STRONG INFLUENCE OF PERIOD OF BIRTH ON THE OCCURRENCE OF RHEUMATOID FACTOR

July 10, 2001

STRONG INFLUENCE OF PERIOD OF BIRTH ON THE OCCURRENCE OF RHEUMATOID

FACTOR : RESULTS FROM A 30 YEAR FOLLOW-UP STUDY ON PIMA INDIANS.

Alan J Silman, Ian Enzer, Knowler, Graham Dunn, Lennart

sson Manchester, United Kingdom; Phoenix, AZ and Malmo, Sweden

Introduction: The Pima Indians have one of the highest incidences of

rheumatoid arthritis but recent evidence has suggested a marked secular

decline. Epidemiological investigation of the occurrence of rheumatoid

factor in this population could provide supportive evidence of an

environmental influence.

Objective: To evaluate the relative contributions of secular and birth

cohort influences on rheumatoid factor production in the Pima.

Subjects & Methods: Rheumatoid factor data, using both the BFT and SCAT

tests were available on 5345 Pima Indians born between 1886 and 1975,

who were surveyed at two yearly intervals between 1966 and 1995.

Analysis: Logistic modelling was undertaken on the relative

contributions of age, calendar year (period) and birth (cohort) taking

account of multiple observations per subject.

Results: Data were available on 22,721 examinations. Results were

similar for both BFT and SCAT. There was a marked decline in the

proportion of positive tests, in both males and females, during the

period of observation. Thus the SCAT positive rates in males and females

in 1986-1995 were 3.1% and 4.2% compared to 7.6% and 11.6% in 1966-75.

However after adjusting for age and birth cohort this secular trend

became non-significant. By contrast a strong birth cohort trend was

evident with an approximately linear, age-adjusted, tenfold decline from

a peak in those born between 1896 to 1905 to those born between 1966 and

1975.

Conclusion: These data suggest strong environmental influences on the

production of rheumatoid factor in the Pima and may help to explain the

recent decline in RA incidence in this population. The stronger effects

of birth cohort over calendar year suggest that such environmental

influences are likely to have operated early in life. These data have

implications for understanding the epidemiology and etiology of RA.

Session Info. : MiniSymposium: Epidemiology of Rheumatoid Arthritis

(2:15 PM-3:45 PM)

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