Serological Diagnosis of Indian Tick Typhus in and around Puducherry: Application of Indirect Immunofluorescence Assay
Dr. Selvaraj Stephen,
Professor, Department of Microbiology, Mahatma Gandhi Medical College and Research Institute, Pondy-Cuddalore Main Road, Pillaiyarkuppam-607403, Puducherry, India.
Introduction: In the past, tick typhus caused by Rickettsia conorii has been reported from different parts of India. However, during the last two decades, reports are few and far between with only some case reports and very few seroprevalence studies.
Aim: To estimate the seroprevalence of Indian tick typhus among febrile patients of Puducherry and surrounding Tamil Nadu, employing Indirect Fluorescence Antibody (IFA) test, which is considered as the ‘gold standard’ serological test for diagnosing rickettsial diseases.
Materials and Methods: This study was conducted during the period February 2018 to March 2019 at Mahatma Gandhi Medical College and Research Institute and Indira Gandhi Government General Hospital and Post Graduate Institute, Puducherry, India. The study included 114 febrile patients who provided both acute and convalescent serum samples. Forty ante-natal women and 23 voluntary blood donors were included as healthy controls. All sera were examined for IgG antibodies to Rickettsia conorii by IFA test (Fuller Laboratories, Fullerton, California, USA). Mean±Standard deviation with 95% confidence interval was calculated for age of the patients and duration of illness using Graph Pad Quick Calcs Software, USA. Chi-square and Fisher’s-exact tests were performed and p-value =0.05 was considered as statistically significant.
Results: Out of 114 febrile patients screened for Spotted Fever (SF) IgG IFA, 27 were positive in IgG IFA with titres ranging from 1:128 to 1:2048 (23.68%). Among the control group (n=63), only one participant was seropositive for R. conorii IgG IFA (1.59%). Statistical difference in seropositivity between the febrile patients and healthy controls was quite significant (p=0.0001).
Conclusion: Due to large number of false positivity, presence of IgM antibodies in acute SF is of doubtful significance. Presence of significant titres of Spotted Fever Group (SFG) IgG antibodies in IFA (=1:128) in acute serum or four fold increase in titres in paired sera are the recommendation of Centre for Disease Control and Prevention (CDC) to confirm SF.