Clinical diagnosis of
scrub typhus is often difficult because the symptoms are very similar to those of other febrile illness such as
dengue,
leptospirosis,
malaria and other
viral hemorrhagic fevers. Though better diagnostic tests are available for
rickettsial diseases and
scrub typhus elsewhere, the Weil-Felix test is still commonly used in India, mainly because microimmunofluorescence assays (M-IFA) were not available in India till recently and relevant staff had insufficient training. The present study was performed to investigate the performance of M-IFA,
IgM ELISA, and Weil-Felix test on 546 non-repeated serum samples from subjects suspected of having
scrub typhus. One hundred and forty-three of these 546 samples were positive by M-IFA; these cases were also confirmed clinically to have
scrub typhus based on their dramatic responses to
doxycycline therapy.
IgM ELISA was positive in 122 of the 143 M-IFA positive cases and the Weil-Felix test in 96. Though the Weil-Felix test is a heterophile agglutination test, it was found in this study to have good specificity but far too little sensitivity to use as a routine diagnostic test.
IgM ELISA can be a good substitute for M-IFA. Incorporation of multiple prototype
antigens on M-IFA slides is likely one of the reasons for its superior performance. As newer and better diagnostic assays become available for
scrub typhus diagnosis in developed countries, it will be imperative to also use such tests in other endemic countries to prevent over- or under-diagnosis of
scrub typhus.