Viral Infections in May Be Linked to Autism in Boys, Study Suggests

• Scientists may have found a link between serious childhood infections and autism later in life. 
• They prompted young male mice to mount an immune response, then found the mice later had trouble recognizing familiar faces.
• A study of more than 3.6 million hospitalized children bolster these findings.

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Scientists may be a step closer to understanding what causes autism and how to treat it. A study released this month offers evidence that severe infections in childhood might make a future diagnosis of autism spectrum disorder more likely in men who are genetically predisposed to the condition.   

Scientists at the University of California, Los Angeles performed the study on mice, so it's too early to say what its implications are for humans. But other research hints at a similar association: Data collected by researchers at the University of Chicago and used in the same new study found that boys diagnosed with autism were more commonly hospitalized with infections between the ages of 1.5 and 4 than boys who didn't have autism. (That dataset included more than 3.6 million children with a host of different infections, though the UCLA study didn't explore whether any particular virus was associated with autism.)

"These parallels are so striking that they're highly unlikely to be unrelated," Alcino Silva, director of UCLA's Integrative Center for Learning and Memory, said of the mouse and human data.

The research bolsters the idea that genetic factors don't necessarily trigger autism on their own. Environmental factors, like a viral infection, also play a role.

The mouse study even offers a possible explanation as to why: Childhood infections may cause the body to over-express genes that code for microglia, the central nervous system's primary immune cells. That, in turn, can affect brain development, which could be at play in some traits commonly associated with autism, such as difficulty communicating verbally or recognizing familiar faces.

So the researchers experimented with drugs that target microglia, and found that they not only prevented those social issues in adult mice — they might have reversed them. 

Male mice with mock-viral infections had trouble recognizing familiar faces 

Studies in mice are often critical to understanding how viruses affect human health, since scientists can't ethically induce viral infections in people.

For the UCLA study, researchers used newborn mice with a genetic disorder called tuberous sclerosis, which is associated with autism in people. Silva said only a subset of humans with this disorder — around half of them — go on to develop autism. The researchers were attempting to figure out why.

To start, they injected some of the mice with a chemical that stimulates an immune response, much like a viral infection would. The rest of the mice received a placebo.

The results showed that only the male mice that received the chemical as newborns saw impaired social behavior as adults. These mice weren't able to distinguish between a mouse they had seen before and a mouse they had never met.

Another UCLA lab then replicated the study and found that the mice injected with the chemical were less skilled at communicating with other mice — a hallmark of autism.

The researchers concluded that early viral infections, in combination with certain genetic mutations, could lead to an autism diagnosis down the line, but only in men. 

"The female seems to be less affected than the male," Manuel López-Aranda, the study's lead author, told Insider. "Maybe the root of this question is in the microglia."

Boys' and girls' microglia may be in different developmental stages when they're young, López-Aranda said, which could explain why men are more predisposed to autism. 

Scientists are getting closer to identifying possible autism treatments

One of the biggest lessons of the UCLA study is to not underestimate viral infections.

"Something that has to be clear from this work is: please vaccinate your kids," López-Aranda said. "Our results and the human results suggest that if you're not vaccinating your kids for polio and they get polio, if they don't die because of polio, they have a higher chance than other kids to develop autism spectrum disorder."

Silva said severe childhood infections might also be linked to a higher likelihood of psychiatric disorders like schizophrenia, anxiety, or depression — a concept the researchers are studying further.

For now, though, existing data from the UCLA study suggests that rapamycin, a drug approved to treat rare lung disease, either prevented male mice from forgetting familiar faces or reversed this memory deficit after the mice had developed it. In theory, that's a clue that children who get severe viral infections could receive treatments that help prevent them from developing autism later.

But Silva said scientists are still at the beginning stages of this research. 

"We have some of the pieces of the puzzle, but it's only two or three pieces of 1,000-piece puzzle," he said.