Sacroiliitis in the old order Amish

In a study designed to clarify the relationship of HLA‐B27 and sacroiliitis, 45 members of seven Old Order Amish families were thoroughly evaluated for sacroiliitis. The primary study group was three sibs who carried HLA‐B27 and three of their other sibs who inherited the alternate non‐B27 haplotype, their spouses and offspring. The seventh B27 positive family was more distantly related. The study was undertaken with no prior knowledge of disease in these families. These families represented all the B27‐positive subjects who had been ascertained up to the time the study began. In these selected families, 24% of the members were found to have sacroiliitis, irrespective of whether they were B27 positive. Analysis of the relevance of HLA to disease in this kindred was confounded by the presence of Bw39 and B17, antigens associated with psoriatic arthritis. However, not all B27‐negative cases possessed one or both of these specificities. Rather than clarifying the relationship of B27 to disease, this study, in a highly consanguineous kindred, indicated that other nonlinked loci may be epistatic to B27, that other HLA alleles may be involved, or that certain causes are more prevalent in the Amish than in the non‐Amish population.