Many parasites are satisfied with just living off of their hosts, while others decide their hosts must die. But there are also some parasites who can change their hosts' behavior or physiology in ways fit only for science fiction. Here are 12 parasites who manipulate their hosts in incredible ways.
Top image by Dick Belgers via Wikimedia Commons
1. Hymenoepimecis "Build Me a Web!" argyraphaga
Hymenoepimecis argyraphaga is a Costa Rican parasitic wasp that terrorizes the spider Plesiometa argyra. When it's time to procreate, an adult female wasp will seek out a spider, paralyze it and then lay an egg on its abdomen. After hatching, the larva wasp will feed on its host, while the spider goes about its business like nothing's wrong.
Then things get interesting. After a couple weeks of bloodsucking, the larva will inject a chemical into the spider, which causes the spider to build a web like none it's ever built before. The spider sits motionless in its creation — which is far from pretty, but super durable and able to withstand pelts of rain — to await its fate. The parasite then kills the spider with poison, sucks it dry and builds a cocoon that hangs from the middle of the new web.
Image via William G. Eberhard.
2. Toxoplasma "Do That Cat!" gondii
If Tom & Jerry taught us anything, it's that cats and rodents typically don't get along. In fact, rats inherently know the smell of cat urine and run from it like their lives depend on it (because, well, it does). But if a rat is infected by the single-celled parasite Toxoplasma gondii, it loses its instinctual fear of cat pee. Worse yet, the parasite appears to make the rat think it's sexually attracted to the revolting odor. T. gondii does all of this to increase the chances of its host getting eaten by a cat, so that it can happily complete its lifecycle in its new feline friend.
Image by Jitinder P. Dubey via Wikimedia Commons.
3. Dicrocoelium "Climb That Grass, but Only at Night!" dendriticum
The Lancet liver fluke Dicrocoelium dendriticum has a very busy life. As an adult it spends its time in the liver of a cow or another grazing mammal. Here it mates and lays eggs, which are excreted in the host's feces.
A snail eats the poo, taking in the eggs at the same time. The eggs hatch in the snail and make their way into its digestive gland, where they asexually reproduce. They then travel to the surface of the snail's body. As a defensive maneuver, the snail walls the parasites up in cysts and coughs up the balls of slime...doing exactly what the parasites wanted it to do.
An ant comes along and gobbles up the fluke-laded slime balls. The flukes then spread out inside of the ant, with a couple of them setting up shop in the insect's head. When night approaches, the flukes take control. They make the ant climb up a blade of grass and hold tight, waiting to be eaten by a grazing animal. If the ant is still alive at dawn, the flukes release their control and the ant goes about its day like normal (if the ant baked in the sun, the parasite would die, too). At night the flukes take over again and the cycle repeats until the ant becomes cattle food.
Image by Adam Cuerden via Wikimedia Commons.
4. Myrmeconema "Go Red Berry Bum!" neotropicum
When the nematode Myrmeconema neotropicum gets into Cephalotes atratus ants, it does something rather unique: It makes the ant look like a berry. You see, these South American ants are black, but they live up in the tropical forest canopy, where there are a lot of red berries. So the nematode takes advantage of this fact by making the ant's gaster (its bum, basically) look exactly like a red berry. Infected ants also tend to be sluggish and walk around with their bums in the air, making them all the more appealing to fruit-eating birds.
Image by Steve Yanoviak via Wikimedia Commons.
5. Spinochordodes "Jump to Your Death!" tellinii
Spinochordodes tellinii is a nematomorph hairworm that infects grasshoppers and crickets. As adults, the parasitic worms live in water and form writhing masses to breed. Grasshoppers and crickets ingest the worms' microscopic larvae when they drink the infested water.
The hairworm larvae then develop inside of the insect host. Once grown, they release powerful mind-controlling chemicals that sabotage the insect's central nervous system. The evil hairworms force the insect to jump into the nearest body of water, where it subsequently drown. Yes, the hairworms actually cause their hosts to commit suicide. The parasites then escape their deceased host and the cycle begin anew.
Image via Wikimedia Commons.
6. Glyptapanteles "Protect Me!" sp.
Glyptapanteles is a genus of parasitic wasp that often infects Thyrinteina leucocerae caterpillars. The cycle begins when an adult wasp lays its eggs inside of a helpless baby caterpillar. The eggs hatch and develop inside of the caterpillar, as the caterpillar itself grows up. When the larvae are full-grown, they emerge from the caterpillar and pupate nearby. But it seems the larvae somehow induced a kind of Stockholm syndrome in their former host. The caterpillar host stops feeding, but remains close to its parasites and will even cover them with silk. If a potential predator comes by, the caterpillar will defend the pupating wasps with violent head-swings.
Image by José Lino-Neto via Wikimedia Commons.
7. Leucochloridium "Pulse!" paradoxum
Leucochloridium paradoxum is a parasitic flatworm commonly known as the green-banded broodsac (you'll see why it has this cringe-inducing name in just a moment). L. paradoxum spends most of its life in the body of a bird, which doesn't seem to mind the parasite's presence all that much. The flatworm breeds inside of the bird and its eggs get passed through the feathered host's digestive tract.
