Environmental aetiology of rheumatoid arthritis

An individual's environment seems to influence how likely it is that a genetic predisposition to rheumatoid arthritis is clinically expressed. For example, populations with similar or identical genotypes may not be concordant for rheumatoid arthritis:

• African populations in Europe have a similar incidence of rheumatoid arthritis to Caucasians despite the disease being rare in sub-Saharan Africa
• there is less than 50% concordance for rheumatoid arthritis in monozygotic twins

One interesting theory is that appears to be no historical records of rheumatoid arthritis in ancient civilizations. It may be that a new extrinsic agent has arisen in the interim - for example an as yet unknown virus - that acts as a trigger in susceptible individuals.