Spatiotemporal dynamics and environmental trends of reported human leptospirosis in Sri Lanka, 2007-2024

Leptospirosis is a globally significant zoonoses that contributes to 1.03 million infections and 58,900 deaths annually. Animal reservoirs carry pathogenic Leptospira in the kidneys and contaminate the environment via urine. Contact of mucous membranes or abrasions with pathogenic Leptospira commonly leads to infection in humans. Leptospirosis has various presentations, ranging from an acute self-limiting infection to a severe infection with multi-organ involvement requiring hospitalization. In 2008, leptospirosis cases in Sri Lanka spiked, marking a potential emergence event. In the current work, cases of reported leptospirosis were analyzed from 2007 to 2024 identifying an overall increase in leptospirosis over the study period and annual seasonality. Increased precipitation seems to be driving the seasonal spikes in leptospirosis case rates across Sri Lanka. District level case rates over the 18-year time frame increased in the southern districts and indicated a dissemination to northern districts. A low-low spatial cluster, signifying an area of lower disease, was identified in the northern districts from 2007 to 2012, but was not observed from 2013 to 2024. A high-high spatial cluster, indicating an area of increased incidence, was identified from 2013 to 2024. District level precipitation and case rates were correlated over the 18-year study period, suggesting that rainfall is a driver of disease in Sri Lanka. While this correlation was identified to be significant, the correlation weakens over time when analyzed per annum with 2011 and 2024 showing no link. No correlation was identified in 2011, however, analysis of precipitation trends and case rates over the year show that major flooding events drove anomalous increases in leptospirosis case rates throughout the country. In 2024, a record year in reported leptospirosis, there was also no correlation over the entire year but the seasonality of disease still aligns with the overall trend. Investigation of renal carriage in five potential animal reservoirs identified that all carried the lipL32 gene of pathogenic Leptospira species. Together, this work identified temporal and spatial trends of human leptospirosis in Sri Lanka, identifying a link with precipitation as a potential driver of disease, and further characterized localized renal carriage of animal reservoirs linked to environmental contamination.