Last Updated on June 2, 2019

Evolution does funny things

Over millenia of evolution, each creature figures out a way of spreading and procreating. Viruses take over other cells and make their machinery reproduce the virus. Some viruses, though, get more ambitious. The rabies virus spreads itself from animal to animal by making dogs rabid, so they will bite other animals and spread the virus, which is excreted in their saliva. It changes their behaviour completely, so that it can spread. That is taking over completely!

The parasite Toxoplasmosis, on the other hand, needs to spread from rat to cat. And the story of how it does this, and how such a relatively common parasite may affect the human brain is an enlightening evolutionary story.

Why dopamine?

Dopamine is the risk/reward chemical in the brain. It spikes in anticipation of a new experience, and again when we experience it. But it is a cruel master. If we have the same thing a few times in a row, the second spike disappears and we don’t feel as good when we get it, which is why the new toy fades from a child’s interest and the flashy car or new clothes lose their appeal.

Adding dopamine can therefore make us more prone to take risk, to seek novelty and look for something new or enjoy a new experience.

But could you make a rat fall for a cat?

A different type of love story: A Rat and a Cat

Toxoplasmosis produces dopamine and we know that in the infected rat, this drives the exploratory system of the brain, that seeks out novelty, to like cat urine. Rats and cat urine should not mix (the cat probably being not too far – it is marking territory after all), but this way, it is much easier for a rat to get eaten by a cat. This means that the Toxoplasmosis parasite is then ingested by the cat, in which it multiplies and can complete its life cycle.

The Lifecycle of Toxoplasmosis: From Cat to Rat, but with pigs, sheep and humans as side ‘hosts’

The wonders of evolution!

An unintended consequence

However, people have cats as pets, and so, being exposed (to cat droppings) many are also exposed and become a side host for Toxoplasmosis. We don’t get eaten by cats, but it seems we might get the same treatment as the rat… Extra dopamine. This is where the science gets more interesting (and personal) but as yet not completely clear.

Too much dopamine can lead to strange consequences, including hallucinations (seeing or hearing things that are not there) as well as delusions (thinking that is not tied to reality) and we can see this sometimes by giving patients too much medication that works on the dopamine system.

There is a disease that has all these symptoms as well, Schizophrenia. So researchers have looked to see if there is more Toxoplasmosis infection in patients with schizophrenia than others without, and it seems to be the case.

Precise statistics vary but paint a similar picture: some show around 75% of patients with Schizophrenia have the infection, compared to 50% of age- and sex-matched controls (a sample trying to replicate the general population by comparison), others show that around 46% of patients with Schizophrenia have the infection. What’s more, it seems those who have the parasite also tend to have more severe disease that starts later in life: some studies show symptom severity is worse, others show that those whose disease does not settle (chronic) or needs more invasive treatment (like ECT) have a higher chance of having the parasite (75% or so versus 22% with a mild course).

Studies are now looking at the medications used for Schizophrenia and their effects on the parasite itself, with some showing strong anti-Toxoplasmosis effects of the medications as well.

Links between Toxoplasmosis infection and Parkinson’s disease, which is a neurologic disease of slowly reducing levels of dopamine, are less clear, with some studies suggesting more Toxoplasmosis in those affected, but many others not finding such a link.

The research continues…


Folia Parasitol (Praha). 2015 Jan 1;62. pii: 2015.015. doi: 10.14411/fp.2015.015

Eur Arch Psychiatry Clin Neurosci. 2014 Mar;264(2):179-83. doi: 10.1007/s00406-013-0413-4. Epub 2013 Jun 15

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