I must say, I am incredibly fascinated by this topic lately. Nature’s demonstrations of  a parasite’s ability to control the mind of its host is just. so. cool. (in a terrifying sort of way)…Here are two examples that excite that bejeezsus out of me:

1. Leucochloridium paradoxum

This parasitic flatworm uses snails of the genus Succinea as its intermediate host, and birds as it’s terminal host. Here’s how it works: birds spread this parasite’s eggs through its feces, and the feces just so happens to be one of the succinid snail’s favorite meal. So once the eggs are inside the snail, they hatch and little larvae begin making their way into the snail’s antennae. The antennae become swollen, and since they are transparent, you can visibly see the worm pulsing inside. Next is when the mind control takes place. Normally, these snails are shade-loving animals, and do not ever venture into open exposed areas in the sun. Once the parasite gets a hold of them, they influence the snail to crawl up plants until they are openly exposed in the sun. Once they are on top, the pulsing worms in the antennae closely resemble squirming maggots; some of the bird’s favorite food. Birds see the antennae, take a bite, the worm lays eggs inside the bird and bada bing it all starts over again.

If you want to be even more grossed out, here is a video demonstrating the whole thing: http://www.youtube.com/watch?v=EWB_COSUXMw

The next one on the list especially warms my heart: 

2. Dicrocoelium dendriticum

If you thought that last one was freaky, check these out. These types of brain invaders are called Lancet liver flukes (Dicrocoelium dendriticum), and they have an impressive series of intermediates hosts. Their ideal environment for reproduction occurs inside the bile ducts of grazing cattle’s liver, but this is not their ideal environment for growth and development, that has to be done elsewhere.

So here is how they pull it off: First, the D. dendriticum lays its eggs inside grazing cattle, which are then excreted in the feces. Cattle feces and the D. dendriticum eggs happen to be a good food source for terrestrial snails, so they happily feast on the feces. Once ingested by the snail, the eggs hatch and form cercariae (the free-swimming larval stage of the lancet liver fluke), which will now live in the snail’s respiratory chamber. What is especially interesting in this relationship is that the presence of the fluke’s cercariae doesn’t actually kill or even harm the snail, the snail will just become irritated by the cercariae and their bodies will coat them with slime and release them into the environment. Job done, snail moves on to eat some more crap.

Next is where things get real crazy. Ants are especially attracted to these slime balls as a source of moisture, and they devour them, cerceriae and all. So, about 50-100 of these are now growing and developing into metacercariae inside the ant, and they all have one mission: to get to the brain of the ant.  They all begin migrating together, until one of them reaches the subesophageal ganglion of the brain.

Once one metacercariae has infiltrated the ganglion, the others turn around and nest quietly inside the ant. The subesophageal ganglion plays a huge role in the processing of sensory information and motor control of the mandible, maxilla, and labium. The metacercariae is now controlling this part of the ant’s brain, and it will demand some pretty remarkable behavior now.

Typically, ants will recede into their nest at night or when temperatures drop, but when under control of the metacercariae, the ant will now climb up vegetation, clamp its mandibles on the tip of the plant and wait, paralyzed. The paralyzed ant is now in the perfect position at the top of the plant to be ingested by grazing cattle. Once ingested by the cattle, the metacercariae makes its way to the bile ducts of the liver, matures into the adult lancet liver fluke, lays eggs, and the whole process starts over again. If the ant is not ingested by dawn, the metacercariae will release control of the brain and allow it to continue life as normal. Until the sun goes down again. (This is because the parasite would die it allowed to be sitting in the direct sun all day).

This is an incredible process. What type of signaling mechanism occurs when one fluke has reached the brain, and the others know to turn around? And exactly what type of processes are occurring when the fluke is controlling the ant’s brain? Is a type of enzyme secreted by the fluke that orchestrates this perfect behavior? What is also worth noting is that the single metacercariae that reaches the brain and does all the heavy work actually doesn’t survive. It sort of goes on a suicide mission in hopes of 72 virgin flukes.