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ORIGINAL ARTICLE
Year : 2017  |  Volume : 61  |  Issue : 4  |  Page : 239-242  

Dengue fever in a municipality of West Bengal, India, 2015: An outbreak investigation


1 Scientist C, Division of Epidemiology, ICMR – National Institute of Cholera and Enteric Diseases, Kolkata, West Bengal; MPH Scholar, ICMR – National Institute of Epidemiology, Chennai, Tamil Nadu, India
2 Scientist E, ICMR School of Public Health, ICMR – National Institute of Epidemiology, Chennai, Tamil Nadu, India
3 Chief Medical Officer of Health, Department of Health and Family Welfare, Government of West Bengal, West Bengal, India

Date of Web Publication6-Dec-2017

Correspondence Address:
Dr. Falguni Debnath
Division of Epidemiology, National Institute of Cholera and Enteric Diseases, P-33, Cit Road, Scheme-Xm, Beliaghata, Kolkata, West Bengal
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijph.IJPH_309_16

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   Abstract 


Background: In November 2015, death due to fever and increased number of fever cases were reported from Baranagar Municipality of North 24 Parganas district of West Bengal. Objectives: The episode was investigated with the objective to (1) confirm the existence of an outbreak, (2) describe it in terms of time, place, and person, (3) determine the cause of outbreak, and (4) recommend control measures. Methods: Monthly incidence of dengue from 2012 to 2014 was calculated and compared with 2015 to confirm the outbreak. We used Integrated Disease Surveillance Programme definition and searched for suspect dengue cases going door-to-door in ward number one of Baranagar Municipality. Active case search was done in health facilities also. Information on date of onset, symptoms, sociodemographic, serological reports, and clinical outcome for suspected and confirmed dengue cases was collected. Blood specimens were collected for NS1 ELISA/monoclonal IgM antibody capture-ELISA test. Environmental and entomological surveys were done. Results: Six hundred and seventy-one dengue cases (Overall attack rate = 3/1000), two deaths (Case fatality = 3/1000) were reported during September 14, 2015, till December 12, 2015. Out of 34 wards, attack rate was highest in ward number 1 (0.7%) and was 3 per 1000 among females. All age groups were affected. Thirty-two percent required hospitalization. NS1 ELISA was positive for 612 cases. Out of interviewed 31 dengue cases, 94% had headache, 90% had myalgia, followed by arthralgia, rash, and retro-orbital pain. Only in ward number 1, house index was >5%. Conclusion: We confirmed dengue outbreak. All age groups got affected. Deaths occurred in this outbreak. Potential breeding sources were present in ward number 1.

Keywords: Dengue, monoclonal IgM antibody capture-ELISA, NS1 ELISA, outbreak, West Bengal


How to cite this article:
Debnath F, Ponnaiah M, Acharya P. Dengue fever in a municipality of West Bengal, India, 2015: An outbreak investigation. Indian J Public Health 2017;61:239-42

How to cite this URL:
Debnath F, Ponnaiah M, Acharya P. Dengue fever in a municipality of West Bengal, India, 2015: An outbreak investigation. Indian J Public Health [serial online] 2017 [cited 2017 Dec 11];61:239-42. Available from: http://www.ijph.in/text.asp?2017/61/4/239/220062




   Introduction Top


Globally, dengue fever has shown a 30-fold increase in the past five decades.[1] Worldwide, 3.9 billion people of 128 countries are at risk of infection with dengue virus.[2] Reports of dengue fever outbreak from different parts of India have increased in the last decade.[3]

Baranagar Municipality is situated in North 24 Parganas district of West Bengal. It is an urban area with a population of 247,866. The municipality has 34 administrative wards and caters to a sociodemographically diverse population. One death due to fever in November 2015 in ward number 1 and increased number of fever cases warranted this investigation. The episode was investigated with the following objectives: (1) to confirm the existence of an outbreak, (2) to describe the outbreak in terms of time place and person, (3) to establish the cause of outbreak, and (4) to recommend control measures.


