Nepal has seen the outbreak of several emerging and re-emerging diseases in recent years, including dengue fever, rickettsial fevers, and other vector borne diseases . The emergence of these diseases has been attributed to ecological changes, climate change, dispersion of mosquito vectors  and human population dynamics . Nepal has three major ecological zones: the tropical Terai region, a subtropical and temperate mid-hill region, and the subalpine to alpine Himalayan region [3, 4].
Dengue fever, malaria, and Japanese encephalitis (JE) are among the most common vector borne diseases (VBDs) in low- and middle-income countries (LMICs) . The endemicity and overall burden of VBDs in LMICs is strongly related to infrastructural weaknesses, including poor water systems, sanitation, and hygiene; and the health system to respond . Studies suggest the co-circulation of similar VBDs like dengue and Japanese Encephalitis (JE) for years in Nepal . Recent outbreaks of dengue fever in Nepal in 2019 have alarmed public health authorities with unprecedented spread, morbidity and mortality.
Dengue is a viral infection transmitted by female Aedes aegypti and Aedes albopictus mosquitoes . The causative agent, dengue virus (DENV), belongs to the genus Flavivirus of Flaviviridae family of single-stranded RNA virus [9, 10]. DENV has four main serotypes: DENV-1, DENV-2, DENV-3 and DENV-4 . Infection with any one of these serotypes likely confers lifelong immunity to that specific serotype . Infection by a new serotype may result in severe disease . Most dengue infections (up to 60%) are self-limiting , and are characterized by acute fever, frontal headache, vomiting, myalgia, joint pain, and macular skin rash . However, some patients may develop life-threatening conditions such as acute dengue hemorrhagic fever (DHF), dengue shock syndrome (DSS), and (multi-)organ failure . In the absence of effective vaccines and antiviral drugs, symptomatic treatment and vector control programs are currently the only viable strategies for dealing with dengue infections. [17, 18]. Studies so far have suggested that timely diagnosis and clinical management with intravenous rehydration are critical to mitigate the severity of infection . Transmission can be reduced through protection from blood feeding Aedes mosquitoes.
The laboratory diagnosis of dengue is supported by the clinical suspicion followed by diagnostics that include rapid diagnostic tests (RDT), enzyme linked immunosorbent assay (ELISA) and complete blood counts (CBC) . A CBC profile demonstrating leucopenia, thrombocytopenia, increased hematocrit and liver enzymes are some of the parameters that aid in clinical suspicion . More specific and sensitive diagnostic tools such as viral isolation and culture, and detection of viral genome by polymerase chain reaction (PCR), are not routinely performed in Nepal.. Moreover, serological tools are used even during epidemic outbreaks, which further limits the proper diagnosis of disease in Nepal, as such tests are not the gold standard and DENV virus may not be detected prior to the development of antibodies, severely limiting diagnosis during outbreaks .
Previous studies from Nepal have explored the seroprevalence in various regions since the first potential outbreak of dengue in Nepal in 2006. Overall seroprevalence of 10.4% (anti-DENV-IgG) was found among suspected cases of dengue fever (DF) and DHF in south-west region of Nepal 2006 . Seroprevalence studies targeting smaller geographic locations have found 7.7% in Kathmandu in 2007 , 29.3% in south-western Terai between 2007 and 2008 , 9.8% in 2009 in the same region , 12.2% in Kanchanpur , 11.8% Bharatpur and Rapti Zonal Hospital in 2011 [26, 27], and 19.3% in Chitwan and Dang in 2013. ELISA was the choice of technique in all these studies. Rapid diagnostic tests and particle agglutination tests were used for primary screening. In few studies, molecular techniques such as reverse transcriptase PCR were also used [20, 23]. Despite of these various methods, seroprevalence in the range of 10 to 30% in Nepal.
Although the Government of Nepal has developed an Early Warning and Reporting Systems (EWRS) to issue warning on potential outbreaks, the response to dengue outbreaks have not been sufficient to prevent outbreaks. In 2019 there was a large dengue epidemic in Nepal , coinciding with outbreaks of dengue and other Aedes-spread diseases throughout much of the tropical world. There are several challenges for prevention and control of dengue infection in Nepal, among which robust mechanism to respond to the outbreak has been constrained by lack of updated epidemiological data. In addition, Nepal has recently entered into a federal system with three tiers of government: federal, provincial and local which lack effective coordination that has adversely impacted the management of human resources, logistic chain management and surveillance . To mitigate these challenges, the federal system has devised an integrated vector control strategy (that includes diseases such as malaria, and kalaazar), that is currently under preparation. Nonetheless, variation in characteristics of vectors, mechanism of disease transmission and epidemiology may remain as major challenges.
Countering these challenges is critical for designing an effective dengue control and prevention program which largely relies on effective detection of the cases, diagnosis and prevention based on the surveillance data. There are no previous studies systematically exploring the epidemiological trends and distribution of the dengue cases at a nationwide scale. The main objective of this study was to explore the epidemiological patterns of dengue fever since its first outbreak (2006) through 2019 in Nepal so that future public health efforts can be appropriately targeted.