Dicrocoelium dendriticum

The Lancet liver fluke (Dicrocoelium dendriticum) is a parasite fluke that tends to live in cattle or other grazing mammals.

Life cycle
D. dendriticum spends its adult life inside the liver of its host. After mating, the eggs are excreted in the feces. The first intermediate host, the terrestrial snail (Cionella lubrica in the United States), eats the feces, and becomes infected by the larval parasites. The larvae (or cercariae) drill through the wall of the gut and settle in its digestive tract, where they develop into a juvenile stage. The snail tries to defend itself by walling the parasites off in cysts, which it then excretes and leaves behind in the grass. The second intermediate host, an ant (Formica fusca in the United States), uses the trail of slime as a source of moisture. The ant then swallows a cyst loaded with hundreds of juvenile lancet flukes. The parasites enter the gut and then drift through its body, eventually moving to the sub-esophageal ganglion (a cluster of nerve cells underneath the esophagus). There, the flukes, now as metacercariae, take control of the ant's actions by manipulating these nerves. As evening approaches and the air cools, the infested ant is drawn away from other members of the colony and upward to the top of a blade of grass. Once there, it clamps its mandibles onto the top of the blade and stays there until dawn. Afterward, it goes back to its normal activity at the ant colony. If the host ant were to be subjected to the heat of the direct sun, it would die along with the parasite. Night after night, the ant goes back to the top of a blade of grass until a grazing animal comes along and eats the blade, upon which the lancet flukes will be back inside their host. They live out their adult lives inside the animal, reproduce, and the cycle continues.

Ecological role
The lancet flukes are important to grasslands. To prevent infestation, cattle avoid patches of the greenest grass, which has been fertilized by manure. The danger for them is not the feces themselves, but rather the possibility that tiny ants might be at the tops of the blades, carrying the parasite. Therefore, those patches grow undisturbed as cows feed on the remaining grass, which is not enough for all of them. The cows are then forced to eat the green grass, and for a while the cow population rises. Eventually, the parasites infect the cows. Infected cows eat less and fail to reproduce as frequently. The cow population drops, allowing the grass to regrow. The cow population eventually recovers and returns to being wary of the greenest grass, completing the cycle.