Molecular and Entomological Characterization of 2023 Dengue Outbreak in Dhading District, Central Nepal

Abstract

In 2023, Nepal faced its second largest dengue outbreak ever, following a record-breaking number of dengue cases in 2022, characterized by the expansion of infections into areas of higher altitudes. However, the characteristics of the 2023 circulating dengue virus (DENV) and the vector density remain poorly understood. Therefore, we performed DENV serotyping, clinical and laboratory assessment, and entomological analysis of the 2023 outbreak in central Nepal. A total of 396 fever cases in Dhading hospital suspected of being DENV positive were enrolled, and blood samples were collected and tested by different techniques including PCR. Of these, 278 (70.2%) had confirmed DENV infection. Multiple serotypes (DENV-1, -2, and -3) were detected. DENV-2 (97.5%) re-emerged after six years in Dhading while DENV-3 was identified for the first time. Dengue inpatients had significantly higher frequency of anorexia, myalgia, rash, diarrhea, nausea, vomiting, abdominal pain, and thrombocytopenia (p < 0.05). In this area, Aedes mosquitoes largely predominated (90.7%) with the majority being A. aegypti (60.7%). We also found high levels of Aedes index (20.0%) and container index (16.7%). We confirmed multiple DENV serotype circulation with serotype re-emergence and new serotype introduction, and high vector density in 2023. These findings call for the urgent initiation and scaling up of DENV molecular surveillance in human and mosquito populations for dengue control and prevention in Nepal.

Keywords: Aedes spp.; Nepal; dengue outbreak 2023; dengue virus serotypes; entomological investigation; vector density.

Figure 1

Nepal map showing provincial incidence of dengue and the current study area (Dhading district with its 13 municipalities). 1, Rubivalley; 2, Gangajamuna; 3, Khaniyabash; 4, Tripura Sundari; 5, Netrawati; 6, Jwalamukhi; 7, Nilkantha; 8, Siddhalek; 9, Benighat Rorang; 10, Gajuri; 11, Galchii; 12, Tharke; 13, Dhunibesi. Dengue incidence rate (IR) per 10,000 population in all seven provinces of Nepal is presented with different color codes [18].

Figure 2

Flow diagram of the study. Out of 8017 febrile patients, 2814 (35.1%) were dengue positive.

Figure 3

Seasonal trend of dengue and monthly climatic parameters in Dhading, Nepal, 2023.

Figure 4

Map of Dhading district showing multiple DENV serotypes circulation in human in 2023.

Figure 5

Trend of dengue cases in Nepal with available DENV serotype distribution during 2004 to 2023 [8,10,11,12,13,14,15,18,19,40,41,42]. DENV serotypes are given in the box above the bar for each year with data availability. Y-axis indicates the number of cases in log-scale and X-axis denotes year.

Figure 6

Density of mosquito vectors in Dhading during 2023 dengue outbreak. (a) Mosquito density by species. (b) Ward-level map of Nilkantha municipality showing vector density in the bar diagrams for each of 3 wards (wards 3, 4, and 6) that underwent entomological investigation. Numbers (1 to 14) in the map indicate ward numbers (sub-municipality divisions) of Nilkantha municipality.

Figure 6

Density of mosquito vectors in Dhading during 2023 dengue outbreak. (a) Mosquito density by species. (b) Ward-level map of Nilkantha municipality showing vector density in the bar diagrams for each of 3 wards (wards 3, 4, and 6) that underwent entomological investigation. Numbers (1 to 14) in the map indicate ward numbers (sub-municipality divisions) of Nilkantha municipality.