The bird poops out the eggs and — you guessed it! — a snail comes along and eats it. In its larval stage, the parasite travels to the digestive system of the snail, where it develops into the next stage, the sporocyst. They rapidly reproduce and form long tubes of swollen "broodsacs." As the broodsacs grow, they spread out into the snail's eyestalks, preferring the left tentacle for some insane reason. Here, the broodsacs pulse green and yellow, causing the snail's eyestalks to resemble caterpillars, which birds love.
But the parasite's manipulation doesn't stop there. Snails prefer the dark, so the broodsacs override this behavior and cause the snail to seek out light. Once in the light, the broodsacs twitch, becoming absolutely irresistible to birds.
Image by Thomas Hahmann via Wikimedia Commons.
8. Ophiocordyceps "Zombify!" unilateralis
Some species of carpenter ants prefer to build nests high up in the canopy of trees and only come to the forest floor to forage. But that plan gets shot to hell when the fungus Ophiocordyceps unilateralis gets involved. If the fungus infects an ant, the insect becomes compelled to leave its treetop home. It climbs down to one of the lower leaves and clamps down with its mandibles until it dies. The fungus consumes the ant's tissues — all except for the muscles controlling the mandibles — and grows inside of it. After a couple of weeks, the fungal spores fall to the ground to infect more ants. Ants infected by this particular fungus are often called "zombie ants."
Image by David P. Hughes & Maj-Britt Pontoppidan via Wikimedia Commons.
9. Sacculina "Serve Me!" carcini
Sacculina carcini barnacles start life as tiny free-swimming larva, but once they find their crab host, they become so much more. The female larva is the first to colonize its crustacean host — she attaches herself to the underside of a crab, forming a bulge in its shell. She then spreads root-like tendrils throughout her host, which she uses to draw nutrients.
As she grows, the bulge in the crab's shell turns into a knot. A male Sacculina then comes along and implants himself inside of his mate, where he produces sperm. The pair then continuously reproduces. At this time, the helpless, now-castrated crab essentially becomes the barnacle's servant. It stops molting and growing, and actually begins to take care of the barnacle's eggs as if it were its own. And this doesn't just happen with female crabs.
When the barnacle infects a male crab, it sterilizes the crustacean and alters its body to resemble that of a female crab by widening and flattening the abdomen. It then forces the crab's body to release certain hormones — the male crab begins to act like a female crab, even to the point of performing female mating dances. It also takes care of the barnacle's eggs.
Image by Hans Hillewaert via Wikimedia Commons.
10. Schistocephalus "Turn Up the Heat!" solidus
The bird tapeworm Schistocephalus solidus is not one to stick around in a single host for too long. As an adult, it reproduces inside of the intestines of fish-eating water birds. The tapeworm's eggs get delivered to water in a nice package of bird feces. Once in water, the eggs hatch into their larval stage and get eaten by small crustaceans called copepods. Sticklebacks then eat the copepods.
When inside a stickleback, the tapeworm shows its true power. It makes the fish seek out warmer waters, which it needs to grow rapidly. And grow it does. The tapeworm can actually get so big that it outweighs its host.
When it's time to make its way into a bird's gullet, the tapeworm causes the fish to pull a Jeckyll-Hyde transformation: the stickleback becomes bolder and more solitary, which essentially makes it a more appealing prey for a fish-eating bird.
Image via Wikimedia Commons.
11. Euhaplorchis "Dance for Me!" californiensis
Like several other parasites on this list, the parasitic worm Euhaplorchis californiensis has several hosts. The worm's life begins in the horn snails found in the salt-water marshes of Southern California. Inside of their sterilized hosts, the worms produce several generations of offspring, which then leave the aquatic snail in search of killifish.
Once the parasite finds its new host, it latches onto the gills of the killifish, and then makes it way to the fish's brain cavity, where it forms a carpet-like layer over the fish's brain. Here it releases chemicals that mess with the fish's central nervous system. Infected killifish perform a complex dance routine involving the shimmy, the jerk, the flash and the surface. With these cool moves, the fish are 10 to 30 times more likely to get eaten by birds than uninfected fish. Inside of the birds, the fish breed and their eggs are pooped out, to be eaten by unsuspecting horn snails.
Image via Wikimedia Commons.
12. Heterorhabditis "Back Off!" bacteriophora
Heterorhabditis bacteriophora is a nematode that has set out to break the mold. Most parasites, it would seem, manipulate their hosts in ways that make them get eaten; this allows the parasite to complete its life cycle. H. bacteriophora, on the other hand, tells hungry predators to back off.
When the nematode infects an insect larva, it gradually changes the color of its host's body from white to red. This vivid color is a warning color to predators — in fact, robins avoided eating the red larva in experimental studies. The parasite needs the larva, which it liquefies and feeds on, and would actually die if its host got eaten.
Image by Peggy Greb.