Zombie Snails and the Fungal Apocalypse

When video game developer Naughty Dog (through Sony Distribution) released the blockbuster The Last of Us on June 14, 2013, it was met with nearly universal critical acclaim. For the non-gamers out there, The Last of Us (TLOU) is an action-adventure survival horror game made exclusively for the Play Station 3. The player controls a man named Joel who inherits the responsibility of guiding a young girl named Ellie through a “zombie” ravaged post-apocalyptic America of 2033. Zombies and other bands of human survivors are equally dangerous.The human "resistance" in the game believes Ellie is connected to a potential cure to the global “infection.” Such is the plot. Game controllers are accessed. Buttons are pressed. Carnage ensues.

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The lucrative trope of “the zombie apocalypse” in popular culture (see AMC’s The Walking Dead) shows no signs of abating. However, TLOU features a clever spin on the genesis of zombie creation. Rather than the usual runaway outbreak of a militarily designed biohazard, TLOU posits a world where humans are frighteningly turned into murderous, rabid, unthinking monsters due to a fungal infection called Cordyceps. As the game ominously suggests, the potential for zombie transformation may already exist in nature and just be waiting the right mutation to infect a human host, spread exponentially, and eventually demand Brad Pitt’s intervention. In fact, nature has already managed several “actual” zombies without the assistance of screenwriters or marketers. One such example is the Ophiocordyceps unilateralis, a kind of parasitoidal fungus which effectively render ants as zombies. The infected ant drunkenly bobs and weaves as it is forced away from its colony, up a tree, and finally onto a overhanging leaf where it unconsciously sinks its mandibles into said leaf. Here the fungus finally kills the ant and then proceeds to grows spores out of the ant's decaying remains. This really happens.

My favorite example, however, is reflected in the charmed life of Leucochloridium paradoxum. Leucochloridium is a kind of parasitic flatworm typically found inside of Succinea, or amber snails. What the Leucochloridium does is effectively turn the snail into a zombie. Once inside the snail, the Leucochloridium takes over the functioning of the snail’s brain, controlling its directional momentum and causing it to move erratically, very much like a zombie. It then fills the snail’s tentacles with moving larvae that strongly mimic the appearance of a caterpillar or grub worm, a favorite food of predatory birds. Forcing the ill-begotten snail high up into the trees where it’s easily seen by such birds, the flatworm makes the snail an easy, hapless target. The birds then unwittingly digest the snail with the flatworm and its eggs inside. Later on, the birds droppings--now filled with new flatworm larvae--are subsequently eaten by other snails, beginning the cycle anew.

Snail with parasitic infection. 

To watch a snail being taken over by this parasite is a humbling experience. Surely, no such parasite could do this to a human?