Three mind-blowing examples of parasites that hijack their hosts’ brains and bodies

Species of parasitic wasps, worms and single-celled organisms, among others, all affect their hosts’ behaviour to aid their own reproduction and survival. This can include; suicide of the host, decreasing fear of predators so they get eaten, and making them defend or nurture the invader (think cuckoos). There are possibly effects on infected humans, too, including increased risk taking.

Parasites, by their very definition, live on or in another organism, the host, causing it some harm. When it comes to mind control, it isn’t always clear if behavioural change has evolved by the parasite, the host, or is an accidental side-effect of infection. For some host-parasite pairs though, behavioural manipulation is clearly a tactic evolved by the parasite to facilitate its continued, often nauseating, existence…

This parasite makes rodents attracted to the smell of cat urine, something they usually avoid, innately¹. The effect is specific - infected rats don’t become generally unafraid of cats; if they see one before it’s too late, they will still run away.

But it does increase the chances an infected rodent will get eaten by a cat. When it does, the parasite can continue its lifecycle, where it reproduces in the cat’s gut. The eggs are then excreted into the environment to be consumed by rodents, where they develop, and the cycle continues.

T. gondii does infect humans too. This is usually harmless – up to 30 per cent of the world’s population are thought to carry the parasite - though it can be dangerous for pregnant women, or anyone who is immunocompromised.

However, some studies have suggested that latent infection is associated with an increased risk of schizophrenia, aggressiveness, and even car accidents², though the link is not strong, and far from conclusive. It is thought that the parasite is able to change the levels of certain neurotransmitters in the brain.

T. gondii is part of the apicomplexan group of organisms, which are all parasites. These single-celled organisms have been studied by Sanger researchers over the years, to understand their genes, gene activation and how these differ between species to determine which hosts they can infect, their transmission strategies, virulence and zoonotic potential³.

The apicomplexan group also includes the Plasmodium parasites that cause malaria. Perhaps the most researched of apicomplexans, malaria parasites also have complex life cycles, living in human blood and livers, morphing into different forms, before being transmitted via mosquito.

Research has focused on understanding how these shape-shifting parasites hijack the human body⁴, as well as their rapid evolution in response to antimalarial treatments and controls⁵.

Parasitoid wasps inject their eggs into other arthropods, usually caterpillars, using a stinger-like probe. The eggs hatch and eat the caterpillar from the inside, mostly feasting on bodily fluids and avoiding the vital organs, to keep it alive for as long as possible. Some species then burst out of the caterpillar, killing it, and continue on with their lifecycle.

But other species go further. As some species of Glyptapanteles sp larvae exit a caterpillar they ‘plug’ the holes behind them, using an exoskeleton they have moulted, keeping the caterpillar alive. Then, the caterpillar helps them pupate. It stops feeding and stands guard over the clutch of wasp larvae, defending them with violent head swings to knock off any predators⁶. Not behaviour that any peace-loving caterpillar usually exhibits. Eventually, the caterpillar starves to death.

It is not clear how the wasp larvae make the caterpillar do this.

A Thyrinteina leucocerae caterpillar with parasitoid wasp pupae. Image credit: José Lino-Neto, CC BY 2.5, via Wikimedia Commons

Around 25 wasp species in Britain specialise in parasitising spiders. The adults paralyse the spider temporarily and lay an egg where the spider can’t reach. The spider recovers, continues to catch insects, and the wasp larva slowly grows by feeding on spider’s haemolymph (like blood).

Other wasps are ‘hyperparasitoids’ – feeding on other parasitoid wasps. Some species can stick their egg-laying probe in a caterpillar and ‘taste’ whether there is another parasitoid wasp larva inside. They then lay their own egg directly into that wasp larva.

One species of wasp, Amblyteles armatorius, is susceptible to infection by parasitic fungus from the Ophiocordyceps genus. Ophiocordyceps unilateralis, a.k.a the zombie-ant fungus, also manipulates behaviour. It makes its insect host climb up a blade of grass or to the underside of a leaf, grip on, and never let go. A perfect place for the fungal spores to grow out of the insect and then be nicely distributed by the wind.

Several parasitoid wasp species have now had their DNA sequenced for the first time by Sanger Institute scientists, as part of the Darwin Tree of Life project⁷. It may be that the genomic information can be used to find out how the wasps manipulate their hosts – one of nature’s more macabre mysteries.

Sequencing the genomes may also lead to discoveries of new chemical compounds useful to humans. Gavin Broad, from the Natural History Museum, who is also working on the Darwin Tree of Life project said, “Currently, some of the most interesting drugs have come out of things like cone shells, spiders, snakes… But venomous wasps must be a huge untapped resource,”

“They’re doing really complicated things to their hosts, such as arresting development temporarily or permanently, and introducing antibiotics or antifungal agents.”

These worms induce the suicide of their hosts – in this case, insects. The adult worms are free-living in water, where they mate. The larvae then infect crickets or other unsuspecting critters. Once grown (and they are big), the worm causes the insect to find water, jump in, and drown itself, and so returning the worm to water⁸. One species can even survive if their host gets eaten, being able to wriggle out of the predator.

Horsehair worms are included on our list not because Sanger Institute scientists have sequenced their DNA, though they will, but because of the amazing, stomach-turning, videos available to watch: