Originally published over at SiMF.
Literally. Parasites hijack our bodies, manipulate our thoughts and emotions, and even use us as incubation chambers for cute little baby parasites. And it terrifies us! The very idea of some outside entity invading us…not just our world, but you and me specifically…gets inside of our heads and makes our skin crawl. Which is why they make such great Sci-Fi monsters! They’re everywhere! Movies, television, literature…we can’t get enough of the creepy little guys. The Xenomorphs from the Alien franchise are arguably one of the most iconic parasitic organisms ever, with a complex life-cycle that requires a human host. Stargate SG-1 has the Goa’uld, parasites that integrate with their human hosts rather than bursting out of their chests. Even in children’s literature you have the Yeerks; evil space slugs from the Animorphs novels who tried to enslave humanity via the ear canal.
Noticing a bit of a trend? If you look at AMC’s list of the Top Ten Parasite Movies, more than half of them feature an alien parasite. Even though parasites are common right here on Earth, their life cycle is so radically different from ours it feels alien. Especially to those of us living in developed countries with treated water and clean food, where parasitic infections have been largely eliminated. In many parts of the world however, especially in areas with poor sewage treatment, parasites are still a day-to-day problem.
What most people imagine when they think of the term ‘parasite’ are intestinal parasites. Hookworms, roundworms, pinworms and the like colonize the intestinal tract as adults, laying eggs that are shed in feces. The eggs and larvae then spread via contaminated water to other animals, including humans. Gross. They don’t reach full maturity until they find a host though, so their life cycle actually depends on obtaining this ideal host environment. Different species of parasites have adapted a wide variety of techniques for improving their odds of finding the host they need.
For example, Toxoplasma gondii is a parasitic germ whose primary host is cats. It can infect other animals, but is only capable of sexual reproduction in cats. When T. gondii infects rats it actually alters their brain chemistry to make them attracted to the scent of cat urine, thereby increasing the likelihood that the parasite will end up inside of a cat, along with its current host. It’s a very subtle manipulation of the intermediate host to get the parasite where it wants to be. Kind of like mind control.
Opiocordyceps unilateralis, known as the zombie-ant fungun, functions in a way that is entirely like mind control. When this parasitic fungus infects a carpenter ant, it manipulates the host into leaving the colony and attaching itself to the underside of a leaf. The fungus prefers to grow in this environment. When the ant dies, the fungal stalk erupts from its body and produces spores…which drift down to the forest floor and infect other ants.
Most parasites try to avoid killing their hosts. Why mess with a free ride, after all? But that is certainly not always the case. Certain species of wasp, known as parasitoid wasps because they ultimately destroy the host, lay their eggs inside other insects. When the eggs hatch, the juvenile wasps eat their way out of the living host without so much as a ‘thank you’. Parasitoid wasps are often cited as the inspiration for the Xenomorph life cycle.
So you see, we don’t need to look to outer space or science fiction to be creeped out by parasites. There’s plenty of creepiness to go around here at home.