INDIRECT IMMUNOFLUORESCENCE FOR SEROLOGICAL DIAGNOSIS OF DENGUE VIRUS-INFECTIONS IN SWEDISH PATIENTS

Background: An increasing number of travellers to dengue endemic areas has emphasized the need for an easy and reliable serological test for diagnosis of dengue virus infections. Objectives: Indirect immunofluorescence (IF) and hemagglutination inhibition (HI) tests were compared for serological diagnosis of dengue fever (DF). Study design: Sera from patients with clinical symptoms compatible with DF and a travel history comprising dengue endemic areas were included in the study. Paired serum samples from 24 patients and single convalescent sera from five patients were investigated by HI and IF, Paired sera from ten patients were investigated by plaque reduction neutralization test (PRNT) in order to confirm the HI/IF results. Results: In twelve of the patients with paired sera a sere-conversion or a significant (greater than or equal to 4-fold) antibody titer rise was seen with HI as well as with IF. Four patients with stationary HI-titers had significant titer rises when investigated by IF. Stationary or declining HI- and IF-titers were found in the remaining eight patients. The five single sera were antibody-positive by both methods. Investigation by PRNT of paired sera from ten patients confirmed the IF results. A total of 298 serum samples (paired sera or early convalescent samples) were investigated for dengue antibodies by IF. A sere-conversion, significant titer-rise or an IF-titer of greater than or equal to 320 was generally seen 7-13 days post onset of disease. During the years 1991-1993 altogether 161 cases of dengue were diagnosed by IF in Swedish patients, a majority of which had travelled to Southeast Asia. Conclusion: Antibody detection by indirect IF proved to be at least as reliable as HI for diagnosis of dengue infections in Swedish patients. The specificity of the IF was ensured by testing the sera of ten patients for serotype specific neutralizing dengue antibodies.