The lancet liver fluke, Dicrocoelium dendriticum, is a parasitic flatworm that targets the liver and bile ducts of grazing mammals, such as cattle and sheep. It is also known as the lesser liver fluke due to its small, slender, lancet-shaped body. It is one of the most widespread trematode infections globally, distributed across Europe, Asia, Africa, and North America. This parasite is often overlooked in veterinary medicine because it typically causes less severe disease than other liver flukes. The life cycle of D. dendriticum is complex, requiring two different intermediate hosts to reach its final mammalian host.
The Complex Life Cycle and Transmission
The transmission of Dicrocoelium dendriticum relies on two intermediate hosts: a land snail and an ant. The cycle begins when the final host, usually a sheep or cow, passes embryonated eggs in its feces. A terrestrial land snail ingests the egg, which hatches into a miracidium that penetrates the snail’s gut wall.
Inside the snail’s tissues, the parasite develops through two generations of sporocysts, eventually producing numerous cercariae. The snail then expels these cercariae in masses of sticky mucus, known as “slime balls,” which are consumed by the second intermediate host, an ant.
Once inside the ant, most cercariae encyst in the abdominal cavity, developing into the infective metacercariae. However, one or two metacercariae migrate to the ant’s nervous system, specifically the subesophageal ganglion, effectively taking control of the ant’s behavior. This phenomenon is a form of parasitic manipulation, sometimes referred to as the “brain fluke” effect.
The infected ant exhibits abnormal behavior, especially as temperatures drop in the evening or early morning. The ant is compelled to climb to the tip of a blade of grass or other vegetation and clamp down with its mandibles, becoming immobilized. This strategic positioning increases the probability of the ant being accidentally ingested by a grazing mammal during feeding. After the definitive host consumes the infected ant, the metacercariae are released in the small intestine and migrate directly up the bile duct to the liver, where they mature into adult flukes and begin reproducing.
Clinical Manifestations in Hosts
Upon reaching the bile ducts and gallbladder, the adult flukes cause pathology that is directly related to the parasite burden. In livestock, which are the primary hosts, light infections are often asymptomatic and do not cause severe clinical signs. However, heavy, long-standing infections can involve tens of thousands of flukes, leading to significant health issues.
The presence of numerous flukes causes chronic irritation to the bile ducts, leading to thickening, distension, and the formation of fibrous tissue around the ducts. This condition can progress to biliary fibrosis and liver cirrhosis, resulting in a loss of liver function. Symptoms in animals may include emaciation, anemia, weight loss, and reduced milk production, although economic loss is primarily due to the condemnation of infected livers at slaughter.
Humans are considered accidental hosts, and true infection occurs only through the accidental ingestion of an infected ant. Human dicrocoeliasis is rare and often mild or completely asymptomatic. If symptoms do occur, they are typically related to the physical presence of the flukes in the bile ducts and can include abdominal pain or biliary colic. In heavy human infections, there is a risk of bile duct obstruction, chronic inflammation, and an enlarged liver, sometimes accompanied by an elevated count of eosinophils in the blood.
Detection, Treatment, and Prevention
Diagnosis of dicrocoeliasis traditionally relies on the identification of the parasite’s small, thick-shelled, yellowish-brown eggs in the host’s feces. For humans, finding eggs in stool samples can lead to a misdiagnosis known as “false dicrocoeliasis.” This spurious infection occurs when a person consumes the raw or undercooked liver of an infected animal, and the fluke eggs simply pass through the digestive tract.
To confirm a true infection in humans, physicians often require the patient to maintain a liver-free diet for several days before re-examining the stool, or they may examine bile or duodenal fluid directly. Specific antihelminthic drugs are used to eliminate the adult flukes.
Treatment
In livestock, drugs like albendazole and netobimin are effective, often requiring higher doses than those used for other fluke infections. In humans, treatment for a true infection typically involves a drug like praziquantel or albendazole.
Prevention
Prevention for livestock focuses on grazing management, such as avoiding pastures with high ant populations or preventing grazing during early morning and late evening when infected ants are attached to grass. For humans, prevention centers on avoiding the consumption of raw vegetation or water that may be contaminated with infected ants.