Today, I present to you a parasite that has been studied exhaustively because of its commercial significance and because of its interesting life cycle. This parasite is a liver fluke most often found in ungulate (sheep, goats, cows, pigs, etc.) mammals. It has a blade-like tapered body giving it the common name of "lancet fluke". I'm speaking of course, of Dicrocoelium dendriticum.
The lancet fluke belongs to the phylum platyhelminthes ("flatworms") along with free-living turbellarians (planarians), cestodes (tapeworms), and other members of class trematoda, (a.k.a. "trematodes" or "flukes"). Within class trematoda lies subclass digenea, a group characterized by life cycles with two or more hosts (typically including a molluscan host). D. dendriticum belongs to order plagiorchiformes within this subclass. The adults of this order are quite diverse, but the larval and juvenile stages are fairly conserved among its members. Members of this order have small eggs that are often eaten by a snail and cercariae are simple with a finfold on the dorsal side. The family of this parasite is family dicrocoeliidae, which is one of the three major families of liver flukes (along with fasciolidae and opisthorchiidae). Members of this family rarely parasitize humans, but are known to parasitize other mammals, especially domestic animals. All members possess a subterminal oral sucker and an anterior acetabulum.
The life cycle of this parasite is an eloquent complex of evolutionary wonder. It begins, as many life cycles do, with the feces of an infected mammal falling to the grass laden with Dicrocoelium eggs. Along comes a hungry snail, which devours the delicious droppings. Inside of the snail, the miracidium hatches from the egg and undergoes a variety of bodily transformations. The parasite then finds its way (at this point in the form of a cercaria) into a slimeball that gets excreted by the snail. Snail slimeballs make tasty snacks for unsuspecting ants. After ingesting the cercaria, the parasite forms a metacercarial cyst within the body of the ant. This is when the most interesting aspect of this life cycle comes into play. The parasite, through processes not completely understood, is able to manipulate the behavior of its new host in order to continue its life cycle. For whatever reason, infected ants will climb onto tall vegetation in the evenings and lock their mandibles onto plants. This behavior is totally uncharacteristic of uninfected ants. The next day, the ants are eaten accidentally by grazing ungulates. Within the bile duct of the ungulate, the parasite joins thousands of other liver flukes and matures into adulthood. From there, the parasite mates with another and after about a month begins releasing eggs to perpetuate the life cycle and preserve the species.
Infection with this parasite typically presents as general dysfunction of the bile ducts due to irritation and over population in a finite area. Symptoms often include inflammation of the bile ducts, liver cell death, and fibrosis. Many types of ungulates, including sheep, deer, goats, pigs, and cattle, have been reported as having this parasite, making it agriculturally important to study.
There have been reports of D. dendriticum infections in humans, but most were instances of false parasitism. This means that people passed eggs after eating an infected liver but did not actually become infected themselves. True infections have been reported from Asia, Africa, Europe, and one case in New Jersey.
There is not currently a good method for preventing the spread of this disease. After all, land snails and ants are never in short supply in the pastures where livestock are allowed to graze. For treatment of infected livestock, praziquantel and various benzimidazoles are the drugs of choice. Unfortunately, without adequate control measures, infection rates will continue to be high, meaning that treatment might not be economical for ranchers. With human infections, praziquantel is most often prescribed to take care of the infection.
Moral of the Story
As with so many other fascinating parasites, behavioral modification of hosts continues to be a critical survival strategy. Ants must be easy targets for such manipulation due to their abundance and reliance on chemical stimuli. This is demonstrated time and again with how easily parasites seem to be able to zombify these colonial insects! More to the credit of the lancet fluke, the evolutionary processes involved in the development of its life cycle are impressive, to say the very least. The fact that this fluke not only managed to dupe ants into becoming kamikaze vessels for the benefit of the fluke's species, but the fluke also evolved beyond the reliance on water that is so commonly seen amongst other trematodes. What a beautifully crafted life cycle for such an interesting and unique organism! Just for funzies, here's a link to a sweet comic about this parasite from The Oatmeal. (You know a parasite is cool if it makes The Oatmeal! :p) Enjoy!