   Materials and Methods Top


To confirm outbreak

To confirm existence of a dengue outbreak, data on dengue cases for the year 2012–2014 were collected, and we also collected data on dengue cases for January to November 2015 were collected. Monthly incidence of dengue per lakh population for 2012–2014 was calculated and compared that to 2015. The investigation was started on November 24, 2015. Ethical clearance was not awaited as it was an outbreak investigation.

Descriptive epidemiology

Door-to-door active case search in ward number 1 of the municipality was conducted to identify suspect dengue cases. Suspect dengue cases from indoor/outdoor of Baranagar State general hospital were also identified, and information on date of onset, symptoms, sociodemographic characteristics, and awareness related to dengue was collected. Suspect dengue case was defined as any case of fever of 2 days or more with two or more of any of the following: headache, retro-orbital pain, myalgia, arthralgia, rash, hemorrhagic manifestations, and leukopenia.[4] Confirmed dengue case was defined as any suspect dengue case with one or more of the following: positive monoclonal IgM antibody capture-ELISA (MAC-ELISA) test in a serum specimen from the late acute or convalescent phase, or positive NS1 ELISA test in acute phase.[4] Blood specimen of suspect dengue cases was sent to Sagore Dutta Medical College for dengue confirmation. Some of the blood specimens of the suspect dengue cases were also tested at other private laboratories. They used MAC-ELISA and NS1-ELISA test for confirming dengue virus infection as these two types of tests are recommended by Government of West Bengal for confirmation of dengue. Records were also reviewed and information on the date of onset, sociodemographic characteristics, serological reports, clinical outcome for confirmed dengue cases from January to November 2015 was collected.

Entomological and environmental survey

Principal investigator along with municipal health inspector and trained health workers conducted entomological survey in ward 1, 4, and 27. Total number of households in ward no. 1, 4, and 27 were 3600, 2450 and 2069 respectively. Hence, purposive sampling method was opted for entomological survey as surveying all the households in mentioned wards was not possible due to logistic constraints. We searched for mosquito breeding sources at and around the house and looked for Aedes aegypti larvae. Aedes larvae were identified by visual inspection of their appearance and movement in water by the trained health workers of the municipality.[4],[5] House index (%), container index (%), and breteau index per 100 households were calculated to understand density of the vector (A. aegypti) which is responsible for dengue virus transmission. A house index value of >5% and breteau index value of >20 per 100 households in any ward was considered threshold to say that ward to be dengue-prone.[4]

Environmental survey was also conducted in ward number 1. We observed the sanitation practices of the residents, their water collection and storing habits, mosquito breeding sites, and drainage system.

Data collection and analysis

Principal investigator along with trained health workers collected all the information using self-developed dengue case search format. The used case search format was developed following Centers for Disease Control dengue guidelines. We also reviewed records, collected necessary information on dengue cases, and plotted an epidemic curve by week of onset of the dengue cases. Age- and gender-specific attack rates were calculated. Hospitalization rate and case fatality rates were also calculated. We calculated ward-specific attack rates and described the clinical profile of the dengue cases that we identified during our survey. Proportion of different responses regarding awareness related to dengue was also calculated.


   Results Top


Confirmation of outbreak

Monthly incidence (per lakh populations) of dengue in Baranagar Municipality for the month of January to November 2012–2014 was calculated and it ranged in between 0 and 2. Although no case of dengue was reported from January to August 2015, in September, incidence was four per lakh population, and in October and November, it reached to 159 (/lakh) and 102 (/lakh), respectively. We compared the monthly incidence of dengue of 2012–2014 to that of 2015, which clearly indicated the existence of outbreak.

