“It is important to emphasize that it is rare for these infections to be transmitted from mother to child. Although these infections represent a very small proportion of all cases of autism in the population, we see that children who are affected are clearly at increased risk for both autism and intellectual disability,” says René Gardner, a researcher at Karolinska Institutet’s Department of International Public Health who contributed to the study.

What are TORCH infections?

The study focused on a group known as TORCH infections. This category includes cytomegalovirus, rubella, toxoplasma and herpesviruses. Although most common infections do not reach the developing fetus directly, these pathogens can cross the placenta and cause congenital infections.

Study tracked 3.7 million people

Researchers examined health and education records for 3.7 million people born in Sweden between 1987 and 2021. Among them, 975 people had been diagnosed with a congenital TORCH infection.

Participants were followed for up to 30 years, allowing the research team to investigate whether congenital TORCH infections were associated with later neurodevelopmental diagnoses and educational outcomes.

Children with congenital TORCH infections were about three times more likely to later receive an autism diagnosis than children without these infections. They were more than seven times more likely to be diagnosed with intellectual disability.

Among people with severe to profound intellectual disability, the difference was even greater, with a 30-fold increased risk.

Higher relative risk, but few cases overall

Although these findings represent a significant increase in relative risk, congenital TORCH infections are rare. As a result, they account for only a small proportion of autism and intellectual disability cases in the overall population.

Researchers estimate that approximately 1.2% of severe intellectual disability cases in Sweden may be associated with congenital TORCH infection. They estimate that about 0.034% of autism cases in the country can be attributed to these infections.

At an individual level, however, researchers estimate that approximately one in five children born with a TORCH infection may later develop autism.

“It has long been known that these infections can cause intellectual disability. However, in previous studies the link with autism was less clear. Studies were often based on small patient groups. This study is the largest in this field to date and is based on national registry data covering almost the entire population of Sweden,” says Hugo Sjöqvist, PhD student and lead author of the study.

Sibling comparisons support the findings

The researchers also used sibling comparisons to investigate whether the results could be explained by shared characteristics within families.

They compared children who experienced a TORCH infection with their siblings who did not. The pattern remained similar in these comparisons, suggesting that common family factors alone are unlikely to explain the association.

Researchers did not identify a clear link between congenital TORCH infections and other neuropsychiatric disorders, such as ADHD or obsessive-compulsive disorder.

However, they found differences in school performance. Even among children who were not diagnosed with autism or intellectual disability, those with congenital TORCH infections had lower average performance than those without infections.

Congenital infections may have long-term effects on brain development

“Our results suggest that certain infections transmitted to the fetus during pregnancy can have long-term effects on brain development. Although these congenital infections are rare, they are important from a public health perspective because some are preventable,” says René Gardner.

Researchers say the findings highlight the importance of preventing infections that can be passed to the fetus during pregnancy.

Rubella is one example where prevention can make a significant difference. The infection has virtually disappeared in Sweden since the introduction of the national vaccination program. Researchers say it remains important to maintain vaccination programs and other preventive measures to reduce infections that can be passed from pregnant women to their developing fetuses.

This study was funded by the Swedish Research Council. Co-author David Mataix-Cols receives author fees from UpToDate Inc. and is a partner in Scandinavian E-Health AB, but says he is not affiliated with this publication.