Cognitive Manipulation by parasites

Parasite-host interactions have been perfected over millions of years of evolution and reached surprising and even unnerving preciosity. The diversity of parasites that can manipulate behavior ranges from viruses to insects and even worms. In some of the most fascinating manipulations, the parasite taps into the host brain's neuronal circuits to control the host's cognitive functions. To exert long-term behavioral manipulation of the host, the parasite must secrete compounds that can modify gene expression with long-lasting effects. This way, parasites hijack the decision-making ability behavior of their host. Mind-altering manipulation can occur in practically all animals. Nevertheless, the phenomenon is often studied in insects due to their simple nervous system and their ability for goal-directed behavior. For example, when presented with two very near concentrations of the same odor, it takes longer for fruit flies to decide, and mistakes can occur. 

Suicide manipulation: 

Fungus infection manipulates insect behavior. The fungus (Ophiocordyceps unilateralis) infects ants. The fungus feeds on the ant's innards. The fungi produce chemicals that alter their hosts' navigational sense; the infected ant is compelled to leave its nest and seek a plant in a humid area to climb to its top. The ant clamps its mandibles around a leaf or leaf stem, a behavior that has never been reported for uninfected ants. The fungus feeds on the ant's brain until it finally produces spores, killing the animal. The spores explode and spread from the top of the tree to infect other ants and start another cycle. 

In another example of parasitic infection, the Lancet liver fluke takes over the ant's navigational skills by migrating to the ant's head and settling near one of the cerebral ganglia. Unlike normal behavior, the ant climbs to the tip of a grass blade to be eaten by a grazing animal. The mature Lancet expels its eggs via the digesting system, allowing ants to be infected. 

Another example is a hairworm, which requires water for its reproduction. The hairworm infects crickets and other terrestrial insects and develops inside their bodies. The mature hairworm causes the insect to seek out water. When the insect falls into the water, the parasite emerges to start reproduction.

Offspring Care:

Although solitary insects are not known to provide care and safety to their offspring, one of the parasites' most fascinating behavioral manipulations is to coerce a host to care for the parasite's offspring. Developmental symbiosis occurs when individuals of one species cannot reach maturity without another's aid. For example, a wasp sting injects her eggs into a caterpillar. The wasp larvae mature by feeding on the host and emerge after two weeks. However, one or two larvae remain within the caterpillar. These siblings activate the caterpillar's muscles to violently move its upper body back and forth. The behavior protects the pupae by deterring predators.

The caterpillar's aggressive guarding behavior can also occur in other insects without parasites in the host's body. For example, a single wasp larva develops inside the body of a ladybug, and it spins a cocoon between its legs. The ladybug is alive, but its programming triggers twitching and convoluting body movements, which keep the wasp pupa safe from potential predators. It is thought that the parasite co-infests with a virus, which induces a severe neuropathy and antiviral immune response that correlates with the symptoms of motor twitches. 

Another example is the butterfly Maculinea arion, whose larvae mimic the smell of those of a particular ant species, causing its members to carry them into their nests, caring for them as their own. The sophistication and effectiveness of parasite infection might have given the idea for the simulation hypothesis. 

Infections in People

The fungal pathogen Candida auris can cause bloodstream infections that can be fatal in 60% of cases. Toxoplasma infection can be linked to mood disorders and mental health issues, including schizophrenia and behavioral changes, such as promoting risky behaviors.

Watch a fascinating video: in praise of parasites

https://youtu.be/eOCiIgYQaFo

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