Largest study yet links so-called cat-poo parasite to schizophrenia

House cats and other animals in the cat family, including leopards, can spread the infectious disease toxoplasmosis via their droppings.

The disease is also known as rabbit fever and pregnant women, in particular, are at risk. If a pregnant woman is infected with the parasite that causes toxoplasmosis, it can have serious consequences for the foetus and may cause brain damage. For this reason, pregnant women should avoid emptying litter trays and eating raw meat because cats can also transfer the parasite to pigs and other animals that end up on our dining tables.

Although it is not yet clear how, toxoplasmosis may contribute to the development of schizophrenia in human beings. This is confirmed by a scientific study recently published in the research journal Brain, Behaviour and Immunity.

The study was conducted by a large team of Danish researchers, led by post doc Kristoffer Burgdorf and Professor Henrik Ullum, both from the Department of Clinical Immunology at Rigshospitalet, University of Copenhagen. Researchers from Statens Serum Institut (SSI), Skejby University Hospital and Aarhus University also took part.

‘Our study isn’t the first to investigate the assumption of a link between toxoplasmosis and schizophrenia. But it has never been proven using the large volumes of data we work with,’ says Kristoffer Burgdorf, who was funded by the Lundbeck Foundation.
85,000 blood donors

Kristoffer Burgdorf and his colleagues based their investigation into potential links between toxoplasmosis and psychiatric disorders, including schizophrenia, on around 85,000 blood samples from the Danish Blood Donor Study (DBDS).

These blood samples can be linked via the Danish civil registration (CPR) system – although solely in pseudonymised form – to data available in the Danish health system about the health of the individual blood donor. We can, for instance, see whether a pseudonymised donor has a schizophrenia diagnosis. (In this context, ‘pseudonymised’ means that the data are coded so that it is not possible to identify the person in question.)

The research team headed by Kristoffer Burgdorf and Henrik Ullum worked with samples from a total of 11,500 donors in DBDS. 2,500 of these donors had a psychiatric diagnosis.

‘On the other hand, we didn’t know whether any of the 11,500 people had been infected with toxoplasmosis. For this we had the help of a research team from the Stanley Research Centre in Baltimore, USA. They’re experts in this type of analysis. And when we got their results, we began to look at toxoplasmosis in relation to a range of issues,’ says Kristoffer Burgdorf.

The answers showed that 26% of the 11,500 donors were infected with the so-called cat-poo parasite – although, in most cases, the infection was dormant. This percentage is broadly equivalent to the results for the population as a whole.

When Kristoffer Burgdorf took a closer look at the data from DBDS, he noticed some remarkable connections:

‘For one thing, the incidence of toxoplasmosis was higher in the 151 individuals with a schizophrenia diagnosis than in the other donors. And we also noted that many of the 151 subjects had developed schizophrenia after they had given their original blood sample. This suggests that the toxoplasmosis with which they had already been infected could, potentially, have provoked the subsequent development of schizophrenia. And this has never been shown before,’ he says.

It is not certain why toxoplasmosis may be a contributing causal factor for developing schizophrenia. However, it is a known fact that the parasite initiates a biochemical process in the brain which inhibits the neurotransmitter serotonin – and it is the concentration of this chemical that we are trying to increase when we prescribe antidepressants.

Kristoffer Burgdorf stresses that much more research is needed in this field, and no correlation has been found between psychiatric disorders and owning a cat:

‘The majority of cases of toxoplasmosis are due to eating meat that has not been sufficiently cooked.’