Changes in the incidence and prevalence of rheumatoid arthritis in Kamitonda, Wakayama, Japan, 1965-1996

Objective-To evaluate secular trends in the incidence and prevalence of rhe umatoid arthritis (RA) in Japan. Methods-The incidence and prevalence of RA were determined in a longitudina l population based study in the Kamitonda district, Wakayama, Japan, from 1 965 to 1996. Results-In the study area consisting of about 3000 inhabitants, 16 incident cases, satisfying definite RA by the Rome criteria were detected during th e study period. The age and sex adjusted incidence in both men and women co mbined and the age adjusted incidence in women significantly decreased (p<0 .025 and p<0.01, respectively). The age and sex adjusted prevalence in all inhabitants tended to decrease (p<0.1), and the age adjusted prevalence in women significantly declined (p<0.025). In men, however, neither incidence nor prevalence showed significant change. Conclusions-The decline of incidence and prevalence of female RA may be red ucible to some environmental changes preferentially occurring more obviousl y in Japanese women than in men. Because the use of oral contraceptives has been extremely low in Japan, the decline should be explained by other fact ors.