Abstract
Background
Dengue is one of the most important vector-borne disease in India. It has been linked to monsoons when Aedes aegypti mosquitoes breed profusely in containers. No study exists in Armed Forces wherein a community-based sero-survey has described the epidemiology of dengue. The present study tries to fill this knowledge gap.
Methods
A total of 422 participants were studied for one transmission season of July–December. Blood samples were collected for testing dengue IgG and IgM at the beginning and at end of the study period. The study participants were interviewed at least twice within this period of 6 months to assess clinical condition and follow-up. Point prevalence and incidence were measured. Distribution of presence or absence of symptoms was noted for positive as well as negative cases.
Results
All participants were males. Average age was 31.75 years. Point prevalence at the beginning of transmission season was 11.6% (95% CI: 8.4%–14.6%) and 15.6% (95% CI: 12.1%–19.1%) towards the end. Incidence was found to be 147.4 per 1000 for 6 months. Forty percent of incident cases were asymptomatic.
Conclusion
Healthcare planners and hospital commanders in stations across Armed Forces can use the prevalence and incidence figures obtained in this study as a general guide while planning for prevention and control of dengue. Also, this study points to the fact that dengue transmission in Delhi may have shifted earlier to months of April/May than the conventionally accepted season of July–December.
Keywords: Dengue, Transmission, Sero survey, Point prevalence, Epidemioloy
Introduction
There is an old African proverb: If you think you are too small to make any difference, you probably haven't spent a day with a mosquito. Maybe it was this philosophy that inspired the 2015 World Health Day theme as ‘Small Bite, Big Threats’. And as far as ‘small’ individuals making difference is concerned, no disease epitomises this truth in modern society than dengue, because being a manmade disease, prevention of Aedes breeding by source reduction at household and individual level is the best answer to this communicable disease. Mosquito-borne diseases in general and dengue in particular are gaining such notoriety in the tropics that graffiti slogans such as I wish Noah had swatted those two mosquitoes are not uncommon.
Dengue is turning out to be one of the most important vector-borne disease in recent times; its outbreaks occurring almost every year in many parts of India.
Delhi has become endemic for dengue. The transmission season is July to December, which has been authenticated in two separate studies.1,2 Armed Forces troops and families located in the station are at constant risk of infection throughout the transmission season.
The envelope protein of dengue viruses elicit the IgM and IgG responses against the infection in populations at risk. IgM is mounted as an acute response and stays positive for 2–3 months postrecovery. The IgG response is comparatively muted initially, but it stays for much longer durations.3 Measurement of these antigens in community based serosurveys can provide a wealth of information as far as public health planning of dengue prevention and control is concerned.
There is a paucity of evidence regarding point prevalence, incidence, and occurrence of subclinical cases of dengue, based on community based serosurveys in Indian Armed Forces. This study aims to fill this knowledge gap with an aim to describe the extent of dengue infection in a large cantonment during one transmission season. The study had twin objectives of determining the seropositivity of dengue in a defined military population in Delhi at the beginning of transmission season and to estimate the incidence of clinical and subclinical dengue infection in the population during the same period.
Materials and methods
This prospective observational study was carried out in serving healthy personnel of the Armed Forces from various units stationed in Delhi. Period selected was from July to December of the year of study, which completes one transmission cycle in Delhi. All serving Armed Forces personnel serving in Delhi-NCT formed the reference population. The study population comprised of suitably sampled troops from units where a medical officer is authorized.
Units stationed in Delhi were selected randomly from a list available at the local Station Headquarters. A consolidated nominal roll of serving personnel from earmarked units was prepared. The study population was then selected by simple random sampling.
Sample size for the study comes to 384 assuming a prevalence of 50% (for maximum sample size at given level of significance), alpha error as 5% and 10% error of margin. Thus, a sample size of 400 was studied to get adequate power for estimating both, the incidence as well as point prevalence of the infection.
Those personnel who were likely to move out of the station within 6 months from the commencement of the study were excluded.
