Autism (incidence)

Overview
The reported incidence of autism varies considerably among countries and is complicated by varying criteria for diagnosing autism, different standards for reporting public health problems, and other possible variations.

Background
Autism was first characterised in 1943 by psychiatrist Dr. Leo Kanner of the Johns Hopkins Hospital in Baltimore, and almost simultaneously, in German, by Dr Hans Asperger. Both published case series of children with strikingly similar features.

The number of children born in each year diagnosed with autism in countries reporting figures is larger now than then. The populations of those countries have also increased but it is unclear what change in the incidence fraction has occurred. Public health organizations and researchers are not yet confident they have completely determined causes for all changes in the rate of reports of Autism (vide infra). Potential factors listed by the UK National Autistic Society include:
 * The underlying condition may have a changing incidence with time, i.e., more children (and/or more per thousand born) are affected by it;
 * More complete pickup of autism (case finding), as a result of increased awareness and funding;
 * Attempts in the US and UK to sue vaccine companies may have also increased case-reporting.
 * The diagnosis being applied more broadly than before, as a result of the changing definition of the disorder, particularly changes in DSM-III-R; and DSM-IV
 * Successively earlier diagnosis in each succeeding cohort of children including recognition in nursery (preschool) - this would affect apparent prevalence but not incidence

There has been public concern that the MMR vaccine or the vaccine preservative thiomersal have contributed to an increase in the incidence of autism; the consensus of the medical and scientific community is that there is no scientific evidence to support these hypotheses, but decreasing uptake of vaccines has led to outbreaks of serious childhood diseases.

Incidence
The incidence of a condition is the rate at which new cases occur in a population during a specified interval, e.g. "10 per year" or "12 in 1982". The prevalence of a condition is the proportion of a population that are cases at a point in time, e.g. "1 in 1000".

Examples of the way information is collected to specifically measure incidence rather than prevalence include:
 * Annual and age specific incidence for first recorded diagnoses of autism (that is, when the diagnosis of autism was first recorded);
 * Annual, birth cohort specific risk of autism diagnosed

Incidence in sub-groups
There have been suggestions that the incidence of autism may vary amongst particular groups defined by occupation, lifestyle or genetic isolation. Changes that made travel and communication easier, and the growth of the technological industries during the past decade, have been suggested as means for increase in the proportion of couples likely to produce an autistic child. None of these have been established.

Denmark
In November 2002, a study reported a lower incidence of autism in Denmark than in the US and other countries. An incidence of 1 in 727 (738 out of 537,303) was reported, compared with up to 1 in 86 among primary school children in the United Kingdom and around 1 in 150 children in the USA. Danish authorities also reported a continued increase in the incidence of autism after 1992 after withdrawal of thiomersal-containing vaccines. Data presented in 2003 shows a clear increase in incidence between 1990 and 1995 (before the criteria changed). Thus, the increased incidence of autism after the removal of thiomersal was not a measurement artefact.

Japan
The Yokohama study in Japan (2005) examined the incidence of autism before and after the 1993 withdrawal of MMR, reporting 48 and 86 cases per 10,000 children in two sequential years before withdrawal, doubling to 97 and 161 per 10,000 afterwards in the two years afterwards.

It studied over 30,000 children (278 cases) born in one district of Yokohama.

United Kingdom
The National Autistic Society regarded the incidence and changes in incidence with time as unclear. A 2001 review, by the Medical Research Council, yielded an estimate of one in 166 in children under eight.

The reported autism incidence in the UK rose starting before the first introduction of the MMR vaccine in 1989.

United States
The number of diagnosed cases of autism grew dramatically in the U.S. in the 1990s and early 2000s. For example, in 1996, 21,669 children and students aged 6–11 years diagnosed with autism were served under Part B of the Individuals with Disabilities Education Act (IDEA) in the U.S. and outlying areas; by 2001 this number had risen to 64,094, and by 2005 to 110,529.