|Year : 2019 | Volume
| Issue : 2 | Page : 56-61
Rainfall and dengue occurrences in India during 2010–2016
Bioinformatics and Data Management Group, ICMR - National Institute of Virology, Pune, Maharashtra, India
|Date of Submission||10-Jun-2019|
|Date of Decision||27-Aug-2019|
|Date of Acceptance||15-Oct-2019|
|Date of Web Publication||22-Nov-2019|
Dr. Pratip Shil
ICMR - National Institute of Virology, 130/1 Sus Road, Pashan, Pune - 411 021, Mahrashtra
Source of Support: None, Conflict of Interest: None
Background: A changing climate scenario coincided with the emergence and re-emergence of vectorborne diseases such as dengue, chikungunya, and Zika. The worldwide resurgence of dengue since 2005 has affected millions and generated huge disease burden, especially in the tropical developing countries including India. While India has a huge burden of dengue with all four serotypes causing outbreaks in different parts of the country, reports on climate and environmental effects on dengue are sparse. Aims and Objectives: To understand the influence of rainfall on dengue occurrences across India between 2010 and 2016, with emphasis on the most affected states. Methods: Dengue occurrence data was obtained by data mining from the NVBDCP and IDSP websites. Area-weighted-rainfall (ARF) were computed from the division-wise data. Statistical analyses performed to analyze the association between annual ARF and dengue occurrences. Spatio-temporal analyses of dengue outbreaks was conducted. Results: Spatio-temporal analyses revealed that high rainfall was positively associated with the number of cases in the northern states (Indo-gangetic Plains) whereas, the reverse was true for the southern (peninsular) states. The number of rural outbreaks of dengue had also been modulated by annual rainfall. Conclusion: Our study revealed the effect of rainfall on dengue in India. We conclude that rainfall influence the dengue occurrences differently in the northern and the southern states of India.
Keywords: Dengue, India, Indo-gangetic plains, peninsular, rainfall
|How to cite this article:|
Shil P. Rainfall and dengue occurrences in India during 2010–2016. Biomed Res J 2019;6:56-61
| Introduction|| |
Dengue caused by dengue viruses, belonging to Flaviviridae family, affects millions of people worldwide. The disease, spread by Aedes mosquitoes, can be contracted by one of the four serotypes of the virus: Dengue 1, Dengue 2, Dengue 3, and Dengue 4. According to the WHO estimate, approximately 40% of the global population lives in dengue-endemic areas. Dengue has become a major public health concern in India, as the country continues to experience fatal outbreaks each year, with more than 4 million people being affected by the disease between 2010 and 2016. All the four serotypes either singly or in combination were involved in the major outbreaks.,,,,
There has been a 30-fold increase in the burden of disease over the past 50 years, with urbanization, trade, globalization, travel, demographic change, warming temperatures, and inadequate domestic water supplies being factors favoring the spread of the main vectors Aedes aegypti and Aedes albopictus, the two main vectors of dengue. Elevated dengue risk has been found to be associated with suitable local temperature and high rainfall. In some locations, dengue has been found to be associated with humidity and vapor pressure., Furthermore, proximity to low-income urban and semiurban centers was also associated with greater risk, provided those having good transport connections. Climate changes resulting in increased temperature and altered rainfall, along with urbanization, may be associated with increased dengue incidence and outbreak risk in the tropical countries.
India has mostly tropical weather, but the landmass is divided into many climatic types along with geological diversity of terrain. This diversity reflects in mosquito diversity and distribution as well.,,,, The climate of India is strongly influenced by the southwest monsoon, which brings heavy rainfall from June to September. Enhanced rainfall has been associated with increased Aedes breeding in some areas of India.,, Although dengue has been well studied in India, the role of environmental factors affecting dengue occurrences has not been investigated in details as publications are sparse. This necessitates research to understand the role of environmental parameters in different parts of India.
In the present study, I have analyzed the role of rainfall in dengue occurrences in India from 2010 to 2016 by considering country-wide as well as state-wise analyses of data. The association of dengue occurrences with rainfall has been studied for different regions of India.
| Methods|| |
Data on annual confirmed cases of dengue from across India were obtained from the website of National Vector Borne Disease Control Program, Ministry of Health and Family Welfare, Government of India (https://www.nvbdcp.gov.in/). A total of 364 weekly reports on disease outbreaks were downloaded from the website of the Integrated Disease Surveillance Programme of National Centre for Disease Control, Ministry of Health and Family Welfare, Government of India (http://idsp.nic.in/). The said reports were used for data mining for information on the rural outbreaks of dengue.
