Spatial Patterns of Dengue Incidence in Nepal During Record Outbreaks in 2022 and 2023: Implications for Public Health Interventions
Simrik Bhandari, Jason K. Blackburn, Sadie J. Ryan
American Journal of Tropical Medicine and Hygiene · 2025 · DOI: 10.4269/ajtmh.24-0747
Countries: Nepal
What Was Studied
This study asked where in Nepal dengue incidence was unusually high or low during the country's two largest outbreaks on record (2022 and 2023), and when each district's cases peaked. The researchers digitized monthly district-level dengue case counts for all 77 districts from the Ministry of Health and Population's Epidemiology and Disease Control Division (EDCD) surveillance system, paired with population figures projected from the 2011 and 2021 national censuses. Dengue incidence per 100,000 people was mapped for each district and year, and Local Moran's I spatial statistics (queen contiguity weights, 999 permutations, Bonferroni-corrected significance at α=0.025) were used to identify statistically significant hotspots, cold spots, and spatial outliers. This is an ecological (district-level) analysis of aggregated case counts, not an individual-level study, so its clusters describe where incidence is statistically concentrated, not why.
What They Found
Nepal recorded 54,784 dengue cases in 2022 and 51,243 in 2023 — both record years. District-level incidence ranged from 4.21 to 1,716.89 per 100,000 people in 2022, and from 2.49 to 2,242.44 per 100,000 in 2023. In 2022, a statistically significant hotspot (Global Moran's I = 0.472, p = 0.001) formed around six Bagmati districts (Kathmandu, Lalitpur, Bhaktapur, Kavrepalanchok, Makwanpur, and Dhading), while a cold spot covered eight remote northwest districts (Humla, Mugu, Bajura, Kalikot, Jumla, Jajarkot, Rukum West, and Surkhet); most districts peaked between August and October. In 2023, overall clustering was no longer statistically significant (Global Moran's I = 0.095, p = 0.060), but the hotspots had shifted entirely to new locations — four Gandaki districts (Gorkha, Lamjung, Kaski, Syangja) and two Koshi districts (Dhankuta, Morang) — while peak case months spread out from March to November, with mountain districts peaking both earliest and latest.
What This Means for Nepal
This district-level hotspot mapping gives EDCD (the Ministry of Health and Population's Epidemiology and Disease Control Division, Nepal's federal disease-surveillance authority under the constitution's Schedule 5 communicable-disease mandate) a concrete tool for the 'efficient early-warning system' that dengue researchers have been calling for since the record 2022 outbreak. Because the hotspot moved entirely from Bagmati (2022) to Gandaki and Koshi (2023), EDCD's vector-control resources — larval source management, fogging, diagnostic capacity — should be allocated dynamically each season using real-time LISA cluster analysis rather than fixed year-on-year district plans. Separately, given Karnali's well-documented health workforce crisis (0% of sanctioned physician and consultant posts filled), the low incidence recorded in Karnali's northwest districts should not be read as genuine safety from dengue; EDCD should commission active case-finding or serological surveys there before ruling out transmission, since the study's own authors caution the 'cold spot' may reflect under-reporting rather than true absence of disease.
Contextualisation
Nepal's dengue burden reached record levels in 2022 (54,784 cases, 88 deaths) and remained high in 2023, with EDCD's own surveillance confirming the vector now reaches 2,438m in Jumla — a climate-driven ascent into hill and mountain districts once assumed too cold for Aedes mosquitoes. This study uses that same EDCD case-surveillance system and finds dengue reached all 77 districts in 2022, while the 'cold spot' it identifies in Karnali's remote northwest (Humla, Mugu, Jumla, Kalikot) is a caution flag rather than reassurance: Karnali has 0% of its sanctioned physician and consultant posts filled, so low reported incidence there may reflect weak case detection rather than true absence of transmission.