Instruments
An android application-based questionnaire with a tablet–PC interface was used for baseline information collection and subsequent follow-up data entry. The questionnaire was first field tested on 15 individuals who were not included in the final analysis.
The serological tests for dengue infection were conducted with Novatec kits, NovaLisa ™. The qualitative immune-enzymatic determination of IgG and IgM class antibodies against dengue virus were based on the Enzyme-linked Immunosorbent Assay (ELISA) technique. The kits used for IgM ELISA were 97.6% specific and 91% sensitive whereas that for IgG kits was 93% and 90%, respectively.
Technique
The enrolled service personnel were asked to fill a detailed questionnaire for baseline demographic and service details at the beginning of the study. They were subjected to IgG as well as IgM ELISA testing for dengue at the beginning of transmission season. Seropositivity at this juncture was noted.
The participants were followed up at least twice during the transmission season, that is, upto December, for recent history of occurrence of fever, detailed history pertaining to movement and any change in job profile. During the season, those found with fever were referred to the garrison hospital and usual fever protocol was followed. Fever patients from the study group who were found dengue IgM positive on ELISA were labelled as incident cases. The same individuals were not counted twice as cases during the study period.
After 6 months of the transmission season, IgG and IgM tests in the same individuals were again conducted and the results were accordingly noted.
Although IgM ELISA is the diagnostic test of choice for determining acute illness, since the IgM antibodies generally wither away after 3 months of illness, cases occurring immediately after (or within the months of July, August, September) the initial sampling would have been missed. Hence, IgG positivity at the end of the season (in whom it was initially negative) was also kept as a diagnostic criteria of incident cases. Thus, cases were defined dengue positive as “any individual negative for IgG at the beginning of the transmission season but positive for IgG or IgM at the end of the season”.
Routine anti-larval and anti-adult measures against mosquitoes were followed as per protocol in the station throughout the transmission season. Institutional Ethics Committee approval was obtained. Informed consent was given by all participants. Statistical analysis was performed using MS Excel and SPSS ver 20.
Results
A total of 422 individuals were studied. All the participants were males. Average age was 31.75 years with a standard deviation of 6.93 years. The age ranged from 19 years to 55 years.
Seropositivity at the beginning of the transmission season, that is, July was measured by collecting samples in last week of June. The samples were tested for IgG as well as IgM. The results are as shown in Table 1.
Table 1.
IgG and IgM positivity at the beginning of transmission season.
| Result | IgG ELISA |
IgM ELISA |
||
|---|---|---|---|---|
| Frequency | Percent | Frequency | Percent | |
| Negative | 373 | 88.4 | 379 | 89.8 |
| Positive | 49 | 11.6 | 43 | 10.2 |
| Total | 422 | 100.0 | 422 | 100.0 |
For determining the seropositivity at the end of the transmission season, the samples were collected in the first week of January of the next year. However, 46 individuals not being available in the first week due to service-related exigencies, their samples were collected till late into the month of January. Thus, all 422 individuals were followed up for repeat samples, and the data were complete. Posttransmission season testing results are shown in Table 2.
Table 2.
IgG positivity at the end of transmission season.
| Result | IgG ELISA |
IgM ELISA |
||
|---|---|---|---|---|
| Frequency | Percent | Frequency | Percent | |
| Negative | 356 | 84.4 | 398 | 94.3 |
| Positive | 66 | 15.6 | 24 | 5.7 |
| Total | 422 | 100.0 | 422 | 100.0 |
At the beginning of the transmission season, 15 individuals were positive for both IgG as well as IgM, while at the end of the transmission season, eight individuals were positive for both.
The measures of dengue infection using IgG and IgM are summarized in Table 3. Incidence values have been depicted in Table 4.
Table 3.
Measures of dengue infection found in the study.
| Measurement | At the beginning of transmission season (% with 95% CI) | At the end of transmission season (% with 95% CI) |
|---|---|---|
| Point prevalence | 11.6% (8.4%–14.6%) | 15.6% (12.1%–19.1%) |
| Recent infection | 10.2% (7.3%–13%) | 5.7% (3.4%–7.8%) |
Note: Point prevalence calculated based on IgG and recent infection, using IgM.
