Imagine for a moment that you are a Lancet fluke (Dicrocoelium dendricitum), a tiny species of trematode flatworm that has a very complicated life cycle. In order to get to the host your offspring need (Sheep or other ruminant), you need not be proactive.
You lay a large cluster of eggs in the intestinal track of your host animal, and your work is one. The embryonic eggs, dropped in clusters imbedded in the feces of the host animal are now outside in nature. This is where the journey of a thousand miles (so to speak) begins.
The adult produces both eggs and sperm and can self-fertilize, although probably not necessary, as tens of thousands of these tiny creatures are commonly found in a single sheep. Like many higher forms of animals (including humans), the breeding interval depends on how often one fluke comes in contact with another, or whenever it produces both eggs and sperm to self fertilize.
Now on the outside, nature has provided a method for the egg to develop into a larva, and for that larva to get back into another sheep or other ruminant. What this involves two other tiny creatures, a form of snail and an ant.
The embryonic eggs of the Lancet are covered in a tough layer, and are ingested along with bits of fecal matter by snails. When the eggs are in the digestive track of the snail, they hatch into free-swimming larva called miracidia. The larva’s metamorphose into sporocysts (an adult form) and take up residence in the digestive gland of the snail. After a few months, these sporocysts form cercariae (the final free-swimming larval form), and invades the nail’s simple lung.
As more larva develop and migrate into the snail’s lung, it eventually fills, irritating the lung tissue, causing the snail to cough or expel them along with mucus balls. Hundreds of lancets may be held in a single mucus ball, and the nails mucus prevents the larva from drying out. It has now traveled outside of the sheep as n egg, and through a snail in it’s larval stage. Enter the ant.
The Formica fusca is native to the United States. These ants gather the mucus balls as a source of moisture, however, when they ingest the mucus, they also and just dozens of Lancets. The Lancets create holes in the ant’s esophagus, preventing them from being swallowed. The lancets create a material that closes the hole, keeping the ant alive. It needs a healthy and mobile ant for the next leg of the journey. The Lancets migrate to the posterior portion of the abdomen, and develop into the adult form. The Lancet, now an adult, manipulates the subesophageal ganglion of its ant host. These ganglia control muscles and alter the behavior of the ant.
Instead of staying in the nest with her sisters (except for the drones that die after matting, all ants are female), at sundown it leaves the nest and climbs a tall blade of grass, and clings using us mandibles. If the ant is still alive at sunup, the Lancet releases control, and the ant returns to its normal daily routine. Both the ant and the Lancet would die if left exposed in the sunlight, so allowing the ant to return to its duty keeps them both alive. Temporarily.
Sooner or later, poised at the top of a blade of grass, a grazing animal will come along and eat the grass, ant and all. Now ingested, the ant quickly dissolves in the stomach of the sheep or other grazing animal. Not so the Lancet. Upon reaching the duodenum (the part of the small intestine just beyond the stomach), the lancets burrow into the blood vessels and follow the scent of bile through the bile duct to the capillaries of the liver. There they take up residence, feeding on the nutrients from the host animal the rest of their lives. After a few months in the liver, the lancets reach the adult stage, and soon after begin to produce eggs and the entire process starts over again.
Humans can and do play host to Lancets, and because the bodies of these parasites a long and narrow, infections are most frequently confine to distal part of the bile ducts. Because these infections reside in the biliary tree, symptoms are generally mild and often go unnoticed. In children, these infections can produce biliary colic and digestive disturbances, among them bloating and diarrhea. In extreme cases, the biliary epithelial maybe come inflamed, causing enlargement of the liver, or cirrhosis.