"Cats could be making children stupid," reports the Daily Telegraph. It says that a parasite called Toxoplasma gondii, which is carried by cats, could be affecting performance at school.
Toxoplasma gondii is a common parasite that can be found in many mammals, including cats. It can be contracted by humans if they come into contact with the faeces of infected cats, or by consuming contaminated food or water.
An infection with Toxoplasma gondii is known as toxoplasmosis.
While toxoplasmosis does not usually cause symptoms in healthy adults, some researchers have argued that the parasites might have an effect on the brain. For example, a 2012 study that we discussed linked cat ownership with an increased risk of suicide.
This latest study involved just over 1,700 secondary school age children in the US. It found a link between having been exposed to toxoplasma and lower scores on two cognitive tests – one of reading ability and one of verbal memory.
However, there were no differences in performance on maths or visuospatial tests (the ability to process visual information about the position of objects). While the researchers took into account some factors that could affect results, such as family income, removing their impact completely is likely to be difficult.
The study did not assess how the children had been exposed to toxoplasma – whether through cats or contaminated food. Overall, this study should not cause undue alarm among families with cats. Regardless of the results of this study, good hygiene around family pets is always a good idea. Pregnant women are already advised to avoid cat faeces to reduce their chances of passing an infection to the unborn baby.
Where did the story come from?
The study was carried out by researchers from the University of Iowa and Florida International University. It received no specific funding.
The study was published in the peer-reviewed medical journal Parasitology.
Rather unusually, the headline in the Telegraph is more cautious than some of its article text. This study cannot prove cause and effect, and the headline appropriately talks about the parasite being "linked to" learning difficulties, but the article says that "cats could be making children stupid".
The article does include a note of caution from the authors that longitudinal studies are needed.
Mail Online’s reporting of the study is accurate and provides useful background information about toxoplasmosis. Both papers take the opportunity to show cute pictures of kittens and cats (the real purpose of the internet).
What kind of research was this?
This was a cross-sectional study looking at whether infection with the parasite Toxoplasma gondii is linked to poorer cognitive performance in school age children.
Toxoplasma gondii is a single-celled parasite that is said to affect about a third of the world’s population. As noted in the news, it can be carried by cats and transmitted to humans through contact with infected cat faeces. It can also be passed on by drinking contaminated water, eating contaminated undercooked meat or unwashed vegetables, or from mother to baby.
Toxoplasma can cause serious illness if passed on from a pregnant woman to her foetus, or in people whose immune systems are compromised. However, in most people with healthy immune systems infection does not cause noticeable symptoms, and the infection is considered "latent" or inactive.
There has been some suggestion, however, that infection might be causing more subtle behavioural or cognitive changes that are currently not attributed to the infection. No studies have ever looked at this possibility in children, so the researchers wanted to see whether children with toxoplasma infection might show different cognitive performance to those without the infection.
This kind of cross-sectional study can only tell us about whether certain characteristics (cognitive function in this case) are different in certain types of people (those with or without toxoplasma infection in this case). As it does not assess which factor comes first, we can’t say for certain that infection could potentially be causing any differences seen. That is, we’d need to know whether children’s cognitive performance was different before the infection or just afterwards.
What did the research involve?
Researchers used data from an ongoing cross-sectional study called the National Health and Nutrition Examination Survey (NHANES). Among other assessments, this study tested children in the US aged 12 to 16 years old for signs of toxoplasma infection, and also tested their cognitive abilities. They then compared cognitive test results between those with and without toxoplasma infection.
The NHANES selects a sample representative of the US population as a whole. Data analysed in the current study was collected between 1988 and 1994, as part of the third NHANES study. Children were tested for a specified level of antibodies to toxoplasma, which indicated that they had been infected at some point. Blood samples were also tested for presence of antibodies against other forms of infection (such as hepatitis B and C or herpes viruses), and for levels of various vitamins. The NHANES also collected other information, for example on family income and ethnicity.
The children completed standard reading and maths tests, and tests assessing reasoning and various aspects of memory and other cognitive functions. Children with learning disabilities were not included in the current study. The researchers analysed whether children with evidence of toxoplasma differed in their cognitive test scores from those without evidence of infection. They took into account factors that could affect results (potential confounders), and they also looked at whether results differed in boys and girls, or in those with different levels of vitamins in their blood.
What were the basic results?
The researchers analysed data for 1,755 children. They found that 7.7% of children showed evidence of having been exposed to toxoplasma infection. Children with toxoplasma infection were more likely to be from families whose main language was not English and to have signs of other infections. They also tended to be poorer.
Having been exposed to toxoplasma infection was associated with lower reading skill scores and verbal memory scores, after adjustment for potential confounders. There was no relationship between toxoplasma infection and maths or visuospatial reasoning in these adjusted analyses.
Toxoplasma infection seemed to be associated with a greater difference in verbal memory in children with lower vitamin E concentrations in their blood. None of the other vitamin concentrations or gender seemed to affect the link.
How did the researchers interpret the results?
The researchers concluded that: "Toxoplasma seropositivity may be associated with reading and memory impairments in school age children", and that "serum vitamin E seems to modify the relationship".
This cross-sectional study found an association between toxoplasma infection in secondary school age children in the US and certain measures of cognitive function (reading and verbal memory), but not others (maths or visuospatial reasoning).
The study included a relatively large national sample (just over 1,700 children), which was selected to be representative of the US population as a whole. Its main limitation is its cross-sectional design. As the authors note, this means that they cannot establish that infection with toxoplasma was present before any differences in cognitive function. Therefore they cannot draw conclusions about whether toxoplasma might be directly causing the differences seen. Cohort studies are needed to gain a better understanding of whether the link might be due to a direct effect of toxoplasma.
In addition, while the researchers took into account various factors that might affect results, this may not completely remove their effects. For example, the children with toxoplasma infection tended to come from poorer families. While the researchers took into account one measure of socioeconomic status (family income), removing its impact completely is likely to be difficult.
The researchers also carried out a lot of analyses, and not all were statistically significant. When a lot of significant tests are done, some will find a link just by chance. Also, while there was an association between toxoplasma and some cognitive test results, there was no association for others.
This study should not cause undue alarm among families with cats, as it is not possible to say with certainty that toxoplasma is causing the differences seen. Regardless of the results of this study, good hygiene around family pets is always a good idea. Pregnant women are already advised to avoid cat faeces to reduce their chances of toxoplasma infection and transmission to the foetus.
Pet ownership may have benefits, such as boost a child’s quality of life and teach them about the concept of responsibility. It is important to reinforce good hygiene rules around pets, such as staying away from any animal waste and always washing their hands after handling a pet, especially before eating.
Read more advice about pet hygiene in the home
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Daily Telegraph, 1 June 2015
Mail Online, 1 June 2015
Links to the science
Parasitology. Published online May 20 2015