Dicrocoelium dendriticum, the Lancet Fluke, typically lives in the bile ducts of the livers of grazing animals (and sometimes humans). It has a rather typical trematode (fluke) life cycle with three different hosts but, interestingly, it clearly alters the behavior of one of them, making it more likely to reach the next host. Though this type of behavior control is suspected in many other parasites, it’s often not easy to prove.
Dicrocoelium dendriticum Life Cycle
The animal in which the adult flukes live is called the definitive host—the host in which the parasite multiplies sexually:
- Adult worms in the liver of the definitive host produce eggs that are washed out in the bile, mixed with the stool, and passed from the body.
- Land snails feed on decaying animal droppings and ingest the eggs, whereupon the eggs hatch, releasing miracidia.
- Miracidia migrate through the gut wall into the snail’s digestive gland, where they multiply asexually. Cercariae are produced. The snail is the first intermediate host for the fluke.
- Cercariae exiting the snail’s tissues are encased in a coating of slime, which is left behind on the vegetation that the snail travels over. Transforming to metacercariaie, the parasites can be very numerous (hundreds in one slime ball) and are protected from drying out by the slime encasing them.
- Foraging ants collect the slime balls and carry them back to the nest, where the slime balls are eaten. Metacercariae encyst in the ant’s body cavity and become infective to the definitive host. One metacercaria travels to the ant’s nervous tissue and encysts there, an event which profoundly influences the ant’s behavior from then on - scientists are still unsure of how this works.
- Infected ants crawl to the tops of blades of grass in the cool evenings and early mornings and cling there. This is the time when herbivores are grazing—the ant’s strange behavior makes the insect much more likely to be eaten by a grazing animal! In the heat of the day, when the dew dries up and animals rest in the shade, the ant that has not been eaten resumes its normal activities, only to ascend again when things cool down.
- Metacercariae in ants that have been eaten migrate up the bile duct into the liver and mature to adult flukes in under two months. At about three months after infection, the worms begin producing eggs.
Dicrocoelium dendriticum Infection in Humans
Dicrocoleium dendriticum infection (dicrocoeliiasis) in humans is quite unusual, and many suspected cases are simply the result of the person having eaten the liver of an infected animal—typical eggs from flukes in the liver travel through the intestine and are passed in the stool without actual infection. True cases of human infection with the Lancet Fluke arise when people—accidentally or deliberately—eat ants.
Other Parasites that Control Host Behavior:
Toxoplasma gondii and Behavior
Read Other Interesting Topics in Microbiology
Sources:
Parasites and the Behavior of Animals. Moore, Janice. Oxford: Oxford University Press, 2002.
Foundations of Parasitology 6th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
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