Table 4.
Incidence of dengue infection in the study population.
| No of IgG negative individuals at the beginning of transmission season |
No of IgM positive individuals at the end of transmission season |
No of IgG positive individuals at the end of transmission season |
Total incidence over 06 months |
|---|---|---|---|
| (a) | (b) | (c) | (b + c) |
| 373 | 12 | 43 | 55 |
| 32.2 per 1000 (b/a x 1000) | 115.2 per 1000 (c/a x 1000) | 147.4 per 1000 |
Note: (i) The incidence has been calculated over 6 months.
(ii) None of the 43 individuals in column (c) above were IgM positive.
Of the 49 individuals who were IgG positive at the beginning of the season, 23 were positive even at the end of the season, but all showed declining IgG antibody titres. While the other 26 became completely IgG negative.
Dengue infection and symptomatology
Symptoms of dengue were categorized into present or absent based on history elicited from each individual at the beginning, end, and at least twice in between the study period of 6 months. Symptoms were labelled to be present if the individual had any of the following during the entire duration of the study, including upto 2 weeks prior to beginning of the transmission season (to cover baseline IgM positivity):-
Fever alone or ‘with’ headache or retro bulbar pain AND/OR skin rash AND/OR myalgia, arthralgias AND/OR nausea, vomiting.
Presence of symptoms was dichotomized into present or absent, as depicted in Table 5. It was analysed for the 55 incident cases but the denominator was taken as the whole study population of 422 to include even the initial IgG positive cases who may or may not have had any symptoms. As can be seen from Table 5, 22 out of 55 incident cases (40%) had no symptoms at all, whereas 33 individuals (66%) showed one or the other symptom of dengue. There was a significant association between dengue infection (IgG positivity) and presence of symptoms. However, it is noteworthy that of the 367 individuals not having dengue, 129 (35.14%) had symptoms, which may have been because of other illnesses.
Table 5.
Correlation between symptoms and dengue infection.
| Incident case | Symptoms |
Total | |
|---|---|---|---|
| Absent | Present | ||
| No | 238 | 129 | 367 |
| Yes | 22 | 33 | 55 |
| Total | 260 | 162 | 422 |
Chi Square = 12.48; p = 0.0001.
The data were analysed to find any correlation between age and incidence of the infection (Pearson's r value = 0.015, P = 0.755) as well as age and presence of symptoms (Pearson's r value = −0.074, P = 0.129). There was no significant correlation between either of the variables with age.
Discussion
Dengue virus was first detected in India in a 6-year-old child's serum from Vellore in 1956. The epidemiology of dengue infection in the country has since then undergone significant changes especially in recent decades. After the first reported major outbreak of Kolkata in the 1963, the frequency and intensity of outbreaks are rising every decade.4
The present study portrays the field epidemiology of dengue infection in the Armed Forces community in the large cantonment of Delhi, within one transmission season.
During the first major serosurveillance survey for antibodies against DEN1 and DEN 2 in the country, positivity was found in 40% and 20%, respectively, in 588 serum samples screened from across India.2 In the present study, it was found to be 11.6% (95% CI: 8.4%–14.6%) and 15.6% (05% CI: (12.1%–19.1%) at the beginning and end of the transmission season, respectively. However, since most of the seroprevalence studies on the subject after the above nation-wide study have been hospital-based comparisons are difficult to make. A study in tertiary care hospital in Rajasthan showed seropositivity of 19%.5 Another seroprevalence study at a virology lab in tertiary care hospital in Delhi over a period of 6 years showed a positivity in 30.15% of fever case falling in case definition of ‘suspected dengue’.6 However, it used only acute phase sera of suspected patients and not of healthy population as done in the present study.
