There's no doubt that autism is a disease that affects an entire family. Endless therapy sessions, mounting medical paperwork, and a wide spectrum of symptoms take a toll on the affected kids as well as their parents and siblings. A new study shows that siblings of kids with autism may share more than just their exhausted and frustrated parents — they may also share symptoms of the disease.

A new study suggests that undiagnosed siblings in families that include two or more children with autism often struggle with language delays, social difficulties and other mild symptoms of autism.

The study, conducted by psychiatrist John Constantino of Washington University School of Medicine in St. Louis and his colleagues found that genetics prompt autism symptoms of varying intensity among members of these families, including in some kids who don’t technically qualify as having an autism spectrum disorder.

Constantino's team found that about one in five siblings of children with autism who don’t meet criteria for the disorder display mild or “subclinical” autism traits, such as language delays, the use of odd or repeated phrases and difficulties interacting with others.

Constantino’s team assessed signs of autism in 2,920 children from 1,235 families participating in a national online research registry. Each family in the registry includes at least one child with an autism spectrum disorder and at least one biological sibling. Data came from questionnaires completed by parents.

Their responses indicated that 134 families, or 11 percent, had more than one child diagnosed with autism. About one in four families, including nearly all of those with multiple autism cases, also contained siblings with mild symptoms.

Among mildly affected boys and girls, 20 percent had received a diagnosis of language delay or speech problems early in life, double the prevalence in the general population.

Kids with undiagnosed autism-related social deficits may find it hard to make friends and could experience a worsening of other conditions such as learning disabilities and attention-deficit hyperactivity disorder.

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