Descriptive epidemiology

A total of 671 cases were detected in this outbreak. The outbreak started on September 14, 2015 and continued till December 12, 2015 [Figure 1]. After that no case was reported.
Figure 1: Distribution of confirmed dengue cases by week of onset, dengue outbreak, Baranagar Municipality, North 24 Parganas, West Bengal, September – December 2015.

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Attack rate in ward number 1 was 0.73%. Attack rate in ward 11 and 28 was 0.53% and 0.51%, respectively. In rest of the wards, attack rate was below 0.5%. Out of 671 cases, 54% (363) were female. Attack rate among the females was 2.99/1000. The overall attack rate was 2.71/1000. Among 5 years and less age group, attack rate was 1.53/1000; attack rate was 1.90/1000 in the age group of > 60 years. Thirty-two percent (214) of the confirmed cases required hospitalization. Two patients died of dengue shock syndrome, and the case fatality rate was 3/1000 dengue cases [Table 1].
Table 1: Gender and age specific attack rate in dengue outbreak, Baranagar, North 24 Parganas, West Bengal, India, September to December, 2015

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Confirmation of diagnosis

During our active case search survey between November 24, 2015, and December 27, 2015, blood specimen from 41 suspected dengue patients were sent for laboratory confirmation, and 15 got confirmed as defined criteria. However, during the period of September 14–December 12, 2015, a total of 671 persons residing in Baranagar Municipal area were diagnosed as confirmed dengue cases, and blood specimens from 612 patients were tested positive by NS1 ELISA test. Rests of the specimens were tested positive by MAC-ELISA test.

Clinical profile of dengue case patients

We identified 16 more confirmed dengue cases from Inpatient Department of Baranagar state general hospital during our active case search survey and interviewed them. In total, 31 confirmed dengue cases were interviewed during our survey period. Among them, 94% (29) had headache, 90% (28) had myalgia, 64% (20) had arthralgia, 52% (16) had retro-orbital pain, 45% (14) developed rash on body, 39% (12) experienced abdominal pain, and 3% (1) complained passing of loose stool. Four of them had a history of any one of their family members suffering from similar symptoms in preceding 15 days and 16 of them had a history of neighbor suffering from similar symptoms in preceding 15 days.

Awareness level and water storing practices of the all interviewed suspect and confirmed dengue cases regarding dengue

Out of 41 interviewed suspect dengue cases, 68% (29/41) knew that dengue fever spreads through mosquito, 15% (6/41) knew correctly the time when A. aegypti bites, 27% (11/41) knew correctly where these mosquitoes usually rest, 54% (22/41) knew correctly where these mosquitoes lay eggs, 71% (29/41) store water for household works in vessels without any lid, and 39% (16/41) uses mosquito repellent at home.

Entomological and environmental survey findings

In ward 1, 4, and 27, one confirmed dengue case in previous 1 week before entomological and environmental survey was identified by the health workers. From the household of that confirmed case, 100 consecutive households were searched for water-holding containers in each mentioned ward. In ward 1, total 242 containers were found holding water, among which 7 containers in six households showed A. aegypti larvae. In ward 4, we found 207 water-holding containers, among which three containers in three households showed A. aegypti larvae. In ward 27, we found 215 water-holding containers, among which four containers in four households showed A. aegypti larvae.

During our visit in ward number 1, we found most of the households store water in uncovered tanks and vessels. In many households, water accumulation was seen in discarded cold drink bottles, plastic cups, broken flower vases, and flower pots. After careful examination, some of them were found positive for A. aegypti larvae. The small open drains inside the ward were clogged with supernatant clean water which might have favored the occurrence of the outbreak.


   Discussion Top


Epidemiological investigation and serological reports indicate that the outbreak was due to dengue virus infection. The outbreak continued from September 2015 till December 2015. The outbreak was not localized and almost all the wards got affected. In this outbreak, females were more affected than males. It affected all age group people. One-third of the cases required hospitalization and deaths also occurred. Most of the cases got confirmed through NS1 ELISA test. Potential mosquito breeding sources were present in that area. Only in ward number 1, the HI was >5%. One-third of the suspected dengue cases do not have the awareness regarding spread of dengue fever.