In West Bengal, 6293 ELISA IgM reactive cases were found positive out of 12,059 persons (52.18%) tested in the whole state, involving 18 districts hospitals with 34 deaths.7 A 3-year study testing serum antibody levels in suspected dengue cases in a hospital setting in Tamil Nadu found a seropositivity 25.5% for IgM.8
All such hospital-based studies are bound to be biased (Selection Bias) since the serology is performed in a hospital setting in cases with fever. The pretest probability of IgG/IgM in such samples is high. Further, the incidence of dengue infection in the community cannot be measured from them due to the limitation of finding a definite denominator.
The present study showed dengue incidence of 147.4 per 1000 over a period of 06 months. In 1995–1996, a 1-year cohort study in Indonesia found an incidence of 29.2%. The age group was 4–9 years.9 In 1980, a study in Thailand was conducted by examining pre- and postepidemic serological samples from 3185 children. The incidence among initially seronegative samples (251 children) was 39.4%.10
Another crucial finding from this study is that 22 (40%) out of 55 incident cases had no symptoms of dengue. Several dengue-related community-based studies internationally have found variable asymptomatic case percentage such as 87% among children in Thailand10 and 7.5% in Cambodia.11 A study in five major urban areas of Pakistan among individuals with mean age of 34.5 years using IgG antibody testing revealed that 32.3% of the participants had asymptomatic dengue.12
We found that a total of 43 individuals (10.2%) were detected to be IgM positive at the beginning of the season. For converting this into IgM positivity among people “at risk,” it was measured among those who were IgG negative at the beginning of the season. It turned out that of the 373 individuals at risk with no IgG antibody at the beginning of the season, 28 (7.5%) were IgM positive. This implies that the transmission and infection of the virus must have begun somewhere in the months of April to June. This finding may point towards a shift in the seasonality of transmission of the virus.
The inferences obtained above can be used by healthcare planners at all levels to estimate the logistic requirements while preparing for dengue prevention strategies in a station. For example, it can be inferred that dengue transmission in Delhi starts from April onward and hence larval surveys should begin well in advance accordingly. Similarly, clinicians may keep in mind that about 40% dengue cases will be asymptomatic. They can keep dengue as one of the differential diagnosis accordingly. Various hospital logistics aspects of dengue control such as allocation of hospital beds, procurement of test kits, and so on, can be planned using the incidence rate of 147.4 per 1000 population as obtained from this study.
Limitations of the study
Since the aedes mosquito is almost ubiquitous in the area of Delhi-NCT, a control arm of “Not Exposed” to the mosquito and hence, the virus, was not possible to enrol in the study. Therefore, this project lacks the robust backing up of incidence figures with risk analysis in terms of Attributable and Population Attributable risks. There were no female participants in the study since families of serving personnel did not form the sampling frame. Confounding factors such as individuals moving out of Delhi during the transmission season may have played a role in blunting the accuracy of the above results. Further, since troops had to be sampled from units posted with a medical officer, selection bias may have been introduced in the sample, which cannot be totally compensated by drawing random samples of persons from the selected units. This may affect the external validity of the study. The association between titres of antibody levels and presence of clinical symptoms was not analysed in this study.
Conclusion
There are hardly any community-based IgG/IgM-based epidemiological surveys of dengue in Armed Forces. In an attempt to fill this knowledge gap, the present study found that the point prevalence of infection (IgG presence) was 11.6% (95% CI: 8.4%–14.6%) at the beginning of the season and 15.6% (05% CI: (12.1%-19.1%) at the end. Planning figures for healthcare planners and hospitals as far as incident cases are concerned can be pegged at around 14.7% of the initial IgG-negative individuals and 19.4% (82 of the total 422 studied) of the total population of serving personnel in a given station during a transmission season. Of all the incident cases, 40% were asymptomatic.
Disclosure of competing interest
The authors have none to declare.
Acknowledgment
This paper is based on Armed Forces Medical Research Committee Project No. 4933/2017 granted and funded by the office of the Directorate General Armed Forces Medical Services and Defence Research Development Organization, Government of India.
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