Objective: To determine if the same arthritogenic microorganisms can trigger reactive arthritis (ReA) in both HLA-B27 positive and HLA-B27 negative individuals. To determine risk factors (other than HLA-B27) for the development of ReA.

Methods: Following a common food source outbreak of Salmonella enteritidis, 2 family groups of individuals with dysentery and new onset ReA signs and symptoms were identified. HLA typing was performed for all family members exposed to the dysenteric organism.

Results: ReA occurred in HLA-B27 positive members of Family A. No member of Family B possessed HLA-B27 or CREG phenotype, but 3 members developed ReA. The HLA identical sibling of the proband in Family B developed dysentery but did not develop ReA.

Conclusions: In this epidemic, interaction of the microorganism with an HLA-B27 positive host predictably produced ReA but B27 was not necessary to produce it in other dysenteric patients. Host factors including amount of ingested microorganisms, age and sex but excluding HLA type may account for the differential development of ReA in the siblings in Family B.