Cat parasite found to affect human behaviour, sex ratio

Cats are known to have lived with humans for a few thousand years. People keep and love cats for many reasons but the relationship is not without dangers. Cats can carry a parasite called Toxoplasma which can be easily transmitted to humans. According to recent studies by Czech scientists, the parasite can cause significant changes to our personalities and believe it or not, even influence whether infected women give birth to baby boys or girls.

I spoke to Charles University lecturer, Jaroslav Flegr who heads the team of experts who have been studying the effects of Toxoplasma on humans for many years. I asked him first to describe the parasite and its life cycle.

"Toxoplasma is a very common parasitic protozoon and the life cycle of this parasite is not so complicated. But still it needs two different hosts. The intermediate host can be any warm-blooded animal; any bird or any mammal. The definitive host is always some kind of cat. Not just a domestic cat but it can be any kind of cat - a tiger and so on."

How can people get infected?

"In fact, there are two different ways. One possibility is to eat some raw or undercooked meat and the second one - for vegetarians - one can very easily get infected by eating vegetables contaminated with soil in which some cat faeces containing oocytes are present."

What is the prevalence in the Czech Republic compared to other countries?

"It is moderate - not too high and not too low. In some countries it is around 60-80 percent. In other countries, for example in China, it is much lower, it is less 10 percent. In the Czech Republic it is similar as for example in the USA. It is 30 percent."

Your earlier studies suggest the toxoplasma infection can induce personality changes in humans - how does the human psyche change and does the alteration manifest itself differently in men and women?

"Yes, there is a very huge difference between men and women. The infected men are more suspecting and jealous and they don't like to respect the rules of their society. In women, it is quite the opposite way. They are more outgoing, easygoing, warm-hearted and they like to respect the rules of their society."

Have similar alterations been detected in laboratory animals as well?

"Yes, originally it was observed in laboratory animals and also in wild animals. Of course, we cannot measure the differences in their psychological profile. But there are differences in their behaviour. They are less afraid of danger, for example, especially specific danger connected with cats. They are not afraid of the smell of cats and they are less interested in their surroundings, for example."

Does it have a reason? I guess it is in the parasite's interest to change the behaviour of mice in this way - am I right?

"Yes, of course, it is an evolutionary adaptation to increase the probability of transmission from the intermediate host, usually mice, to the definitive host, some kind of cat predator."

Is there a similar reason for humans to have an altered personality or is that just random?

"Nobody knows... It is difficult to say. When we just think about the current situation - of course, there is no possibility to be eaten by a cat, even if we are not afraid of cats, but in ancient times of our animal ancestors it could have been quite different, so it's also possible that originally it was an adaptive [strategy] also in humans or apes."

Your recent study shows that the sex ratio in humans can be influenced by latent toxoplasmosis. That is women who have been infected by the parasite tend to give birth to more boys than girls - have you worked out how it actually works?

"Again, nobody knows but we suspect there is some connection between immunosuppression and the sex ratio. It is known that many embryos are destroyed during pregnancy. It is at least 30 percent but the number of miscarried embryos is probably much higher. The immune system of the pregnant woman plays a very important role in this destruction of embryos. Toxoplasmosis is known to induce immunosuppression. Therefore more male embryos survive this difficult time during the early stages of pregnancy. It is known that male embryos are more susceptible to this destruction at the very beginning of pregnancy, there is about two times more male embryos than female embryos. But at the end of pregnancy, normally, there is the same number of male and female embryos."

And compared to that, what is the ratio in women who have been infected with toxoplasma?

"In women infected relatively recently this ratio is about 2.5:1, so about 250-260 boys are born per every one hundred females."

Have you found any other effects of latent toxoplasmosis on humans?

"Yes, for example, infected people have much longer reaction times and they have impaired psycho-motor performance. It has also some practical impact on the life of the infected people. For example, we have data showing that infected subjects have about 2.5 times higher probability of a traffic accident. Recently, these results have been confirmed but a different study performed in Turkey."

Is this the same for men and women?

"It seems so, there is no difference."