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The number of children with autism has increased significantly in at least three Canadian provinces and across the U.S. because of wider screening and improved diagnosis, according to new reports.

But some experts caution that environmental factors could also be influencing autism rates.

A report from the National Epidemiologic Database for the Study of Autism in Canada, released this week, found increases in autism diagnoses in Prince Edward Island, Newfoundland and Labrador and southeastern Ontario.

The increases range from 39 to 204 per cent, depending on the region and age group.

Autism Canada spokesperson Laurie Mawlam said she is not surprised by the increase.

"I think we have a public health problem and I think it needs a lot of attention, and until we understand what is going on I am not optimistic that the numbers get better," she said. 

In the U.S., about one in 88 children have the disorder, up from the previous estimate of one in 110, says a report released Thursday by the Centers for Disease Control and Prevention in Atlanta.

The CDC found that actual cases of autism are twice the number reported five years ago. It's now believed one million children and teens in the U.S. have some form of the condition, a developmental disability characterized by difficulties in social interaction, verbal and non-verbal communication and repetitive behaviours.

NEDSAC researchers compared data between 2003 and 2010 in P.E.I. and southeastern Ontario, and between 2003 and 2008 in Newfoundland.

Among NEDSAC's findings:

• The increase in autism diagnoses was higher among boys in Newfoundland

• The prevalence of autism among children between the ages of two and four appears to be leveling off in southeastern Ontario

• Roughly half of children with autism were not diagnosed until after their fifth birthday

• Diagnosing kids at an earlier age does not seem to explain the increasing autism rates

In an email to Â鶹ӰÊÓ, report author Helene Ouellette-Kuntz, a Queen's University professor, said she suspects "up to 2/3 of the increase may be due to increased awareness and changes in diagnostic practices."

But additional research suggests that even after accounting for these factors, "a third of the observed increase in the prevalence of autistic disorder between 1996 and 2004 could not be explained by increased diagnosis alone."

Researchers point out that autism rates are highly variable across regions in Canada, much like in the United States.

But the growing number of autism cases is raising concerns.

"There has been a lot of evidence over the last 10 to 15 years that autism is increasing at a very large rate that cannot be accounted for by changing diagnostic opinions or increased surveillance," University of Western Ontario autism researcher, Dr. Derrick MacFabe, told Â鶹ӰÊÓ.

"We think that something is happening environmentally that is triggering this increase in susceptible individuals," he said.

In their search for answers, MacFabe said researchers have looked at pesticides and various chemicals, as well as increasing antibiotic use. There is a belief that autism could be triggered by the over-use of antibiotics, which can alter bacteria found in human digestive systems.

"A lot of these children have had experiences of long-term antibiotic use which we think is changing the bacteria of their guts - favouring bacteria that may be triggering autism," said MacFabe.

In a statement, CDC researcher Dr. Coleen Boyle said the centre is "not quite sure" why autism rates have spiked.

The CDC study is considered the most comprehensive U.S. investigation of autism prevalence to date.

Health and school records were checked to determine how common autism was in the selected location in each state and its prevalence overall.

Researchers looked at health and school data from 2008 in areas in 14 states targeted specifically at children who were eight years old to determine how common autism was and its prevalence overall.

A 2002 study estimated that about one in 150 children that age had autism. But 2006 data revised that figure to about one in 110 until the most recent statistics were released.

Autism disorders are almost five times more common among boys than girls, with one in 54 boys identified with it in the studies used to formulate the report.

The number of children identified with the disorders ranged from one in 210 in Alabama to one in 47 in Utah. The largest increases were among Hispanic and black children.

"One thing the data tells us with certainty - there are many children and families who need help," said CDC director Dr. Thomas Frieden in the same release, a message echoed by Canadian advocates.

"I actually don't want think there is enough support out there across the country and across the lifespan of individuals with autism I think it is a dollars and sense issue. It is not cheap to support people with autism but it is crucial that we do it," said Mawlam.

In a news release Thursday, Autism Canada called for more studies and immediate government action to help families with autistic children, saying many of them can't afford basic speech and cognitive therapies.

Canadian schools are also "overwhelmed" by the increasing autism rates and more funding is needed for special education programs, the organization said.

"Autism is a public health emergency and governments must work together and react immediately," the advocacy group said in a statement. "A national autism strategy is long overdue."

Experts say the most important thing for parents to do is to act quickly whenever there's a concern about a child's development.

Early identification and intervention in autism disorder cases can significantly improve a child's life, doctors say.

With a report from CTV's medical specialist Avis Favaro and producer Elizabeth St. Philip