Comparative evaluation of elisa, pcr and micro-ifa in diagnosis of scrub typhus

Background: Scrub typhus is a rickettsial infection which is caused by Orientia tsutsugamushi and transmitted by the bite of chigger. It is an important cause of acute undifferentiated febrile illness. Delay in diagnosis may prove to be life threatening.
Objective: To compare IgM ELISA and PCR for 56kDa antigen with Micro IFA-IgM in diagnosis of scrub typhus and perform IgG ELISA, MicroIFA-IgG for prevalence of scrub typhus.
Material and Methods: There were 327 cases clinically suspected of scrub typhus over a
period of 1 year. Commercially provided kits for IgM ELISA (In Bios Inc. USA), IgG ELISA (In Bios Inc. USA), IgM IFA (Fuller Laboratories, USA), IgG IFA (Fuller Laboratories, USA) and PCR for 56kDa antigen (Roche, Indianapolis, IN) were used. Analysis was done using statistical software Epi-info version 7(7.1.1.0).
Results: Out of 327 clinically suspected scrub typhus cases, 227 were IgM and/or IgG
ELISA positive. Out of 177 IgM ELISA positive cases, 174(98.3%), 151(85.3%), 129(73%) and 17(9.6%) cases were positive for IgM IFA, IgG IFA, IgG ELISA and PCR respectively. The sensitivity of IgM ELISA and IgG ELISA as compared to IgM IFA
and IgG IFA was 95% and 89% respectively. The specificity of IgM ELISA as compared to IgM IFA was 93%. The sensitivity and specificity of nested PCR was 9.6% and 100% respectively.
Conclusion: Good specificity and low sensitivity of PCR using plasma needs further evaluation for optimizing the type of sample required for PCR. In resource limited settings, IgM ELISA seems to be a good alternative method for serological diagnosis.