How, then, do these single-celled intestinal denizens exert their influence on a complex multicellular organ such as the brain? Although the answer is unclear, there are several possibilities: the Vagus nerve, for example, connects the gut to the brain, and it’s known that infection with the Salmonella bacteria stimulates the expression of certain genes in the brain, which is blocked when the Vagus nerve is severed. This nerve may be stimulated as well by normal gut microbes, and serve as the link between them and the brain. Alternatively, those microbes may modulate the release of chemical signals by the gut into the bloodstream which ultimately reach the brain. These gut microbes, for example, are known to modulate stress hormones which may in turn influence the expression of genes in the brain.This reminds me of an ant whose mind is seized the parasite Dicrocoelium dendriticum and forced to go climb a grass stem and hang there waiting to be eaten by a grazing animal:
Regardless of how these intestinal “guests” exert their influence, these studies suggest that brain-directed behaviors, which influence the manner in which animals interact with the external world, may be deeply influenced by that animal’s relationship with the microbial organisms living in its gut. And the discovery that gut bacteria exert their influence on the brain within a discrete developmental stage may have important implications for developmental brain disorders.
The second intermediate host, an ant (Formica fusca in the United States), uses the trail of snail 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. Most of the cercariae encyst in the haemocoel of the ant and mature into metacercariae, but one moves to the sub-esophageal ganglion (a cluster of nerve cells underneath the esophagus). There, the fluke takes control of the ant's actions by manipulating these nerves. As evening approaches and the air cools, the infected 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, ingesting the ant along with it, thus putting lancet flukes back inside their host. They live out their adult lives inside the animal, reproducing so that the cycle begins again.To my mind, there is nothing quite as scary as being seized and turned into a helpless robot manipulated by alien life to satisfy its needs while you are trapped in your own body. It combines the horror of being paralyzed along with the horror of being used by another for their own benefit and at the expense of your own interests.
Here is a video of a fungus that seizes an ant and grows from its head. It gives me the shivers to watch:
You can read more about this brain-controlling, zombie ant making fungus here.
It's a jungle out there! Whoever can believe the soft soap story about gentle nature, of the lamb lying with the lion, about the noble primitive. No. The real world is fully of cruelly indifferent agents that sometimes cooperate but can also compete with or exploit their fellow creatures, cruelly exploit in the case of these parasites.