Introduction
Dengue fever (DF) is the most common mosquito -borne tropical viral illness in humans. It is caused by dengue virus (DENV), which is a positive sense single, stranded RNA virus belonging to the family Flaviviridae. The clinical spectrum of dengue can vary from classical dengue fever to severe and life threatening conditions like dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). There is an increasing trend in the incidence of dengue with in the past two decades affecting 40- 50% of the world’s population. With early diagnosis and appropriate treatment, the mortality rate of dengue is as less as 1% whereas the mortality rate rises to 20% when left untreated.
The genome of the virus is cleaved into three structural proteins, namely, C, prM, E and seven nonstructural proteins, NS1, NS2a, NS2b, NS3, NS4a, NS4b and NS5. The tests currently available for diagnosis of early dengue infections are dengue NS1 antigen detection and RT-PCR.
During acute dengue infections when IgM is not readily detectable, NS1 antigen based ELISA shall be considered as an important diagnostic tool. In endemic as well as non-endemic countries several commercial NS1 antigen detection kits are available for the diagnostic purposes. The sensitivity of tests available for dengue diagnosis varies between 63% and 94% based on various factors like the time and type of tests performed, DENV serotype and whether the infection is primary or secondary. 1,2,3,4
Apart from the problem of ever increasing in cidence of dengue, the spread to new areas and explosive outbreaks poses a greater threat to public health. The objective of this study is to determine the epidemiological and demographic factors of dengue in a rural area of Puducherry to improve the localized dengue surveillance and early warning system.
Materials and Methods
At the Department of Microbiology in a tertiary care hospital of Puducherry this hospital based retrospective study was carried out between January 2017 and February 2018. All patients irrespective of age with clinical suspicion of dengue/DHF/DSS with manifestations like headache, retro orbital pain, myalgia, arthralgia, rash, hemorrhagic manifestations, and leucopenia, as described by World Health Organization (WHO)5 were included in this study. A total of 1880 patients were screened for dengue NS1 antigen, IgM and IgG antibodies. NS1 antigen and IgM antibodies were detected using ELISA KITS supplied by INBIOS & JM ithra respectively, where as IgG antibodies were detected using lateral flow assay (J.Mithra) following the manufacturer’s instructions.
Results
Patient’s sample reactive for either of dengue NS1antigen, IgM or IgG antibody was considered as positive. Out of 1880 samples collected from dengue suspected cases 607(32.2%) were reactive to any one of the test. Among the dengue positive cases 58.2%(353) were males and 41.8%(254) were females. The most commonly affected age group was 21-30yrs (9.6%) followed by 31-40yrs (6.5%).Figure 1 Maximum cases occurred in the months of September (124) and October (256)Figure 2 . Positivity for NS1, IgM & IgG alone were found to be 48.3%(293), 22.6%(137) and 2.9%(18) respectively where as 20.1%(122) were positive for both NS1 and IgM, 0.8% (05) were positive for both NS1 and IgG, 2.3%(14) were positive for both IgM and IgG and 2.9%(18) were positive for all three. Figure 3 .
Discussion
The incidence of dengue was found to be increasing for past two decades all over the world. The actual numbers of cases were under reported due to various reasons like under diagnosis / lack of confirmation of dengue / non availability of laboratory tests. It was recently estimated that 390 million dengue infections occur per year (95% credible interval 284 – 528 million), of which only 96 million (67–136 million) manifest as clinical disease.6 Brady et al., reported at 2012 that an estimate of 3.9 billion people, in 128 countries, are at risk of infection with dengue viruses.7 According to the data from National Vector Borne Disease Control Programme (NVBDCP) and National Health Profile 2018, From less than 60,000 cases in 2009, cases increased to 188,401 in 2017 — more than a 300 per cent spike. When compared to 75,808 cases in 2013, it is more than a 250 per cent spike. In our previous study conducted in the year 2016 the prevalence rate was 13.9 %.8 The prevalence rate (32.2%) has increased by two folds for the year 2017 in this study. Dengue incidence rate varies from time to time in a given location due to complex interactions among host, vector, virus and it is also proven that climatic factors also plays a role. We observed an increase in the incidence of dengue every month with maximum diagnosed in the months of September and October which coincides with the monsoon months of our locality. Majority of the people attending this hospital belong to rural areas may be due to unplanned urbanization, lack of awareness, poor sanitation and poor drainage systems, inadequate vector control are common leading to water logging and stagnation favoring breeding of mosquitoes and hence high prevalence rates.
Studies conducted in Brazil, Singapore, India and Saudi Arabia showed that dengue was predominantly seen in persons of age between 21-40 years.9,10,11,12 In our study also, we observed 49.7 %(302) of dengue infection in age group of 21- 40 yrs. A male preponderance was reported in studies from Cambodia, Malaysia, Sri Lanka, Singapore, Philippines and India.13,14 In our study also there was a male preponderance with 58.2% (353) being males. More outdoor activities in adults may be one of the reasons for increased probability of exposure to infected mosquitoes. Our current study showed the highest rate of infection during the month of September and October which is the end of monsoon and beginning of post monsoon period in this region (8). It was also observed that number of positivity increases during monsoon season and immediate post monsoon season, when the environmental condition was favorable for breeding of the vector, i.e., Aedes aegypti. Various studies conducted across India also substantiate this seasonality finding. 15,16
For the effective disease control precise diagnosis is very important. Among the 607 positive cases, 48.3%(293) were NS1 antigen positive which clearly indicates that they were viremic and suffering from a primary infection, i.e. they could effectively transmit the virus to the mosquito. 22.6%(137) were found to be positive only for IgM antibodies representing recent infection, 2.9%(18) were positive for only IgG which represents past in fection. The IgG positivity may be due to subsequent infection with any one of the dengue serotypes during the previous outbreaks and this may have sensitized the individual to the tests. 17 In this study, we reinforce the use of all three tests ie. NS1 antigen and IgM, IgG antibody during the earlier phase of illness as secondary infections can be missed if only IgM antibody detection alone was done.
The surveillance system we follow for dengue in India is a passive system, where the cases diagnosed and confirmed by health centers are reported to district health officer and then surveillance is carried out by the state government and the National Vector Borne Disease Control Program. All our cases have been reported to public health department for effective implementation of the preventive measures like conducting awareness program, draining of water filled ditches, advising to use personal mosquito nets, repellents etc.
Conclusion
Our study showed a seasonal trend in dengue infection commencing from the month of July, reaching its peak in September and starts de creasing from November onwards with a male preponderance. ELISA and rapid diagnostic tests for dengue have augmented our ability to detect new infections and to distinguish them into primary or secondary infections. Social awareness, personal protection and vector control measures plays a major role in controlling dengue epidemics and further improvements in the diagnostics with early treatment may prevent the complications of dengue.
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