According to new research the significant increase in the numbers of children diagnosed with autism in California cannot be explained by changes in the way doctors diagnose the condition but is most likely due to environmental exposures.
Since the early 1990s the number of children diagnosed with autism each year has been climbing steadily in California, across the United States, and across the developed world. In 1990 there were only 205 news diagnoses of autism in the state of California compared to more than 3000 in 2006.
Many experts and commentators have attributed the rise solely to greater awareness of the symptoms and changes in the diagnostic criteria which mean more cases are picked up and children with milder symptoms are now diagnosed as having autism. The authors of a new study however say that this is not the case and that environmental factors that babies and foetuses are exposed to, such as pesticides, viruses and chemicals in toys and household products, should be the focus for more research.
In a statement, Irva Hertz-Picciotto, professor of epidemiology at University of California, Davis, and lead author of the study said: "It's time to start looking for the environmental culprits responsible for the remarkable increase in the rate of autism in California."
In the first study of its kind Hertz-Picciotto and co-author Lora Delwiche of the UC Davis Department of Public Health Sciences looked at 17 years of California state medical data that tracks developmental disabilities. They also used birth records and Census Bureau data to calculate the rate of autism and age of diagnosis. The results are published in the journal Epidemiology.
What they found was that In 1990, 6.2 of every 10,000 children born in the state were diagnosed with autism by the age of five, compared with 42.5 in 10,000 born in 2001, and the numbers have continued to rise since then.
The researchers say that changes in how and when doctors diagnose the disorder and when state officials report it can explain less than half of the increase that has been seen.
In their journal article Hertz-Picciotto Delwiche reveal that doctors are diagnosing autism at a younger age because of increased symptom awareness but that this change is only responsible for about a 24 per cent increase in children reported to be autistic by the age of five.
"A shift toward younger age at diagnosis was clear but not huge," they say.
They also say that the diagnosing of milder cases can explain another 56 per cent increase and changes in state reporting of the disorder could account for around a 120 per cent increase.
However, these factors don't come close to explaining the actual 600-700 per cent increase in cases that the researcher's analysis reveals. This leaves genetics and environmental factors, or a combination of the two, as the likely culprits for the bulk of the increase.
In an interview last week Hertz-Picciotto, a researcher at UC Davis' M.I.N.D. Institute, a leading autism research facility, said: "There's genetics and there's environment. And genetics don't change in such short periods of time."
Essentially what she is saying is that scientists must now figure out what in the environment to which mother's and infants are exposed has changed since the early 1990s. It is the answer to this question that may provide answers explaining the huge rise in the developmental disorder.
The culprits, Hertz-Picciotto said, could be "in the microbial world and in the chemical world."
Many environmental triggers have been suggested as explanation for the rise of autism in recent years and some have caused great controversy such as the combined measles-mumps-rubella (MMR) vaccine and thimerosal, the mercury containing preversavative which until recently was used in most childhood vaccines.
An initial study by Dr. Andrew Wakefield linked the MMR vaccine to intestinal damage caused by the measles virus in autistic children. Although these findings may still stand large epidemiological studies have found no link between MMR and the rise in cases of autism.
Likewise mercury is known to be a highly potent neurotoxin and could certainly damage a developing brain yet, again, epidemiological studies have failed to find a significant link.
Although these two environmental factors may still prove to hold substance there are many others that researchers should be looking at according to Hertz-Picciotto.
Mercury from other sources such as food, water and air contamination, other heavy metals such as lead, pesticides, polychlorinated biphenyls (PCBs), and brominated flame retardants - all of which are neurotoxins.
While exposure to PCBs has decreased in recent decades following restrictions on its use, the opposite is true for brominated flame retardants which are used in furniture and electronics.
A recent study linked rates of autism in California to proximity to farms that conducted pesticide spraying (see here). Now a new study has found a link between autism and phthalates, compounds which soften plastics and are found in vinyl, cosmetics and childrens toys amongst other everyday items.
The culprit could also be an infectious agent. If the foetus or infant encounters a new new microbe such as a bacteria or virus it could alter immune function and development of the brain since the immune and nervous systems are highly integrated.
Hertz-Picciotto is now calling for increased funding for research looking at the environment and autism. Funding for genetic research currently accounts for 90-95% of all money available for autism research - a situation that this study's authors and an increasing number of other researchers believe must be addressed immediately if the autism riddle is to be solved.