Meteorological parameters, namely, area-weighted average annual rainfall (ARF), average maximum temperature, and average minimum temperature for the meteorological subdivision of India, were obtained from the Government of India websites (https://data.gov.in/; accessed on June 2018 and http://www.tropmet.res.in/; accessed on July 2018).
Mathematical analyses of meteorological data
In India, area covered by each state (province) is divided into different meteorological subdivisions (according to the Indian Meteorological Department, Ministry of Earth Sciences, Government of India). Subdivisional annual rainfall for the period 2010–2016 of various states and subdivisional landmass areas is given in [Supplementary Table 1]. State-wise area-weighted average rainfall (ARF) was calculated using the following formula:
where X (i) is the average annual rainfall for any (ith) subdivision, A (i) is the area in Km 2 for that (ith) subdivision, and n is the number of subdivisions covering a state or region of India (Thiessen method).
Spatiotemporal mapping of dengue-affected states and rural outbreaks of the disease was performed. Geographical locations (latitude and longitude) of identified rural outbreak sites were determined with the help of Google Maps and www.latlong.net/. Rendering of images and map plotting were performed using free online GIS and map-plotting servers, namely Mapplottr.com (https://multiplottr.com/). All maps depicted in this study are in accordance with the established map-display norms of the Republic of India.
Reference maps for land use and terrain for the various states were obtained from BHUVAN, the Geo-Portal of Indian Space Research Organisation (ISRO) at the URL: https://bhuvan.nrsc.gov.in/.
Mathematical calculations and statistical analyses were performed in R, and graph plotting was carried out using MS Excel software packages.
| Results|| |
All India scenario
Between 2010 and 2016, a total of 425,560 confirmed cases of dengue were registered in India (http://nvbdcp.gov.in/chik-cd.html; accessed on July 2018), covering all states and union territories. [Figure 1] describes the state-wise distribution, highlighting the most affected states. The highest numbers of incidences were reported from West Bengal (43,843), Punjab (37,895), Delhi-NCR (36,312), Tamil Nadu (33,370), Maharashtra (31,385), Kerala (29,865), Gujarat (29,379), Odisha (28,492), and Karnataka (27,290). It should be noted that incidences were reported from even the Himalayan states such as Sikkim (76), Himachal Pradesh (508), Uttarakhand (4703), and Jammu and Kashmir (2090), with most of the occurrences happening in the forested foothills or the Terai regions which experience warm weather.
|Figure 1: States of India affected by dengue from 2010 to 2016 A. D. N. B.: Lightest gray shade represents 1–5000 cases; moderate gray shade represents 5001–10,000 cases; dark gray represents 10001–20,000 cases, and black shade represents >20000 cases|
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Among the islands of India, the habitable islands of the “Andaman and Nicobar Islands” archipelago in the Bay of Bengal have reported 758 cases during this period. Estimates excluded the out-of-bounds islands inhabited by hostile aborigines,, who resist contact with the rest of humanity. There were a few cases from the Lakshadweep group of islands in the Arabian Sea in 2014.
Between 2010 and 2016, a total of 393 rural outbreaks have been recorded. Of these, 69% (271) occurred in the peninsular states, namely Maharashtra, Chhattisgarh, Odisha, Karnataka, Telangana, Andhra Pradesh, and Tamil Nadu, whereas 23% occurred in the plains of North India covering the states of West Bengal, Odisha, Bihar, Uttar Pradesh, Haryana, Delhi, Punjab, Himachal Pradesh, Uttarakhand, Jammu and Kashmir, and including Rajasthan and Madhya Pradesh. About 7% of the rural outbreaks occurred in the northeastern states. [Figure 2] describes the all India ARF and the number of rural outbreaks of dengue between 2010 and 2016. It is observed that the number of outbreaks increased over years along with a decrease in the amount of annual rainfall (ARF). The Pearson's correlation coefficient was r = −0.732, indicated a strong negative correlation between ARF and number of rural outbreaks. [Supplementary Figure 1] describes the spatial distribution of the rural outbreaks on a yearly basis.
