A retrospective study on clinico-epidemiological characteristics of rickettsioses in Sri Lanka

Abstract

Rickettsioses, are vector-borne infections caused by intracellular bacteria of genera Rickettsia and Orientia. They cause typhus fevers or fevers of unknown origin, (FUO) in Sri Lanka. A retrospective study was conducted at the Rickettsial Disease Diagnostic and Research Laboratory, University of Kelaniya, from 2018-2023 to characterize clinico-epidemiological and hematological features of confirmed typhus fever cases {IgG antibody titres >1:128 for O. tsutsugamushi (kato) or R. conorii antigens by Immunofluorescence assay} referred to as the test and unconfirmed cases as the control groups. Clinico-epidemiological and hematological indices were retrieved from the database. Data was analyzed using chi-square tests followed by a correlation analysis between the variables using Pearson correlation. Of 1,221 referrals, 247 (20.2%) were confirmed rickettsial infections; Rickettsia conorii 66.7% (n=166), O. tsutsugamushi 30.1% (n=75), and mixed infections 3.2% (n=8). Both test and control groups were predominantly male, with male-to-female ratios of 1.33 and 1.18, respectively. Most affected were the younger population (20.6%, < 9 years and 10.8%, 10-19 years). The average duration of fever at referral was 13-14 days. Many cases (35%) occurred during January-February. The Western province, primarily Gampaha district had the most typhus infections, (67.6%, n=167) caused by R. conorii (n=108; 64.7%). A significant leucopenia (P=0.005), neutrophilia (P=0.0004), and an elevated erythrocyte sedimentation rate (ESR) (P=0.034) was noted in the test group. Elevated liver enzymes (ALT P=0.0001, AST P=0.0001) and urine occult blood (P=0.024) were also higher. Eschar was common in O. tsutsugamushi cases (33.4%; n=25) than R. conorii (13.3%; n=13). This study highlights the need to increase diagnostic facilities for rickettsioses which comprise one-fifth of FUO. The presence of an eschar, leucopenia, neutrophilia, an elevated liver enzyme profile and occult blood in urine are markers suggestive of a rickettsial etiology in FUOs.