Children born to mothers who have active genital herpes during pregnancy may be at twice the risk of developing autism spectrum disorder, new research suggests.

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Researchers suggest a link between active HSV-2 in early pregnancy and autism risk in offspring.

Lead author Milada Mahic, of the Center for Infection and Immunity and the Norwegian Institute of Public Health in Norway, and colleagues report their findings in the journal mSphere.

Genital herpes is a common sexually transmitted infection primarily caused by herpes simplex virus 2 (HSV-2).

Around 417 million people worldwide have genital herpes caused by HSV-2, with around 10-20 percent of cases occurring in people who have received a prior diagnosis of the condition.

HSV-2 infection is much more common among women than men; around 20.3 percent of women aged 14-49 years in the United States are infected with HSV-2, compared with 10.6 percent of men.

According to Mahic and team, following an initial HSV-2 infection, the virus can lay dormant in nerve cells. At any time, HSV-2 can become active and cause flare-ups. As the body acquires immunity to the virus, the frequency of flare-ups may reduce over time.

Recent studies have suggested a link between immune responses to infections during pregnancy and increased risk of autism spectrum disorder (ASD) in offspring.

Mahic and colleagues believe their study supports such findings, after finding that a mother’s immune response to HSV-2 was associated with their child’s risk of ASD.

To reach their findings, the researchers analyzed the blood samples of 875 mothers who were enrolled in the Autism Birth Cohort (ABC) Study. Of these women, 412 had children with ASD, while 463 were mothers to children without ASD.

Blood samples were collected at 18 weeks of pregnancy and again during childbirth. The researchers analyzed each sample for levels of antibodies to five pathogens: Toxoplasma gondii, rubella virus, cytomegalovirus, herpes simplex virus 1 (HSV-1), and HSV-2.

The team found that mothers who had high levels of antibodies to HSV-2 in their blood at 18 weeks of pregnancy were twice as likely to have children with ASD. High levels of antibodies to the other four pathogens were not associated with ASD risk in offspring.

The researchers note that 18 weeks of pregnancy is around the time when the nervous system of the fetus goes through rapid development. As such, they speculate that a mother’s immune response to HSV-2 may interfere with fetal nervous system development, which may increase the risk of autism.

The link between high antibody levels to HSV-2 and increased ASD risk for offspring was only seen for boys, the researchers report.

However, they point out that their study only included a small number of females with ASD, so they are unable to conclude whether their findings are sex-specific. Still, they note that ASD is much more common among boys than girls.

While further studies are needed to get a better understanding of the link between HSV-2 infection in pregnancy and ASD risk in offspring, the researchers believe their findings shed light on the underlying causes of ASD.

“The cause or causes of most cases of autism are unknown. But evidence suggests a role for both genetic and environmental factors.

Our work suggests that inflammation and immune activation may contribute to risk. Herpes simplex virus-2 could be one of any number of infectious agents involved.”

Senior author W. Ian Lipkin, Center for Infection and Immunity

Learn how a baby’s sex might influence a mother’s immunity.