According to researchers at the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta, the first-ever national estimate of the prevalence of autism in the United States has found that the disorder affects up to one in every 175 school-age children.

This says the CDC equates to at least 300,000 U.S. schoolchildren with autism, a condition that causes difficulties with learning, socializing and behavior.

The agency’s figures are based on interviews conducted in 2003 and 2004 with the parents of nearly 98,000 children aged 4 to 17.

The results of two national surveys, the National Health Interview Survey (NHIS) and the National Survey of Children’s Health (NCHS) indicate that at least 5 in every 1000 children of school age are diagnosed with autism.

Laura Schieve, an epidemiologist with the CDC’s National Center on Birth Defects and Developmental Disabilities (NCBDDD) says the ratios correspond to about one in every 175 -181 children.

Schieve says as shown in previous studies, boys are nearly four times more likely to have been diagnosed with autism than girls.

The CDC says the surveys confirm that autism is a condition of major public health concern that affects many families, and supports previous estimates of autism, which were based on local surveys done in Atlanta and New Jersey.

Laura Schieve, an epidemiologist at the National Center on Birth Defects and Developmental Disabilities who helped conduct the study said however it could not answer many questions about autism.

The CDC experts emphasise that the results do not reflect on either the underlying causes of autism, or ongoing prevalence trends.

The researchers also say that although autism can often be identified as early as 18 months, many children are not diagnosed until they start school and this suggests that the real prevalence of autism may slightly exceed the numbers supplied by these surveys.

It was evident from the survey that the disorder takes its toll on parents and caregivers.

The NHIS survey revealed that 83 percent of the parents who reported that their child had autism also reported that their child had difficulties with emotional symptoms, conduct, hyperactivity or peer relationships.

Problems interacting with other children topped the list of behavioral difficulties, along with hyperactivity.

Autistic children were also found to need more health care than non-autistic children and nearly 94 percent of parents of autistic children said their child had special health-care needs lasting more than one year, and nearly 93 percent of autistic children were described by their parents as at high risk for developmental delay.

The findings appear in the current issue of the CDC’s journal Morbidity and Mortality Weekly Report.