Dicrocoelium dendriticum, commonly known as the Lancet Liver Fluke, is a parasitic flatworm belonging to the phylum Platyhelminthes. This trematode is a widespread parasite, found in various parts of the world, including Europe, Asia, Africa, and North and South America. Adult flukes are small and lancet-shaped, measuring 6–10 mm in length and 1.5–2.5 mm in width. They reside within the bile ducts and gallbladder of their definitive hosts.
The Lancet Liver Fluke’s Life Cycle
The life cycle of Dicrocoelium dendriticum is intricate, involving two intermediate hosts before reaching its definitive host. The cycle begins when eggs, 40 micrometers long, are passed in the feces of an infected definitive host, such as cattle or sheep. These eggs are then ingested by a land snail, the first intermediate host. Within the snail, miracidia hatch, penetrate tissues, and develop into sporocysts and cercariae over three months.
The cercariae are then released from the snail in sticky mucus masses called “slime balls.” These slime balls are consumed by ants, such as Formica species, which act as the second intermediate host. Inside the ant, the cercariae develop into metacercariae, a process taking up to two months.
A unique aspect of this life cycle is the behavioral manipulation of the infected ant. One or two metacercariae migrate to the ant’s subesophageal ganglion, a cluster of nerve cells, altering its behavior. As temperatures drop, especially in the early morning or late evening, the infected ant climbs to the tips of grass blades and clamps its mandibles, becoming immobilized. This behavior increases the likelihood that the ant will be ingested by a grazing definitive host, completing the parasite’s life cycle.
Infection in Livestock
Livestock animals, including sheep, cattle, and goats, are the primary definitive hosts for Dicrocoelium dendriticum. These animals become infected by ingesting ants carrying infective metacercariae while grazing. Once ingested, the metacercariae excyst in the small intestine, and the juvenile flukes migrate to the bile ducts, mature, and begin laying eggs 10–12 weeks after infection.
While many infections in livestock are asymptomatic, heavy parasite burdens can lead to pathological changes. In such cases, the bile ducts may become thickened and distended, and liver cirrhosis can develop. Although clinical signs like anemia, edema, or emaciation are not always obvious, they can occur in severe infections. The economic implications for agriculture are significant, primarily due to the condemnation of infected livers at slaughterhouses.
Human Infection and Symptoms
Human infection with Dicrocoelium dendriticum is rare and usually occurs accidentally. This accidental ingestion happens when humans consume raw or unwashed vegetables that contain infected ants. Another route is through drinking contaminated water containing infected ants.
Once infected, the flukes reside in the bile ducts, and while infections often produce only mild symptoms, heavier burdens can lead to more severe issues. Common symptoms include abdominal pain, digestive disturbances such as bloating and diarrhea, and biliary colic. In severe cases, there can be an enlargement of the bile ducts, sometimes leading to fibrous tissue around the ducts and an enlarged liver.
Diagnosis and Treatment
Diagnosing Dicrocoelium dendriticum infection in both animals and humans primarily involves identifying the parasite’s eggs in stool samples. The eggs are distinct, measuring 35–50 micrometers, oval, with a thick, yellow-brown shell. However, in humans, the presence of eggs in stool can sometimes indicate a “spurious infection,” meaning the eggs were ingested from eating raw infected animal liver rather than a true parasitic infection.
To differentiate between a true and spurious infection, a repeat stool examination after a liver-free diet for at least three days is recommended. Microscopic examination of bile or duodenal fluid can also provide a more accurate diagnosis. For treatment, anthelmintic medications are used. Praziquantel is a common treatment option. Triclabendazole is also effective.
Prevention Strategies
Preventing Dicrocoelium dendriticum infection involves a combination of strategies targeting the parasite’s complex life cycle. Controlling populations of intermediate hosts, particularly land snails and ants, can help reduce transmission. Introducing animals like ducks, turkeys, or chickens can help reduce snail populations in smaller, localized areas.
Implementing proper pasture management practices is also beneficial for livestock. Avoiding animal grazing during early morning or late evening hours, when infected ants are more likely to climb to the tips of grass blades, can decrease the likelihood of ingestion. Covering ant nests with tree branches can also help keep grazing animals away from areas where infected ants might be concentrated. For humans, general hygiene practices, such as thoroughly washing raw vegetables, are important to prevent accidental ingestion of infected ants.