Emerging evidence of sand fly fever virus transmission in Sri Lanka.

BACKGROUND: Sand fly fever virus (SFFV) infection is an acute self-limiting febrile illness caused by phleboviruses and transmitted by sand flies of the Phlebotomus species. The disease has been reported in the Mediterranean region, North Africa, the Middle East, and the Indian subcontinent, and is now considered to be expanding into new geographic areas. Clinically, SFFV infection closely resembles acute dengue fever. AIM: To detect other possible infection including SSFV in patients with clinically suspected dengue fever with negative serology results for NS1 antigen. METHODS: In this study, a total of 757 serum samples were collected from patients clinically suspected of dengue fever and admitted to three major hospitals in Sri Lanka between January and August 2023. Patients were from the Colombo and Kandy District, and demographic information was obtained from laboratory request forms, although detailed individual-level demographic data were limited due to anonymization in this laboratory-based study. All samples were initially screened for dengue NS1 antigen negativity and further tested for a panel of pathogens including SFFV immunoglobulin (Ig) M, hantavirus IgM, chikungunya IgM, and leptospira IgM antibodies, as well as molecular detection of hantavirus, flavivirus, and alphavirus RNA. RESULTS: From tested samples, two samples were serologically positive for SFFV-specific IgM using a commercial immunoblot assay, while all other tests for dengue, chikungunya, leptospira, hantavirus, and related RNA viruses were negative. These two SFFV-positive cases were from Kandy District and included one child (6-8 years) and one adult (40-50 years), both presenting with uncomplicated dengue-like illness. CONCLUSION: This study provides the first evidence of SFFV infection in Sri Lanka. Given the high endemicity of dengue in the country, SFFV infections may be clinically misdiagnosed as dengue and therefore remain under-recognized. Further studies involving febrile patients and sand fly vector surveillance are required to determine the true burden and epidemiological significance of SFFV in Sri Lanka.