EVALUATION OF AN ENZYME-LINKED-IMMUNOSORBENT-ASSAY IN THAI SCRUB TYPHUS PATIENTS

We report the development of an improved enzyme-linked immunosorbent a ssay (ELISA) to detect Orientia (formerly Rickettsia) tsutsugamushi an tibody in human sera. Results were compared with a standard test, the indirect immunoperoxidase assay (IIP). Control serum samples were coll ected from 96 American soldiers and 198 Royal Thai Army soldiers with no recent history of clinical illness. Sera were examined from 79 febr ile, Thai scrub typhus patients presenting at Chiang Rai (76) and Bang kraui Nontaburi (3) Provincial hospitals (cases confirmed by elevated ITP IgG levels greater than or equal to 1:1,600, IgM levels greater th an or equal to 1:400, or presence of an eschar). The mean + 2 SD, used for the upper limit of normal reactions in the IgG ELISA, was 0.10 fo r U.S. soldiers and 0.42 for Thai soldiers. Using the 0.10 cutoff valu e, 29% of the asymptomatic Thai soldiers would be designated as antibo dy positive. Variability of IgG ELISA values was greater in the Thai s oldier group than in American soldiers, possibly reflecting previous e xposure to O. tsutsugamushi. In the Thai patients, there was a signifi cant correlation between LTP titers and single serum dilution (1:100) ELISA values (IgG, r = 0.75, n = 104; P < 0.0005; IgM, r = 0.70, n = 7 5; P < 0.0005) and between IIP titers and ELISA titers (IgG, r = 0.87, n = 103; P < 0.0005; IgM, r = 0.76, n = 75; P < 0.0005). The single s erum dilution ELISA was as effective as the titration in determining p resence of specific antibodies. The O. tsutsugamushi ELISA is a rapid and objective test amenable to accurately testing the large numbers of sera often obtained in seroepidemiologic investigations.