Suppression of adjuvant arthritis by infection with a strain of the rodent malaria parasite Plasmodium berghei.

Hospital data (Greenwood, 1968) and a population survey (to be published) suggest that rheumatoid arthritis is an uncommon disease in Western Nigeria. Furthermore, Nigerian patients satisfying the American Rheumatism Association criteria for a diagnosis of definite or probable rheumatoid arthritis (Ropes, Bennett, Cobb, Jacox, and Jessar, 1959) have a clinically milder disease than European patients with the condition (Greenwood, 1969) and rarely show any of the immunological changes associated with the disease (Greenwood and Herrick, 1970). The limited literature available suggests that rheumatoid arthritis is uncommon in some other parts of tropical Africa and also in New Guinea. However, rheumatoid arthritis is seen frequently in American Negroes (Engel, Roberts, and Burch, 1966), many of whom are of West African origin. It thus seems probable that environmental factors play some part in determining the infrequent occurrence of clinical cases of rheumatoid arthritis in some parts of tropical Africa. It has been suggested (Greenwood, 1968) that the effects of repeated parasitic infections from childhood might be one of these factors. In order to investigate this hypothesis we have studied the effects of malaria infection on adjuvant arthritis in the rat, a widely used animal model for human rheumatoid arthritis.