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

Abstract

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.

Keywords: Epidemiology of scrub typhus; Local transmission; Nepal; Orientia tsutsugamushi; Outbreak; Rickettsial infection; Scrub typhus.

Fig. 1

Overall flow-diagram of the study

Fig. 2

Map of Chitwan district showing the animal and entomological study sites. Investigation was carried out in three VDCs (Mangalpur, Sharadanagar and Shukranagar) (dark filled areas) which are located to the South-West of the district headquarters of Chitwan (the outbreak epicenter district). VDC, village development committee (Source: EDCD data using Arc GIS software)

Fig. 3

Distribution scrub typhus cases during major outbreaks in Nepal. In 2016, a total of 831 scrub typhus cases (based on peripheral reporting) were reported from 47 of the 75 districts (a), of which a total of 401 cases from 42 districts were confirmed at national reference laboratory and included in further analysis (b), while 141 scrub typhus cases from 25 districts were reported to EDCD in 2015 based on aggregated data available from EDCD (c), and a total of 267 cases were confirmed from 30 districts in 2017 (d). Respectively 8, 14 and 3 deaths due to scrub typhus were confirmed in 2015, 2016 and 2017. From 2015 through 2017, 52 out of 75 districts of the country reported cases of and/or deaths due to scrub typhus (Source: EDCD data using Arc GIS software)