Dengue usually affects humans of all age groups. Some studies in the late 90s reported a maximum number of dengue cases in between 5 and 20 years of age group, whereas, in 2003, studies reported that most of the dengue cases were in 21–30-year age group.[6],[7] In our study, all the age groups got affected with the highest attack rate in 17–40 and 41–60 years of age group.

Environmental factors are always critical for the development of infectious diseases. Many countries reported dengue epidemics during the warm, humid, and rainy seasons, as they favor abundant mosquito growth and shortening of the extrinsic incubation period as well.[8],[9] In our study, clustering of dengue fever cases started in postmonsoon season, which is also in agreement with previous studies.[9] Presence of discarded cold drink bottles, flowers pots, plastic cups, and earthen cups might have favored breeding of mosquito in our study setting. Water storing practice of the residents due to lack of continuous water supply was another habit which might have favored mosquito breeding. Small clogged drains in the wards were another potential area for mosquito breeding. All these factors might have aided in the occurrence of dengue fever outbreak and its existence over months.

The entomological survey was done in December 2015. Hence, low A. aegypti larval indices indicate toward the cessation of vector transmission, and this was in coherence with our epidemiological findings.

On the other hand, municipal authority concentrated mostly on thermal fogging in community which is an adult mosquito control measure. Larva control measures were not in place, and reduction of the breeding sources did not take place in all the wards in timely manner which might explain the existence of the outbreak over months.

Although awareness campaigns done by the municipality regarding mode of spread of dengue fever and how to stay protected, still the awareness of the residents needs to be improved to prevent such outbreaks.

Our outbreak investigation has limitations; due to logistic constraints, we could not do active case search and entomological survey in all the 34 wards. There is a chance of selection bias in this study as during our investigation; we interviewed cases from ward number 1 and Baranagar state general hospital only due to logistic reasons. Information bias might have occurred as we collected information through interview of the suspected/confirmed dengue case. Entomological survey was carried out by trained health workers of municipality, and laboratory examination was not done for species identification of larvae.

Data of epidemiological and serological investigation indicated that the fever outbreak at Baranagar Municipality was due to dengue virus infection. All most all the wards were affected. Ward number 1 was the worst affected area. Data of larval investigations indicated that there must have been vector abundance in ward number 1 of the municipality which is also in coherence with our epidemiological findings. Potential mosquito breeding sources were present in that area. Awareness regarding dengue fever was poor among the suspect and confirmed dengue cases.

This situation in Baranagar municipality needed some preventive action to catch hold on this dengue fever outbreak. Although municipal authority had already initiated dengue control measures, they mostly concentrated on organizing fever clinics and thermal fogging in the community which were not very timely. We ensured continued thermal fogging using insecticide technical malathion in kerosene by the municipal workers in early morning between 5.30 and 7.30 am and in the evening between 4.30 and 6.30 pm. However, as it was an adult mosquito control measure did not affect larvae. Source reduction was done in the 300 households of mentioned three wards of the municipality where entomological survey was carried out. However, due to lack of awareness, there was a hindrance from community to participate in the source reduction. To resolve that campaigning was done in the community to ensure their participation in source reduction. The residents were also oriented toward how to prevent themselves from dengue fever. The small clogged drains were also cleaned to ensure water flow.

From the experience of this dengue outbreak investigation, we suggest to conduct awareness spreading campaigns regarding dengue fever and prevention during premonsoon, monsoon, and postmonsoon season. We recommend conducting breeding source reduction activities during monsoon and postmonsoon season and ensuring community participation in that. Adult mosquito control measures should be continued throughout the year. Small drains inside wards need to be cleaned periodically to ensure water flow. Interdepartmental coordination is necessary for reducing/preventing such outbreaks in future.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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