FRIDAY, May 7 (HealthDay News) -- Infections during infancy or childhood do not seem to raise the risk of autism, new research finds.

Researchers analyzed birth records for the 1.4 million children born in Denmark between 1980 and 2002, as well as two national registries that keep track of infectious diseases. They compared those records with records of children referred to psychiatric wards and later diagnosed with an autism spectrum disorder. Of those children, almost 7,400 were diagnosed with an autism spectrum disorder.

The study found that children who were admitted to the hospital for an infectious disease, either bacterial or viral, were more likely to receive a diagnosis of autism spectrum disorder.

However, children admitted to the hospital for non-infectious diseases were also more likely to be diagnosed with autism than kids who were never hospitalized, the study found.

And the researchers could point to no particular infection that upped the risk.

They therefore conclude that childhood infections cannot be considered a cause of autism.

"We find the same relationship between hospitalization due to many different infections and autism," noted lead study author Dr. Hjordis Osk Atladottir, of the departments of epidemiology and biostatistics at the Institute of Public Health, University of Aarhus in Denmark. "If there were a causal relationship, it should be present for specific infections and not provide such an overall pattern of association."

The study was published in the May issue of the Archives of Pediatrics & Adolescent Medicine.

Autism is a neurodevelopmental disorder that is characterized by problems with social interaction, verbal and nonverbal communication, and restricted interests and behaviors.

The prevalence of autism seems to be rising, with an estimated 1 in 110 children affected by the disorder, according to the U.S. Centers for Disease Control and Prevention.

Despite significant effort, the causes of autism remain unclear, although it's believed both genetic and environmental factors contribute, said Dr. Andrew Zimmerman, director of medical research at the Center for Autism and Related Disorders at Kennedy Krieger Institute in Baltimore.

Previous research has suggested that children with autism are more likely to have immune system abnormalities, leading some to theorize that autism might be triggered by infections, Zimmerman said.

Some parents of children with autism have also reported that their children have more frequent infections. While a few studies have shown children with autism may suffer slightly more ear and respiratory infections compared to normally developing children, others found no such connection, Atladottir said.

In addition, there are anecdotal reports of children developing autism after serious infections such as meningitis or encephalitis, Zimmerman said.

In the study, researchers searched for any connection between those particular illnesses, as well as a host of others, including bacterial, viral and fungal infections, and respiratory illnesses, herpes virus and urinary tract infections, specifically. They came up empty handed.

"Yes, there is an increased rate of hospitalization preceding the diagnosis of autism, but it doesn't support a causal relationship between autism and infections," Zimmerman said.

There is a wide range of reasons why children with autism may be more likely to be hospitalized for an illness, the study authors said. For example, autistic children could be more prone to physical illnesses, either due to autism or other medical conditions.

Parents of children with autism frequently report that their children are prone to gastrointestinal problems, such as chronic diarrhea and constipation. Some estimates put the number of kids with autism and gastrointestinal difficulties at 40%, Zimmerman said.

Another reason kids with autism might be more likely to be hospitalized for infectious or other illnesses is that their parents are worried about their child's development and are therefore more likely to seek out medical care.

More medical visits might also help prompt an autism diagnosis, Atladottir said. "It could be that medical professionals see the developmental problems in the child and refer the child further to a child psychiatrist," he explained.

Although this study found no link between autism and childhood infections, prenatal infections -- particularly during the first and second trimesters -- may up the chances children will have autism, prior research has found.

A study published online April 23 in the Journal of Autism and Developmental Disorders by the same group of researchers found a link between autism and hospitalization for maternal viral infection in the first trimester, such as flu, and bacterial infection in the second trimester.

Children whose mothers had a viral infection requiring hospitalization during the first trimester had nearly three times the risk of a later autism diagnosis, according to that study.

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