include generalized /abdominal discomfort. Occasional animals that harbor these flukes have subcutaneous masses/enlargements.

Cattle rarely show signs, but there is some evidence that decreased liver function in sheep (due to damage from the flukes) can cause toxemia in pregnant ewes and mastitis in lactating ewes when combined with other risk factors.

At necropsy a heavily infested animal may have liver abscesses and thickened bile ducts.

Signs

Most infected animals show no signs;

Severe infections may cause

• Anemia
• Edema
• Abdominal discomfort
• Weight loss

Cause

There are several types of liver flukes, and some create more liver damage than this one. Ruminants, especially cattle and sheep, are the normal definitive hosts for Dicrocoelium dendriticum, but other herbivorous animals, carnivores, and humans can be accidental hosts.

Most infections, especially in cattle, are asymptomatic (without signs) but the effect on the liver depends on the number of flukes and the length of time the animal has been infected. Infections are usually without signs, but some animals may show anemia, edema, and emaciation.

With hundreds of flukes present in the bile ducts, there is no change in the liver tissues, but when thousands of flukes are present there may be fibrosis of the small bile ducts and subsequent cirrhosis (scarring). There are usually no clinical signs except with liver cirrhosis—which can result in edema and emaciation.

After entering the grazing animal, immature flukes migrate up the bile duct (where they mature and produce eggs), but do not penetrate the gut wall or liver tissue. Long-standing infections may cause enlargement of the bile duct and liver lesions. This fluke spends its adult life inside the liver of its host. After mating, the flukes’ eggs are produced and excreted in the host’s feces.

The first intermediate host, a terrestrial snail (Cochlicopa lubrica in the U.S.), consumes the ruminant feces, and becomes infected by the larval parasites. The larvae (miracidium) drill through the wall of the snail’s gut and settle in its digestive tract, where they develop into a juvenile stage.

The snail walls the parasites off in cysts, which it then excretes and leaves behind in the grass or wherever the snail was traveling.

The second intermediate host, an ant (Formica fusca in the U.S.), uses the trail of snail slime as a source of moisture and follows the slime. In doing so, the ant swallows a cyst loaded with hundreds of juvenile lancet flukes. These immature forms of the parasites enter the ant’s gut and migrate through its body.

Most of them encyst in the body cavity of the ant and mature into metacercariae, but one moves to the sub-esophageal ganglion (a cluster of nerve cells). There, the fluke influences the ant’s actions by manipulating these nerves.

As evening approaches and the air cools, the infected ant is drawn away from other members of the colony and upward to the top of a blade of grass or some other vegetation, where it clamps its mandibles onto the top of the blade or leaf and stays there until dawn.

Afterward, it goes back to its normal activity at the ant colony. Night after night, the ant goes back to the top of a blade of grass (and can live for at least a year doing this) until a grazing animal comes along and eats the blade of grass, ingesting the ant along with it, thus putting lancet flukes back inside their definitive host.

Infected ants may contain 100 metacercariae, and a high percentage of ants in a colony may be infected. After an ant is eaten by a suitable definitive host, the metacercariae move into the duodenum (the first part of the small intestine). 

These juveniles penetrate blood vessels and travel up the common bile duct to the capillaries of the liver. They bore through the walls of the capillaries and migrate to the bile duct in the liver where they ingest nutrients from the host and continue to do so for the rest of their lives.

Once they reach the liver, the immature flukes mature in 6 to 7 weeks, find a mate, and produce eggs. Eggs can be produced about 10-12 weeks after the host animal ingests the parasitized ant. These eggs pass down the common bile duct to the intestines and, once excreted in the feces of the host, will start this cycle again.

These flukes spend their entire adult lives inside the grazing animal, producing eggs.

Successful internal migration is almost always dependent on the parasite being in the correct host. If it enters the wrong host, the immature fluke might die immediately, or it might be killed by the host’s immune response while migrating.

But in rare cases the parasite evades the host’s immune response for an extended period. In these situations, the parasite almost always causes severe damage, and sometimes even death of the host.

Humans can serve as definitive hosts after ingesting infected ants (such as on contaminated food items)

The diagnosis of lancet liver fluke infection is mainly from finding adult flukes in the liver during necropsy, or detecting eggs in animal feces. Finding the small, brownish eggs on fecal examination or adult worms in the bile ducts at necropsy is the common method but today an ELISA test using a Dicrocoelium dendriticum antigen can also be used to confirm a diagnosis.

Prevention

The complex life cycle makes control of intermediate hosts (land snails and ants) almost impossible; widespread chemical use is impractical and harmful to other organisms, but keeping ducks, turkeys, or chickens to eat the snails can effectively reduce intermediate host populations in small areas.

Because infected ants are usually found within 12 to 20 inches of the base of their nest, covering ant nests with tree branches to keep animals away from the base can also be useful.

Grazing management practices can also decrease the incidence of infection. This includes avoidance of animal grazing early in the day or late in the evening, when ants are more likely to climb to the top of the grass blades.

Treatment

Treatment can be difficult due to the fluke’s complex life-cycle. Various deworming drugs have been shown to be effective when an entire herd is infected, but most of them must be administered at dosages higher than those recommended for Fasciola hepatica (the common liver fluke).

Effective treatments in both cattle and sheep are albendazole at in a single large dose or two smaller doses on successive days, or netobimin. Another treatment is benzimidazole given at higher rates than for other internal parasites or Praziquantel at double the dose for tapeworms.

Two basic reasons to administer an anthelmintic are to aid the individual animal (either to save its life or keep it healthier—for better production of meat, milk, fiber, or for better reproductive efficacy--and to decrease the numbers of parasites in the environment so the at-risk animals will not be exposed to a high level of infection.

With Fasciola hepatica, treatment is only effective against adult flukes, and too late to help the individual animal very much. If a drug is effective during migration of immature parasites, the individual animal may benefit.

The use of an effective product administered to livestock before the snail population has built up (after their seasonal hibernation) will lower the infection rate in snails. Then the snails can’t pass the infection to the next crop of calves or lambs.