Although OCD was once considered rare in both the adult and pediatric populations, improvements in the recognition of this disorder have shown this illness to be a major worldwide heath problem. The first large-scale epidemiological study to include OCD as a separate category and to provide information about its incidence and prevalence was the U.S. Epidemiological Catchment Area (ECA) study (Robbins et al., 1981). This study was conducted on 18,500 adults using the Diagnostic Interview Schedule

(DIS) at five separate sites across the United States. The lifetime prevalence for OCD in this study ranged from 1.9 to 3.3 cases per 100 across the five sites. These rates were 25 to 60 times higher than had been estimated on the basis of clinical populations (Karno et al., 1988).

The most comprehensive study on cross-national epidemiology of OCD combined data from seven international epidemiologic studies done in the United States (the ECA study), Canada, Puerto Rico, Germany, Taiwan, Korea, and New Zeland (Weissman et al., 1994). The lifetime prevalence was remarkably similar among the different countries ranging from 1.9 per 100 in Korea to 2.5 per 100 in Puerto Rico, with the exception of 0.7 cases per 100 in Taiwan. It should be noted that the Taiwanese study reported lower rates for other psychiatric disorders as well (Hwu et al., 1989). The lifetime prevalence of OCD was found to be higher in women than men, with the exception of the German study, which had large standard errors in a smaller sample. In the New Zealand sample the female-male ratio was 3.8, while in Germany it was found to be 0.8. The samples from the other countries found OCD to be 1.2 to 1.8 times more likely in women than in men. The mean age at onset was found to be between 21.9 and 35.5 years across the studies (Weissman et al., 1994).

The rates of OCD in a younger population were assessed by (Flament et al., 1998) and colleagues, using trained mental health professionals and previously validated instruments to assess obsessive-compulsive symptomatology in an adolescent population. As part of a two-stage study of 5596 adolescents (Whitaker et al., 1990), the Leyton Obsessional Inventory was administered (along with other questionnaires on general mental health, anxiety, and eating disorders) to the entire high school population of a county 80 miles from New York City (Flament et al., 1988). Adolescents scoring above the clinical cut-off were interviewed by child psychiatrists with extensive clinical experience with OCD. A total of 20 subjects received a lifetime diagnosis of OCD (18 current and 2 with past illness). The weighted prevalence figure (without exclusion) for OCD was 1.9 percent, a figure that is in close agreement with the ECA estimates for adults. A 2-year follow-up demonstrated that the obsessive-compulsive symptoms were clinically significant, as the majority of subjects remained symptomatic (Berg et al., 1989).

In recent years, several additional epidemiological studies have been conducted in children and adolescents in the United States as well as abroad. In virtually all of these reports, the rates ascertained from direct child reports were higher than those derived from parent reports, supporting clinical data that children with OCD often hide their illness. Secrecy and difficulties of utilizing lay interviewers may have contributed to the low (0.5 percent) prevalence of OCD found by Wittchen and colleagues (1998) in a sample of 3021 adolescent subjects in Munich, Germany. Other investigators, such as Valleni-Basile et al. (1996) in the southeastern United States, Douglass et al. (1995) in New Zealand, and Zohar et al. (1992, 1999) in Israel, used mental health interviewers and semistructured clinical interviews and found more comparable rates of 2.9, 4.0, and 2.3 percent, respectively. A 2-year follow-up evaluation of the Israeli study found that the children who met the diagnostic criteria for OCD remained symptomatic (Zohar et al., 1992).

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