Can a common parasite cause behavioural changes in humans?

Over the years the parasite Toxoplasma gondii has made the headlines on several occasions. It is considered one of the most successful parasites as it infects a wide range of birds and mammals, including humans, causing the disease Toxoplasmosis. It is estimated to infect approximately 30% of the world’s population, with some countries estimating up to 80% of infected population. It achieves part of its life cycle in the intestinal cells of cats, and when it infects other hosts, T. gondii can reside in their bodies for their life time where it can infect many other tissues, including the Central Nervous System and its major organ, the brain.

Many scientific studies that have focused on the interaction between T. gondii and brain cells suggest that it may be responsible for many brain disorders, including schizophrenia, and behaviours, such as risktaking. This has certainly captured the attention of many sensational headlines, such as: ‘A game of cat and mouse’- (The Economist, June 2010), where it presented the case that the nations that have won the world cup multiple times have a higher incidence of people who have antibodies to T.gondii, ‘Invasion of the mind controlling zombie parasite’ (NPR- October, 2011), ‘Extreme PMS is your cat’s fault’ (The Sun, January 2018) and ‘Can cat poo parasite turn you into a millionaire’ (The Guardian, 2018). These are quite scary headlines and recently T.gondii has hit the headlines once more, highlighting mechanisms by which T.gondii can cause Schizophrenia. 

The evidence that T. gondii manipulates behaviours and brain function is plentiful. Since the late 1970s, Scientists have observed behavioural abnormalities in mice and in the last 15 years, observations have been made that mice infected with T. gondii are attracted to cat’s urine, are not afraid of open spaces and take more risks. In these studies, these behaviours are suggested to be the result of T. gondii manipulation of the brain in order to promote its life cycle. Recall that part of its life cycle occurs in the cat, and mice and birds are often eaten by cats. Hence, its life cycle is complete. However, T. gondii infects a wide range of other animals and humans, so how can this manipulation in behaviour be explained in these organisms? It is reported that humans, who are T. gondii positive (having a new or old infection) appear to have more risk-taking attitudes. One study observed that persons involved in road traffic accidents are more likely to have T. gondii. Other studies observed that there is a correlation between T. gondii positivity and Schizophrenia. The most recent headlines focus on a study by Carrillo et al., (2020), which demonstrates that a group of immune cells in the brain, called microglia, important for brain health maintenance and defence against infectious agents, can be manipulated by T. gondii.This work follows on from a previous study by the same group that illustrates that T. gondii infection can alter an important enzyme in GABA synthesis, a molecule that inhibits certain signals in the brain. Carrillo et al., (2020) concludes that T. gondii can also induce microglia-neuron contact, allowing the microglia to surround the neuron, thus preventing synapse function. It is clear that T. gondii is exerting an influence on brain biochemistry and physiology. Other studies have also evidenced a suite of different mechanisms, by which this parasite can change our brain. However, in addition to the direct T.gondii influence on brain biochemistry, which is how cells and molecules interact in the brain, there is also an indirect route that must be considered, which is that of the immune response. Since, T.gondii is an infectious agent, our body’s immune reaction against it will promote inflammation and this can also alter brain biochemistry. For example, interferon-gamma is an important immune molecule made in response to infection. To make it, the body needs an amino acid, which are the building blocks to proteins, called tryptophan. Tryptophan is also important in making serotonin, the molecule that contributes to the feeling of well-being. When our bodies make interferon-gamma, they make less serotonin. Fluctuations in serotonin levels can lead to episodes of depression. 

We have evidence that T. gondii can manipulate our brain chemistry in a number of ways, but we must remember that T. gondii is a tiny microscopic organism and the human brain is very large. How a tiny parasite can influence a large brain still remains to be elucidated. Its influence on behavioural change in the host may depend on the location of T. gondii infection within the brain, neuronal death, immune response and a local imbalance of neurotransmitter as presented in the recent study. It is great that the media highlight such an interesting parasite and how successful it is. However, headlines that report on T. gondii and its potential to manipulate behaviour in their hosts could also factor that we do not yet have evidence of a change in brain chemistry in humans. Responsible reporting would also include that there is no significant difference in getting Toxoplasmosis if you are a cat owner as opposed to non-cat owners and that infection can be acquired through eating contaminated meat (sheep, cows and pigs can be infected) and vegetables contaminated by cat faeces.