6 Of Nature's Most Horrifying Mind-Controlling Parasites

Parasites have it hard, in some ways. Lots of them can only reproduce in certain animals, or certain places. But they can't move around on their own – they have to rely on their hosts to move around for them. So some of them take matters into their own hands and control the brain of their host, like fleshy puppets.

Ophiocordyceps unilateralis is a fungus. It lives on forest floors, but to reproduce – to spread its spores – it needs to get higher up. So it controls ants' brains. Any unfortunate ant which brushes the fungus is doomed: Ophiocordyceps slowly grows into its brain and begins controlling it with remarkable precision.

The ant begins wandering at random, up and down plants, until it hits a spot on the underside of a leaf about 25cm, give or take a centimetre or two, above the forest floor, where the fungus stops it. Having walked its zombified host to the right spot, the fungus makes the ant bite into a vein on the leaf, and stay there until it dies – and then, a few days later, a fungal stalk bursts out of the dead ant's head, and sprays spores onto the forest floor, to infect the next generation of zombie ants.

The 25cm is important: Much lower, and the dead ant gets eaten by scavengers before it can sprout; much higher and it messes up the reproduction. "Like, I'm not very good at estimating 25cm, but somehow the fungus manages to get the ant to die pretty close to 25cm off the forest floor by remote control," says Dr Kelly Weinersmith, a Huxley fellow at Rice University in the US who studies how parasites control host's behaviour. "Freaking amazing."

The California killifish is a very common fish around southern California and Baja California. The trematode worm Euhaplorchis californiensis, which infects it, has a tricky task: getting out of the brain of the fish and into the gut of a predatory bird, where it can reproduce. So it makes the fish dance.

"Adult killifish can have up to 8,000 of these parasites packed in around its brain," says Weinersmith. The worms make the fish act very oddly. "My colleague Alejandra Jaramillo described it best by saying that infected fish appear to be 'dancing' through the water, rather than swimming through the water."

This conspicuous behaviour brings the infected fish to the attention of herons and other birds. "Infected fish are 10 to 30 times more likely to be consumed by predatory birds than are uninfected fish," says Weinersmith, and research has shown that the more parasites a fish has, the more likely it is to get eaten. Kelly's own research is into how the parasites change the fish's behaviour.

The Sacculina barnacle castrates crabs. Then, as if that weren't enough, it takes over their brains.

It infects both male and female crabs, but what happens to the males is particularly interesting. The barnacle (which looks nothing like a barnacle) grows in the spot where female crabs carry their eggs – and convinces both sexes to care for it as if it were its own children. Since the male crabs don't usually care for children at all, that involves some brain manipulation. "It feminises male crabs, and gets them to engage in the kinds of behaviours that the females crabs typically do to protect their offspring," says Weinersmith. "Except now they're protecting the parasite offspring instead."

If you're a sexually transmitted infection, you need your host to have sex. Of course, animals will tend to have sex of their own accord, eventually – but much better to make your host extra horny, and move the whole thing along a bit.

The snappily named "insect iridovirus IIV-6/CrIV" infects a species of cricket, Gryllus texensis. Then it sterilises it. The females stop producing eggs; the males' sperm stop swimming and become useless. Then it kills the cricket. But in the intervening time, it works its magic.

A researcher, Dr Shelley Adamo, whose stock of lab crickets was accidentally infected with the virus, noticed that in the days before it killed them, the crickets had more sex than usual. Normally, ill crickets – like ill humans – are less likely to be interested in sex. She carried out an experiment, and found, sure enough, that "infected males were quicker to court females than uninfected controls".

"There's growing interest in parasite manipulation of reproductive behaviours," says Weinersmith – not just in crickets. Adamo, the author of the cricket study, says there is evidence that other sexually transmitted infections suppress "sickness behaviour", so that they remain sexually active (and sexually attractive).

• Note: The crickets in the above GIF are completely the wrong species and are just being used as illustration for those of you who wondered what two crickets having sex looks like.

The flatworm Leucochloridium paradoxum spends part of its life in snails. But to reproduce, it needs to be in the gut of a bird. So – like Euhaplorchis californiensis, above – it needs to get one host eaten by another host.

Leucochloridium takes a double route. First, it controls the behaviour of the snail – driving it to the exposed topside of leaves, where it can easily be spotted by birds. (This was first claimed in the 19th century, but not confirmed by proper scientific research until 2013, as Ed Yong recounts here.)

Second, it does something completely disgusting: It creates a large brood sac inside the eye-stalk of the snail, and pushes its larvae into it, one after another. The larvae twitch and wriggle inside the eye, making it look (to birds, apparently) like a tasty caterpillar – although, as Yong points out in his piece, while that has been the accepted wisdom for 140 years, there haven't actually been any studies that prove it conclusively.

Toxoplasma gondii might be the most famous brain-controlling parasite of all. It's a single-celled organism that can infect almost any warm-blooded animal, although it only reproduces sexually in cats. In rats, infection by T. gondii has been shown to affect their behaviour – making them attracted to the smell of cat urine and fur, and less afraid of new things in their environment, two behaviours which may make them more likely to be killed by cats. (The parasite can also be transmitted between rats sexually – and there is evidence that infected male rats are more sexually attractive to female ones.)

But where it gets unsettling is the possibility that the parasite, which is hugely common in humans, affects our behaviour too. "There are lots of studies finding correlations between [human] infection and behaviour," says Weinersmith. "There was also a study that found a correlation between country-wide neuroticism scores and the percentage of the population infected by Toxo, suggesting that the parasite influences culture."

One researcher, Jaroslav Flagr – himself a carrier of the parasite – found when testing Czech students (40% of Czechs are carriers), that people infected by T. gondii had slower reactions than the uninfected. And, interestingly, men who were infected were more likely to be introverted and unsocial, while women who were infected were more outgoing and gregarious. There's a great long piece on Flagr's research into human T. gondii infection in The Atlantic.

Weinersmith thinks it's likely that Toxo really does manipulate human behavior, but says it will be much harder work to prove it conclusively. "We'll never be able to conduct the kinds of experiments that will really nail down the effects of Toxoplasma gondii on human behavior," she says. "That would require infecting a random subset of folks with the parasite, and then quantifying how their behavior changes. That pesky ethics thing makes this kind of experiment impossible." The best we can do is follow people who have been infected with it and compare them to people who haven't, but that makes it harder to tease out cause and correlation. After all, people who own cats are probably more likely to be infected by a cat parasite. "If people who own cats have different behaviours than non-cat owners, this could mess up our interpretation of the results," says Weinersmith.

However, there is at least one other ironclad example of a parasite which controls human behaviour: rabies. Humans and other mammals infected by the rabies virus develop hydrophobia – a fear of water – and aggressive behaviour, including biting. Since the virus is spread by saliva, that makes sense: Biting for obvious reasons, while water could wash infected saliva away and make the bites less infectious, so a fear of water is good for the virus. Clearly, humans are not beyond the reach of behaviour-controlling parasites.