The lancet fluke Dicrocoelium dendriticum, forces its ant host to clamp itself to the tip of grass blades, where a grazing mammal might eat it.


It’s in the fluke’s interest to get eaten, because only by getting into the gut of a sheep or some other grazer can it complete its life cycle.

Another fluke, Euhaplorchis californiensis, causes it’s host fish to jump out of the water, greatly increasing the chance that wading birds will grab them.

If you think that flukes make their hosts behave differently, wait until you meet Toxoplasma gondii.


This single-celled parasite lives in the guts of cats, sheddding eggs that can be picked up by rats and other small animals that can be eaten by cats.

Toxoplasma forms cysts throughout its intermediate host’s body, including the brain.

And yet a Toxoplasma-ridden rat is perfectly healthy.

That makes good sense for the parasite, since a cat would not be particularly interested in eating a dead rat.

But scientists discovered that the parasite changes the rats in one subtle but vital way.

Toxoplasma-carrying rats, for the most part indistinguishable from healthy ones, were more likely to get themselves killed.


They are not in the least bit afraid of cats!

Scientists have speculated for years that Toxoplasma secreted a substance that was somehow altering the patterns of brain activity in the rats. This manipulation likely evolved through natural selection, since parasites that were more likely to end up in cats would produce more young.

The scientists were only too aware that humans can be hosts to Toxoplasma, too.

People can become infected by its eggs by handling soil or cat litter.

For most people, the infection causes no harm. Only if a person’s immune system is significantly weakened does Toxoplasma grow uncontrollably.

It’s the reason pregnant women are advised not to handle cat litter and why Toxoplasmosis is a serious risk for people with HIV/AIDS for example.

Otherwise, the parasite lives quietly in people’s bodies (and their brains).

It’s estimated that about half of all people on Earth are infected with Toxoplasma.

Given that human and rat brains have a lot of similarities (they share the same basic anatomy and use the same neurotransmitters), the question becomes inevitable:

If Toxoplasma can alter the behaviour of a rat, could it alter human behaviour?

Some schools of thought believe that Toxoplasma changes the personality of its human hosts, having a different effect on men and women.

Parasitologist Jaroslav Flegr of Charles University, Prague, administered psychological questionnaires to people infected with Toxoplasma.


Those infected, he found, show a small, but statistically significant tendency to be more self-critical and insecure.

Paradoxically, infected women, on average, tend to be more outgoing and warmhearted, while infected men tend to be more jealous and suspicious.

Though controversial and disputed by some, it attracted the attention of E. Fuller Torrey of the Stanley Medical Research Institute in Bethesda, Maryland.

Torrey and his colleagues had noticed some intriguing links between Toxoplasma and schizophrenia.

Infection with the parasite has been associated with damage to a certain class of neurons (astrocytes).

Very similar to schizophrenia.

Pregnant women with high levels of Toxoplasma antibodies in their blood were found to be more likely to give birth to children who would later develop schizophrenia.

It’s conceivable that exposure to Toxoplasma causes subtle changes in most people’s personality, but in a small minority, it has more devastating effects.

So Fuller and other scientists combined their research to find an answer to the next logical question…

Can drugs used to treat schizophrenia help a parasite-crazed rat?

They reported their results to the Royal Society B.


They treated the rats with haloperidol and several other anti-psychotic drugs.

They discovered that the drugs made the rats more frightened.

They also found that the antipsychotics were as effective as pyrimethamine, a drug that is specifically used to eliminate Toxoplasma.

So what is Toxoplasma releasing into the brain to manipulate its hosts?

And how does that substance give rise to schizophrenia in some humans?

The research continues..

Read more about Toxoplasmosis here:

Toxoplasma and Rhesus Negative Bloodtypes




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