Acute Dengue Infection in the Western Terai Region of

Methods: The study was conducted between August 2007 and July 2008 in patients visiting hospitals of the western terai of Nepal with chief complains of fever. The sero-diagnosis of acute dengue infection was determined by enzyme linked immunosorbent assay among 239 patients visiting Lumbini Zonal Hospital, Butwal; Bheri Zonal Hospital, Nepalgunj; Bardiya District Hospital, Bardiya and Mahakali Zonal Hospital, Mahendranagar.


INTRODUCTION
Dengue is an acute febrile illness caused by a virus that is transmitted from humans to humans via species of infected Aedes aegypti mosquito.Four dengue virus (DENV) serotypes, i.e.DEN1-4, belonging to the genus fl avivirus, are responsible for the disease.It manifests as dengue fever (DF), dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). 1 Specifi c symptoms vary according to the severity of dengue. 2 Previous studies have shown that all the serotypes of DENV are found in Nepal. 3,4,5Nepal reported indigenous transmission of dengue cases 3 , although DF had already been observed in foreign visitors earlier. 3,6Because of the open border, Nepal is vulnerable to more serious consequences of DENV infection as infected people can easily come across and the virus might be transmitted by the vector, which is already evident in several locations of the terai region.The aim of the study was to determine the frequency of acute dengue infection using the serological method.The present study would be helpful by providing information on the epidemiology of the disease in the western terai region of Nepal.

METHODS
This was a cross-sectional study conducted between August 2007 and July 2008 in which samples were collected during the post-monsoon period (August -November 2007).The serum samples were collected from Mahakali Zonal Hospital, Mahendranagar; Bardiya District Hospital, Bardiya; Bheri Zonal Hospital, Nepalgunj and Lumbini Zonal Hospital, Butwal.Patients with high fever and clinical symptoms suspected of DENV infection (according to WHO guideline) 7 visiting the hospitals were included in the study, while patients infected with malaria and enteric fever were excluded.Similarly, subjects with previous Japanese Encephalitis immunizations were also excluded from the study.Patients' personal details about the symptoms and age, sex, etc, were obtained through a questionnaire method by direct interview after taking the consent.
Dengue antibodies were measured with enzyme linked immunosorbent assay, i.e.IgM capture ELISA (PanBio, Australia), and results interpreted according to the manufacturer's instructions.The cut-off value was obtained by multiplication of the average absorbance of calibrators and calibration factor (provided in the kit, i.e. 1).
The collected data was analyzed to fi nd out the age, sex, hospital and month wise distribution of the cases.Chi square test was used as a statistical tool.The statistics was done using Win Pepi software version 7.9, 2008.
The dengue positivity was 29.3 % (70/239).Out of the total 70 positive cases, the highest numbers of positive cases were in the age group 15 -50 years and the highest numbers of positive cases were in Lumbini Zonal Hospital, Butwal.The ratio of dengue positive cases in male to female was 1.1:1 (37:33).The association of the disease with age (P = 0.2) and sex (P = 0.6) was statistically insignifi cant (Table 2).Among four months, the highest number of positive cases 37 (52.9 %) were in October.Likewise, 17 (24.3%) in November, 10 (14.3 %) in September and 6 (8.6 %) in August (Fig 1).The association between the month and the occurrence of disease was statistically signifi cant (P = 0.01).

DISCUSSION
A major proportion of the study subjects had been exposed to DENV as refl ected by the increasing seropositivity of DENV infection.Information on the seroepidemiology of DENV specifi c IgM among Nepalese patients would provide a clue to the extent of DENV infection in Nepal.In this study, 29.3 % were found to have been suffering from acute dengue infection.However, DHF and DSS cases were completely absent.
The result coincides with a previous study carried out in Nepal. 8 The change in demography and society due to urbanization, migration, transportation, deforestation, ecology, etc, creates an environment that helps in the emergence of dengue. 9Moreover, dengue has already been evident in neighbouring countries -China and India.The Aedes vector can easily come across the territory because of modern transportation, although the vector is already found in Nepal.The open border in the terai region also increases the transmission of the disease as infected people can come across with the vector and transmit the disease.These might be the reasons for the higher positivity of dengue in the study.
The study was based on the commonly used ELISA test (99 % sensitivity and 84.4 specifi city), which also detects cross-reacting antibodies to other fl avivirus, such as Japanese encephalitis virus (JEV). 10,11 he false positive results might be minimal because IgM capture ELISA is highly specifi c as only 10.7 % DENV infection cross-reacted with JEV antigen. 12Subjects with previous JE immunizations were excluded from the study; therefore, false positive results because of cross reactivity were minimal.The cases coincide mainly with the post monsoon period of subnormal rainfall because the relative prevalence and distribution of Ae. aegypti larval indices are highest during the post monsoon period.Heavy rainfall subsequently leads to a decrease in temperature during the later part of the monsoon period.The temperature remains almost constant and subsequent rainfall further leads to a decrease in the temperature.The result is an increase in relative humidity and the abundant stocks of fresh water reservoirs generated due to rain develop optimum conditions for mass breeding and propagation of the vector and transmission of the virus. 13e highest numbers of positive cases were in Lumbini Zonal Hospital, Butwal.The comparatively higher positive cases might be due to the abundance of the vector, the Aedes species mosquitoes, and the circulation of multiple dengue viral strains.Butwal is one of the busiest stations from where people go to other parts of the country as well as to India and vice versa.So, DEN infected people might come across with the vector and transmit the disease.
The cross reactivity to JEV was not performed.Additionally, paired serum samples were not taken during laboratory tests.These remained the limitations of the study.

CONCLUSIONS
The positivity of dengue was estimated in acute patients from hospitals of western Nepal by IgM capture ELISA and the most affected group was the 15 -50 years.

Figure 1 .
Figure 1.Month wise distribution of dengue cases.

Table 2 . Distribution of total positive cases
The age wise distribution showed that the highest positive dengue cases were in the age group of 15 -50 years which constitutes 75.7 % of the total positive cases followed by the age group <15 years comprising 15.7 % and 8.6 % in the > 50 years age group.The result is in harmony with the data obtained in the outbreak of dengue in Nepal in the year 2006 in which the highest dengue positive cases were recorded in the age group greater than 15 years.3