Mindless, wandering creatures whose only remaining purpose in life is to serve their parasitic hosts. No, not the terrifying subject of a new George A. Romero flick, but a genuine natural occurrence caused by fiendish little organisms that have evolved brain-controlling abilities in order to survive.

Neuroparasitology is the scientific study of parasites that control the neurological systems of their hosts. By infecting animals in this way, some neurological parasites effectively turn their hosts into real-life zombies.

It seems too weird to be true, but there are several examples of different species that use brain control as a survival tactic. Some experts believe that they are only just scratching the surface when it comes to understanding what these parasites are capable of.

To find out more about these unnerving but fascinating organisms, we spoke to three neuroparasitologists whose innovative research aims to find out more about this bizarre natural phenomenon.

Why do certain parasites choose to affect the host animal’s brain?

Professor Joanne Webster (researching parasites and behavioural alterations at Imperial College London): The brain, or central nervous system, is what is called a ‘privileged site’ for parasites and pathogens, hidden away from the full bombardment of the host’s immune system due to the blood brain barrier. For this reason, many parasites are naturally attracted to the brain and eyes.

Professor Shelley Adamo (researching invertebrate behavioural psychology at Dalhousie University): Some parasites, through a fortuitous mutation or other change, started to alter their hosts brain function—for example, via a secretion. This alteration has increased the success of the parasite, therefore the gene has spread throughout the population.

How many types of brain-affecting parasites are there?

Shelley: No one knows, but we think it’s been underestimated.

Joanne: There are lots of parasites that affect the central nervous system. Classic ones include Taenia solium, the pig tapeworm, where you tend to find the larval stages in the brain. This is a major cause of epilepsy in many parts of the world. Another example is Toxoplasma gondii. Until the advent of superior antiretrovirals for patients with terminal AIDS, it was the reactivation of Toxoplasma gondii that was associated with the insanity and large holes in the brain.

How is the host animals behaviour usually affected?

Shelley: This is highly variable, as it depends on the system. Some parasites reduce behaviours—for example, they halt their host from feeding. Some induce behaviours inappropriately, such as parasites that cause caterpillars and ants to climb to the top of a plant, fasten there and die before raining parasitic spores on more insects.

Charissa de Bekker (researching behavioural manipulation by fungal parasites at Ludwig-Maximilians-Universität): In the case of Ophiocordyceps, also known as zombie ant fungi, we have some clues that serotonin and dopamine levels in the ant’s brain might be altered and certain receptors might be activated or blocked by alkaloids that the fungus produces. Furthermore, we found some clues that the ability of the ants to communicate through chemosensing might be impaired. This is a very important method of communication for social insects such as ants. In addition, the fungus causes inflammation in the brain and might be regulating neuronal stress responses.

Does the host animal ever benefit from its relationship with the parasite?
Charissa: Ants do not benefit at all from being infected. The entire manipulation is tailored to benefit the fungal parasite. Making an infected ant leave the nest and climb up the vegetation is all to make sure the fungus can release its spores to infect new ants and perpetuate its life cycle.

Of all neural parasites, which pose the greatest threat to humans?

Joanne: In terms of altering behaviour and free will, the most dangerous to humans is probably Toxoplasma gondii. Effects on the brains include decreased reaction times and increased risk taking behaviour.

Once infected, how does the host animal regain control of its brain?

Joanne: It doesn’t. We can prevent the behavioural alterations developing, but once established they appear to be permanent.

A stick insect is attacked and parasitised by Cordyceps fungus in Madagascar. Spores enter nervous system, taking control and killing the animal. Fruiting bodies can be seen erupting from dead insect. Photo by Ryan M. Bolton / Shutterstock
A stick insect is attacked and parasitised by Cordyceps fungus in Madagascar. Spores enter nervous system, taking control and killing the animal. Fruiting bodies can be seen erupting from dead insect.
Photo by Ryan M. Bolton / Shutterstock

Could a highly evolved or scientifically engineered parasite take over functions of the human brain, effectively turning people into zombies?

Joanne: Perhaps. We have seen this in the invertebrate systems.

Shelley: It is not likely. Humans have very large brains and parasites are very small. Human brains also have lots of parallel redundant connections, unlike invertebrate brains. It is more likely that humans would experience subtle effects like the decreased reflex speed caused by Toxoplasma gondii. At the other extreme, there could be complete behavioural derangement, such as uncoordinated movements and lapsing into coma.

Charissa: The parasite-host interactions that lead to changed host behaviours are all very specific and unique. This because it is a result of a close co-evolution between that particular parasite and its host for millions of years. The angle of a zombie apocalypse is a popularised one that the public likes to talk about, but it misses the point of the important biology of parasites in general and skips a lot of steps to speculate on something that is very unlikely.

So the chances of a zombie apocalypse being brought about by these parasites are pretty slim, then?

Joanne: Yes. The effects are usually subtle, and if the selective pressures were strong enough we’d likely see counter-evolution in the human host.

Shelley: There is zero chance of this happening. I suspect the odds are greater that the planet will be struck by a life-destroying asteroid.

Phew. In a less extreme scenario, could parasites like these hypothetically be turned into biological weapons using the powers of modern science?

Shelley: Not easily. The known parasitic manipulators cannot be aerosolised.  

Joanne: The effects would probably be too slow and subtle for this to work.

That’s a relief! Thanks for sharing your expertise Joanne, Shelley and Charissa.

@mttyrs

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