Parasite Making Mice Unafraid of Cats

The parasite toxoplasma stops mice being afraid of cats, but what happens when the parasite is removed?
23 September 2013

Interview with 

Wendy Ingram, University of California, Berkley


Chris - About a third of people around the world are infected with the parasite toxoplasma.  This could be picked up from undercooked meat or contact with soil that has been contaminated with cat faeces.  There are a range of health consequences for humans, but when mice are infected, it alters their behaviour and makes them lose their fear of cats.  But does eliminating the parasite from the cat's body reverse this effect?  Wendy Ingram is at the University of California at Berkeley - she did the work.  Hello, Wendy.

Wendy -   Hello.

Chris -   So, why does toxoplasmosis, the parasite toxoplasma, affect mice like this?

Wendy -   Well, the parasite naturally wants to be in a cat.  The cat is its Cat and Mouseprimary host and it's in the cat gut where it's able to sexually reproduce and create billions of infectious cysts that then can go on and move throughout the environment and soil or water, and spread much better than if when it's infecting any other mammal or bird.  Like a mouse, it can only move from one animal to one animal.  So, cats are the target locations for these brain parasites.  And in order for it to complete its life cycle, once it's in a mouse or a rat, it means that mouse or rat to be eaten by a cat.  So, the behaviour manipulation is a perfect way for this parasite to get to its target location.

Chris -   So, by changing the behaviour of the mouse and making it lose its fear of cats, because mice are normally very scared of the smell of cats, then it increases the chance that a cat will eat the mouse and then the parasite will go back into the cat where it wants to go.

Wendy -   Correct, yes.  That's precisely right.

Chris -   So, what's the big question that you're trying to answer with your research?

Wendy -   Well, we were very interested in how this brain parasite is manipulating mammalian fear.  There are so many different things we could learn about: either the mouse brain, fear in general.  Also, so many people are infected with this brain parasite.  We don't really know what implications it might have on our behaviour.  These are all just open questions.  So, looking at it from a basic science standpoint, we want to know more.  And so, for this particular study, we wanted to use a parasite that is weakened.  It's not a normal parasite and it doesn't form cysts in the brain and we wanted to see what kind of effects that had on the behaviour of mice.  And if the animals were able to recover from the infection and no longer - the parasite can disappear from the immune system working well, the behaviour effects go away as well.  And we very surprisingly found that that wasn't the case.

Chris -   So, you take some mice, you infect them the same way as they would be infected normally with toxoplasma and do they initially when infected, even though this thing doesn't last for a lifetime in their brain, does it change their behaviour nonetheless and make them initially frightened of cats or lose their fear of cats rather?

Wendy -   Yes, absolutely.

Chris -   And then when the parasite leaves the body, for some reason, that altered fear remains with the mouse afterwards?

Wendy -   Yes.

Chris -   Do you have any theories as to how the parasite is therefore changing the brain of the mouse, so that it is when it's first infected, able to lose its fear of cats?

Wendy -   Well, you can imagine a number of different ways in which either most simply, the parasite is somehow able to go in to the olfactory system, the smell part of the brain and maybe target the specific neuron that's responsible for smelling cat urine.  Basically, they no longer smell the cat, so therefore, they're no longer afraid of the cat.  We don't think that that's the case.  The olfactory system does seem to be pretty good still.  In the mice, we used a test called 'The Hidden Cookie Test' where we did exactly what it sounds like we did.  We hid a cookie under some bedding and then see if the mice can find it and they do.  Another way that it could be affecting the brain in a permanent way would be if the parasite somehow is creating an immune response that the immune system changes permanently and the parasite doesn't have to persist.  It gets in to nitty-gritty detail pretty fast, going into that theory, but that's the one that I'm going to be following up on - looking at the mouse immune system.

Chris -   So, what are the implications then for humans because with so many of us being infected, we're mammals too - we know these organisms, toxoplasma gets into our brains as well when we're infected.  We know some people have suggested there might be a link with mental illnesses including things like Schizophrenia.  Does this suggest then that even if we treated people for toxo, there might be some kind of legacy effect just through having been infected at some time?

Wendy -   Yes, absolutely.  So the researchers who are studying this potential link between Schizophrenia and toxoplasma have suggested that all we need to do is figure out how to cure the parasite.  Get rid of the parasite and we'll cure Schizophrenia.  It's kind of a bit of a brazen statement and this study really showed that that may not be true, that we may have these longstanding changes in the neurobiology that are not curable just like getting rid of the initial cause.

Chris -   Which is a bit of a worry, but at least you've warned us about it.  Wendy, thank you very much.  That's Wendy Ingram.  She's from the University of California at Berkeley and she published the work she was describing there this week in the journal PLoS ONE.


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