When Rats Chase The Cats

The Naked Scientists spoke to Dr Joanne Webster, Imperial College London
14 May 2006

Interview with 

Dr Joanne Webster, Imperial College London


Chris - Your research looks at the parasite Toxoplasma gondii, which most people have probably never heard of. What actually is it?

Joanne - It's a very very common parasite that humans sometimes hear about when they're pregnant. It's a parasite we're told to avoid cleaning out the cat litter for to avoid catching toxoplasmosis.

Chris - But tell us about it's life cycle. What does it do?

Joanne - It's an indirectly transmitted parasite, which means it's got two stages to its life cycle. We've got the cat as the final host for the parasite, and it will have an intermediate host such as a rat or a mouse. What the parasite wants to do is make sure it's transmitted from this rat or mouse into the cat, and that's what it does. It changes the behaviour of its intermediate host to enhance this transmission rate and complete its life cycle.

Chris - How does it change the behaviour of the mouse or the rat?

Joanne - The parasite prefers to exist within the brain of its host and that puts it in a very privileged position in which to achieve manipulation. What it does is to specifically alter those behaviours which are more likely to make it predated by the cat. Cats are immediately attracted to fast moving objects, so the parasite increases the activity of rats and mice. It also makes them less fearful. One of the interesting things is it not only overrides their innate fear of cats but it actually seems to make them attracted to them. So it manipulates the behaviour so that they will actually approach signs of cat presence.

Chris - And is there anything specific about the cat they don't like normally?

Joanne - Normally it seems to be an odour within cat urine. It seems to evoke a very specific physiological response in rats and mice. This is why many scientists use this response when they're developing anxiety reducing drugs. The parasite seems to be interfering with that response.

Chris - How is it doing that and what is the outcome?

Joanne - The mechanism is very much a black box at the moment. It's nothing so crude as having lots of these cysts in the brain. What it seems to be is that it seems to be interacting with dopamine within the brain of these rodents. This is one possibility. Of course the implications are that this is a very common parasite, and about 35% of us in Britain are infected. The parasite infects all warm-blooded animals so the implications are that we'll be seeing these behavioural changes, albeit subtle, in other hosts such as humans.

Chris - And we can get this parasite because it's set up this tissue cystic form in the muscle of all other animals that are infected. So if we eat a piece of meat that's not cooked properly, it can get into us.

Joanne - Absolutely, and that seems to be the most common way that people are infected nowadays. Traditionally it would be coming from cat faeces, but most commonly we're getting it through undercooked meat.

Chris - And the population of France who thrive on cooking their meat at room temperature: how many of them are infected?

Joanne - Yes, it goes up in particular parts of France to 84%. There has been a study finding 92% in Paris, but it's certainly a lot higher in Britain.

Chris - And if there are people picking up this parasite, is there any evidence that it might be leading to behavioural changes or even mental illness?

Joanne - Certainly there are a number of studies by a group in the Czech Republic, and they're doing the same sorts of studies that we do in the rats to see if humans are increase their activity and decrease their reaction times. Indeed they are finding this. In terms of clinical implications there was a study finding that showed you are 2.5 times more likely to be involved in a road accident. What has particularly interested me is that because we've got this parasite within the brain, and if it is indeed altering neurotransmitter levels, then that may have implications in a very few people. A convincing body of evidence is gathering that some mental illness, particularly schizophrenia, may be associated with an infectious origin and toxoplasma seems a prime candidate for this.

Chris - Thanks for joining us and telling us about your research. One of the other points that Joanne made to me on the phone yesterday is that the drugs that can be used to treat diseases like schizophrenia do seem to be toxic to this particular parasite, which suggests that it may have a role to play in diseases like schizophrenia.


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