Thursday, August 23, 2012

Journal Club: Toxoplasma gondii and Schizophrenia

So I touched on something in yesterday's post that reminded me of a fascinating story I heard while still in college in Dublin during my parasitology course. It's time for another one of those sciency posts! Today I'm going to give a brief introduction to Toxoplasma gondii, some various misconceptions regarding it, and it's possible link to schizophrenia - a relatively new hypothesis that many people I have encountered even in the research community have not heard of yet. Let's start with the basics shall we? What is Toxoplasma gondii?

There it is, the little bastard
Toxoplasma gondii is one of, if not the most common parasite known to man, infecting anywhere from 50 to up to 80% of the world's population. Until recently it was believed that this little critter was largely asymptomatic in humans unless a woman is infected during her first trimester, which can lead to devastating consequences to the child once it is born. Ever heard of pregnant women having to stay away from cats? Well this little bugger is why, although it is actually far more common for people to become infected with T. gondii due to infected meat rather than cats, thanks to its ability to infect just about every mammal. Why do cats get the bad rep though? Because cats are the ones that T. gondii really need in order to stick around.

Like many parasites, T. gondii's life is a two-step process, in that it needs two different species to complete it's life cycle. Mice (or other rodents) eat the oocysts, which are basically the "fertilized eggs" that mom and pop T. gondii have made. Once inside the mouse, it busts and out come the babies, ready to grow and multiply inside the mouse. When a cat eats the infected mouse, T. gondii can sexually mature and produce oocysts, which are pooped out by the cat and ready to infect other unsuspecting mice. The problem at this point is that, in order to increase its chances of infecting it's final host, T. gondii has gotten really good at infecting and surviving in a variety of animals, including sheep, pigs, birds and humans. This means that unless a pregnant woman starts getting a mad craving for kitty litter (or is not in the mood to wash her hands after handling cat turds) she's most likely not going to get infected from a cat, but if she eats lamb or pork chops from animals that are infected with T. gondii, she's in trouble.

This is where it gets interesting

I ees controlling your miiiind

Also like many parasties, T. gondii dabbles in a little mind control when it comes to its intermediate host, in order to further increase its chances of infecting its final host (perhaps a review on mind-controlling parasites in the future, if you're interested!). T. gondii can alter mouse behaviour, making it less afraid of open spaces and more likely to come out during the day, greatly increasing its chances of being caught and eaten by a cat.

Here's the thing, if T. gondii can affect behaviour in mice, does it stand to reason that it can also affect certain aspects of human behaviour, even if it is otherwise asymptomatic?

In 1996 a study conducted in Prague showed that humans that are infected with Toxoplasma gondii had significantly different personality profiles than controls when questioned with Cattell's questionnaire. Since then, there have been numerous studies indicating a link between Toxoplasma gondii infection and behavioural changes in humans.

But how does this link to schizophrenia?

Here's the evidence so far:

1. Anti-psychotic drugs used to treat schizophrenia have anti-Toxoplasma gondii properties in vitro (meaning in a petri dish, not inside an organism).

2. Drugs used in the treatment of schizophrenia are actually better at reducing the T. gondii-induced behavioural changes in rats than drugs used to treat T. gondii infection.

3. People infected with T. gondii seem to have a higher incidence of schizophrenia

Of course Toxoplasma gondii is not solely responsible for the development of schizophrenia (if it was, we'd be in trouble). However, being a very complex and multi-factorial disease, it seems possible that T. gondii infection can exacerbate the situation, tipping the scale in favor of the development of the disease. 

So, what do you think? Am I the only one who finds this shit fascinating?

Or is it a no-go on the review of mind-controlling parasites?

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