Dengue: A disease without borders

Dengue fever continues to challenge global health systems, transcending geographic boundaries and posing a threat to millions. Caused by four distinct dengue virus serotypes (DENV1-4), the disease—transmitted through the bites of Aedes mosquitoes—has seen an alarming rise over the past decades with half of the world’s population at risk. In 2023, dengue cases exceeded 6.5m, with over 7,300 deaths, the highest on record. Annually, 390m people are infected, with 96m showing clinical symptoms. Once confined to nine countries before 1970, dengue is now endemic in over 100 nations, predominantly in tropical and subtropical regions. Dengue’s reach is expanding, with cases emerging in new areas due to climate change, highlighting its escalating global impact.

Historical context: A long-standing global issue

The origins of dengue fever can be traced back over a thousand years. While the virus likely emerged from non-human primates and crossed into human populations a long ago, it was not until the 18th and 19th centuries that the disease became more widely recognized. The term “dengue” emerged during the 1828 Cuba epidemic, previously called “dunga”. Suspected outbreaks were reported in Martinique (1635), Guadeloupe, and Panama (1699). Ancient Chinese texts from the Chin (265–420) and Tang (610) dynasties described a similar illness as “water poison” linked to waterborne insects. The disease has thus been a recurrent global health threat for centuries, often emerging in new regions.

Clinical spectrum and diagnosis

Dengue ranges from mild, often asymptomatic cases (80 percent), to severe forms like hemorrhagic fever and shock syndrome. Symptoms last 2–7 days, with complications such as gastrointestinal bleeding and hematuria (blood in urine). Diagnosis varies by phase, using virus detection early and serology later. Treatment focuses on symptomatic management, with most cases managed at home and severe cases requiring hospitalization.

Dengue emergence in Nepal

Dengue outbreaks have evolved significantly over the past two decades, influenced by epidemiological shifts and environmental changes. Since the first imported case, a Japanese national in Chitwan in 2004, Nepal has experienced sporadic outbreaks, initially confined to the Tarai. The 2006 outbreak in Lumbini province, with 32 cases, marked the first notable local spread, with all four serotypes detected. 

The 2010 outbreak with 917 cases, including the first in the highland Kathmandu Valley, signaled dengue’s acclimatization. By 2011, it spread to 15 districts, and in 2013, a major outbreak across 25 districts, dominated by DENV-2, highlighted dengue’s persistence and complexity. The period marked dengue expansion into higher altitudes and the circulation of all serotypes, emphasizing the need for stronger surveillance and public health responses.

Large-scale outbreaks

Dengue outbreaks have shown increasing adaptability and severity. In 2016, a total of 1,527 cases and one death were reported across 30 districts, with Bagmati province and Chitwan district most affected. In 2017, 28 districts reported 2,111 cases with three deaths. In 2018, 43 districts recorded 811 dengue cases and three deaths. The largest outbreak was in 2019, with 17,992 cases and six deaths across 68 districts in all seven provinces, with Bagmati province most affected. Dengue’s spread from tropical to hilly subtropical regions highlighted the virus’s adaptability. 

The Covid-19 pandemic (2020–2021) saw a temporary decline in reported dengue cases with around 500 cases each year, due to movement restrictions, reduced healthcare access, and potential underreporting. However, dengue remained endemic, with the 15–49 age group most affected.

These outbreaks underscored the need for robust public health strategies, continuous surveillance, and climate-adaptive measures to manage the virus’s evolving threat.

Escalating dengue burden: 2022–2024

Nepal faced successive severe dengue outbreaks from 2022 to 2024, with escalating cases and widespread geographic impact. In 2022, the country recorded its worst outbreak with 54,784 cases and 88 deaths across all districts. Bagmati province accounted for 77 percent of cases, with Kathmandu Valley (1,300 meters) heavily affected due to rising temperatures, early monsoon, rapid urbanization and poor waste management. The epidemic peaked between August and November, with September alone contributing 50 percent of the cases.

In 2023, Nepal reported 51,243 cases and 20 deaths, with 75 percent concentrated in Koshi and Gandaki provinces, and 98 percent of the infections occurring between June and November.

By 3 Dec 2024, a total of 34,385 cases and 13 deaths were reported across 76 districts (Humla with no cases). Gandaki and Bagmati provinces contributed over 80 percent of the cases, with Kaski and Kathmandu, most affected, peaking in September and October.

Climate change and spread in highlands

Climate change has aggravated dengue spread to higher altitudes including the Kathmandu Valley and hilly regions. Traditionally confined to the Tarai, dengue now affects subtropical to mid-hills due to rising temperatures, increased rainfall, and extreme weather events like flooding and landslides. Outbreaks in 2016, 2019, 2022-2024 were marked by extreme weather events. 

Nepal’s average temperature has increased by 0.056°C over the past four decades, with higher elevations in the Himalayan regions experiencing more pronounced warming. This shift has created a conducive environment where A. aegypti and A. albopictus can now survive, thrive and breed. The dengue expansion into higher altitudes is particularly concerning, as it exposes new populations to the virus and increases the risk of severe outbreaks. Its spread to higher altitudes underscores its adaptability to diverse ecological niches. The co-circulation of multiple serotypes raises concerns about severe dengue in previously infected individuals. Additionally, poor waste management and inadequate vector control have exacerbated Nepal’s vulnerability. 

The rise of extreme weather events challenges dengue outbreak prediction and management, necessitating climate-adaptive, evidence-based public health interventions. Integrating climate-resilient health systems into national policies and enhancing local capacity is essential to address climate change’s impact on dengue transmission dynamics. A thorough understanding of vector behavior and environmental shifts is critical for effective mitigation.

Dengue’s escalating burden demands a multipronged approach, including vector control, climate adaptation, surveillance, and community engagement with adaptive governance. Community ‘Search and Destroy’ campaigns on eliminating stagnant water, using repellents and fumigation are vital in reducing transmission. A coordinated holistic strategy addressing biological and environmental factors is key to prevent future outbreaks, safeguarding public health across borders.