This parasitic fungus takes over the brain and then ejects its spores out of the ant’s head (Image Credit: Erich G. Vallery, USDA Forest Service – SRS-4552, Bugwood.org, CC BY 3.0 US)

Mind Control: How Parasites Manipulate Cognitive Functions in Their Insect Hosts (2018) Libersat et al.,  Frontiers in Psychology, https://doi.org/10.3389/fpsyg.2018.00572

The Crux

The field of neuro-parasitology is a relatively new field in biology and deals with the study of parasites that manipulate the nervous system of their hosts for their own gain (usually at the expense of the host).  The authors of this review focused on host-parasite interactions between insect hosts and their myriad of parasites, due not only to most studies in this field being done with insects, but also the fact that most animals on the planet are in fact insects. 

Did you know: Human manipulation Toxoplasmosis is an illness caused by a parasite that infects rats and mice (intermediate hosts) until they are consumed by cats (the final host).  The parasite infects the brain and causes a series of changes in the host, the most noticeable being the switch from avoidance of cat urine to an attraction.  This makes the manipulated host much more vulnerable to predation, facilitating the parasite’s transmission to its final host.  Interestingly, humans can get toxoplasmosis as well.  The parasite alters human brain chemistry, affecting both personality and motor skills.  Although humans would nowadays be considered a dead-end host, as we are not at risk of being eaten by the final host, this was not always the case.  In the not-to-distant past, our ancestors were subject to predation by large cats (tigers, lions, etc.).  Research has shown that chimpanzees, our closest relatives and prey items for leopards, show no aversion to leopard urine when parasitized, whereas the healthy chimpanzees avoided the urine.  This makes it possible that this behavioral manipulation evolved because our ancestors were suitable intermediate hosts. 

How it Works

The authors reviewed the available literature on parasitic-manipulation of hosts and separated them into two different categories: manipulations that affected solitary hosts, and manipulations that affected social insects.  For the solitary manipulation, the authors further broke down the types of manipulations into three subcategories: suicidal behavior, bodyguard behavior, and host’s motivation to move. 

What they found out

Suicidal Behavior

Parasites that manipulate the host to kill itself do so in order to increase their own reproductive success.  Parasitic hairworms infecting crickets make the cricket drown itself, where the worm can then emerge and reproduce in the aquatic environment.  This can also be seen in liver flukes, where the fluke makes an ant put itself in a position to be eaten by a grazer, its final host.  Additionally, some parasites are just lazy.  The crypt gall wasp lays eggs in oak stems, where the larva induces the growth of galls (hollow growths) and lives inside until it is ready to emerge as an adult.  Crypt-keeper wasps, however, steals this gall by laying eggs inside of the larval gall wasp after it has already made the gall.  Now the gall wasp, instead of emerging once it grows into an adult, will plug the hole with its head and die, sealing the gall, after which time the crypt-keeper wasp will consume/live inside of its host. 

Bodyguard Behavior

Organisms with complex life cycles tend to be vulnerable during their transitions between stages.  To get around this, parasites manipulate a host into guarding them during this crucial in-between point.  A striking example is that of wasps and caterpillars, where a female wasp will lay her eggs inside of a caterpillar.  These eggs hatch and the larvae grow inside of the caterpillar, eating it from the inside.  When they have grown enough, the larvae crawl out of the caterpillar and spin cocoons around themselves to transition to adults.  Interestingly, not all of the larvae emerge.  Some remain inside of the caterpillar and make it violently thrash around, scaring away any would-be predators from the vulnerable cocoons. 

Motivation to Move

Many of the previously mentioned parasites use host organisms as both incubators and food sources for their offspring, laying eggs inside of the host where the larvae then consume them.  Some parasites do more than just hijack the hosts body, some hijack their thoughts.  Yet another species of parasitic wasp stings cockroaches, injecting venom that makes the cockroach remain stationary.  The cockroach CAN move, but the venom affects the brain in a way to make the cockroach not WANT to move.  This makes the cockroach a perfect food supply for the offspring of the wasp. 

Sociality

Social insects, like ants and termites, depend on a societal structure to operate.  Different ants perform different tasks, like the workers and soldiers, with this division of labor allowing the entire colony to act as a superorganism.  So-called “social parasites”, like a specialist caterpillar, are able to infiltrate these colonies using chemicals to mask their presence, moving into the hive and eating the queen larvae.  Ironically, this social parasite is parasitized by yet another parasite (another wasp, what else did you think it would be?).  The wasp lays eggs inside of the caterpillars, where they will eventually emerge from the caterpillar’s cocoon after consuming it.  Like the caterpillar, the wasp uses chemicals to accomplish its goal, but instead of making itself undetectable its chemicals drive the ants into a frenzy, distracting them while the wasp seeks out and parasitizes the caterpillar. 

Any Problems? 

This study is limited in that it focuses on studies of insect parasitism; however parasites are a ubiquitous force in nature, infecting everything from bacteria to whales.  The authors made the point that most of the animals on earth are insects, thus most studies of behavioral manipulation by parasites are done with insects.  But I would like to know if the general patterns in this review (suicidal behavior, bodyguard behavior, etc.)  are also seen in other systems in which a parasite manipulates its host. 

So What? 

The multifarious manipulations of hosts by their parasites in this review have shown us how changes in brain chemistry affect behavior.  This is important for two reasons, the first being an illustration of the awesome power of evolution and natural selection.  It is sometimes difficult to remember that evolution isn’t working towards some goal, it is simply a process in which whatever works best wins out.  The many different methods by which parasites can manipulate their hosts evolved over time because these traits increased the fitness of the parasite, leading to the remarkable adaptations we see today. 

The second reason this review is important is because we will be able to learn more about the link between brain chemistry and behavior.  We know that chemical reactions determine thought processes and behaviors, and by studying the way these parasites manipulate these processes, and subsequently change the behavior of the host, we may be able to harness that knowledge and use it in the field of human mental health.