Improving recognition of rickettsial infections in Sri Lanka: The value of combined qPCR and ELISA in acute febrile illness.

In Sri Lanka, reliable diagnostics for rickettsial infections are limited. We assessed the burden, seroprevalence, and molecular diversity of rickettsial infections in hospitalized patients with acute undifferentiated febrile illness (AUFI). AUFI patients were enrolled from the Western (n = 540) and Central (n = 260) Provinces. Clinical and exposure data, acute and convalescent sera, and buffy coat/eschar samples were collected. Orientia tsutsugamushi (OT) and Rickettsia spp. were detected using OT-specific (47 kDa) and pan-Rickettsia (17 kDa) qPCR assays. Group-specific IgG ELISAs were performed for scrub typhus (STG), typhus group (TG), and spotted fever group (SFG). Acute infection was defined by qPCR positivity and/or a ≥ 4-fold rise in IgG titres. Genetic diversity was assessed using OT 56 kDa TSA and Rickettsia ompB gene sequencing. qPCR identified 38/800(5%) STG and 25/800(3%) TG/SFG infections. Among participants with paired sera (n = 493), acute infections included 25(5%) STG, 11(2%) TG, and 17(3%) SFG cases. Overall, rickettsioses accounted for 86/800(11%) of AUFI cases. SFG seroprevalence was higher in the Central Province (17% vs 6%), while STG seroprevalence was higher in the Western Province (20% vs 13%). Genetic analysis on OT cases showed clustering with Karp, Kato, Buie, TH1811, TH1826 and Ikeda strains. Two cases of rickettsial infections were speciated as R. felis and R. sibirica. Only 19% of acute cases were clinically recognized and 58% received doxycycline. Rickettsioses were under-recognized, highlighting the need for combining qPCR and ELISA diagnostics to strengthen clinical management and surveillance.