An outbreak investigation of scrub typhus in Nepal: confirmation of local transmission.

BACKGROUND: Scrub typhus is a largely ignored tropical disease and a leading cause of undifferentiated febrile illness in the areas of tsutsugamushi triangle caused by Orientia tsutsugamushi. It is frequently diagnosed in South Asian countries, although clear epidemiological information is not available from Nepal. After the 2015 earthquake in Nepal, a sudden upsurge in scrub typhus cases was reported. The objective of this study was to investigate epidemiology of scrub typhus and its causative agents in humans, animals, and chigger mites to understand the ongoing transmission ecology. METHODS: Scrub typhus cases with confirmed diagnosis throughout the country were included in the analysis. Studies were concentrated in the Chitwan district, the site of a major outbreak in 2016. Additional nation-wide data from 2015 to 2017 available from the government database included to analyse the disease distribution by geographical mapping. RESULTS: From 2015 to 2017, 1239 scrub typhus cases were confirmed with the largest outbreak occurring in 2016 with 831 (67.1%) cases. The case fatality rate was 5.7% in 2015 which declined to 1.1% in 2017. A nationwide outbreak of scrub typhus was declared as the cases were detected in 52 out of the 75 districts of Nepal. Seasonal trend was observed with a peak during August and September. In addition to the human cases, the presence of O. tsutsugamushi was also confirmed in animals (rodents) and chigger mites (Leptotrombidium imphalum) from the outbreak areas of southern Nepal. CONCLUSION: The detection of O. tsutsugamushi in humans, animals, and chigger mites from outbreak locations and wide-spread reports of scrub typhus throughout the country consecutively for 3 years confirms the ongoing transmission of O. tsutsugamushi with a firmly established ecology in Nepal. The country's health system needs to be strengthened for systematic surveillance, early outbreak detection, and immediate actions including treatment and preventive measures.

Mapping Environmental Suitability of Scrub Typhus in Nepal Using MaxEnt and Random Forest Models.

Being a globally emerging mite-borne zoonotic disease, scrub typhus is a serious public health concern in Nepal. Mapping environmental suitability and quantifying the human population under risk of the disease is important for prevention and control efforts. In this study, we model and map the environmental suitability of scrub typhus using the ecological niche approach, machine learning modeling techniques, and report locations of scrub typhus along with several climatic, topographic, Normalized Difference Vegetation Index (NDVI), and proximity explanatory variables and estimated population under the risk of disease at a national level. Both MaxEnt and RF technique results reveal robust predictive power with test The area under curve (AUC) and true skill statistics (TSS) of above 0.8 and 0.6, respectively. Spatial prediction reveals that environmentally suitable areas of scrub typhus are widely distributed across the country particularly in the low-land Tarai and less elevated river valleys. We found that areas close to agricultural land with gentle slopes have higher suitability of scrub typhus occurrence. Despite several speculations on the association between scrub typhus and proximity to earthquake epicenters, we did not find a significant role of proximity to earthquake epicenters in the distribution of scrub typhus in Nepal. About 43% of the population living in highly suitable areas for scrub typhus are at higher risk of infection, followed by 29% living in suitable areas of moderate-risk, and about 22% living in moderately suitable areas of lower risk. These findings could be useful in selecting priority areas for surveillance and control strategies effectively.