|Figure 2: Dengue rural outbreaks and all India area-weighted average rainfall (ARF), between 2010 and 2016|
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Analyses for the most affected Indian states
Based on the geographical features and location, we divide the most affected states into two broad groups: (1) northern states (consisting of Punjab, Delhi, and West Bengal) and Odisha and (2) peninsular states (consisting of Maharashtra, Karnataka, Kerala, and Tamil Nadu) and Gujarat. The association of area-weighted average annual rainfall (ARF) with dengue incidences has been studied using statistical analyses.
[Figure 3] shows the ARF and dengue occurrences for the northern states. For Punjab, the annual confirmed cases showed a moderate positive association with ARF, with Pearson's correlation coefficient, r = 0.427. Low rainfall years 2012 and 2014 led to a decline in the number of incidences compared to the high rainfall years 2013, 2015, and 2016. In Delhi, the National Capital Region, the annual dengue incidence was modulated by ARF, with a positive association indicated by the Pearson's correlation coefficient, r = 0.346. For West Bengal, there was a weak positive association with ARF (Pearson's correlation coefficient, r = 0.155), which indicates that ARF may not be the only factor favoring spread of the disease. In the state of Odisha, no correlation could be established between ARF and dengue incidences. The other states of northern India covering the Indo-Gangetic plains showed a positive association between ARF and dengue incidences, as evident from the Pearson's correlation coefficient, r: Haryana (r = 0.412), Uttar Pradesh (r = 0.405), Bihar (r = 0.38), and the states bordering the Indo-Gangetic plains: Rajasthan (r = 0.152).
|Figure 3: Dengue incidences and ARF for the states of North and East India: (a-c) Punjab, Delhi, and West Bengal (states of the Indo-Gangetic plains). (d) Odisha (on the eastern coast). (e) Geographical regions covered by these states|
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All the peninsular states showed high numbers of dengue annual incidences, with more than 5000 cases per year on an average for each state [Figure 4]. In Maharashtra, there exists a weak negative association between dengue incidences and ARF (Pearson's correlation coefficient, r = −0.165). A strong negative association was obtained for Karnataka (r = −0.60) and Kerala (r = −0.59), which indicated that for these states, the ARF adversely affected the dengue occurrences. In the state of Tamil Nadu which gets rainfall in November to January, there exists a moderate negative association (r = −0.335). Adjoining Maharashtra, in the western state of Gujarat, which has mostly arid hinterlands, there also exists a moderately negative association (r = −0.18).
|Figure 4: Dengue incidences and ARF for the most affected states of peninsular India: Maharashtra (a), Karnataka (b), Tamil Nadu (c), and Kerala (d). The western Indian state of Gujarat also showed high incidences (29,379) (e). Geographical area covered by these states (f)|
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| Discussion|| |
The purpose of the present study was to understand the influence of the rainfall on dengue incidences in India. Based on data analysis, we selected nine most affected states (cutoff ≥25,000 cases), two major groups emerge: (1) northern states covering the Indo-Gangetic plain (Punjab, Delhi, and West Bengal) and Odisha and (2) peninsular states (Maharashtra, Karnataka, Kerala, and Tamil Nadu) and Gujarat.
In the northern states, located in the Indo-Gangetic plain, namely, Punjab, Delhi, and West Bengal, the ARF is positively associated with dengue occurrences. These states have similar geography and land use consisting of vast flat alluvial plains (perennial river watersheds) with heavy cultivation of rice, wheat, and various vegetables (BHUVAN - the ISRO Geo-Portal). As per the Koppen classification, these states experience humid subtropical climate type with hot and humid southwest monsoons. Abundance of water and waterlogged fields is favorable for Aedes breeding. Thus, high rainfall years indicate greater Aedes abundance and more dengue incidences compared to lower rainfall years, similar to that observed in Bangladesh (located at the eastern-most part of the Indo-Gangetic plain). Reports from Pakistan also indicated an increase in the number of dengue occurrences in the high rainfall years in the Indus valley. The land use maps for the northern states are provided in [Supplementary Figure 2].
In case of Odisha (located adjacent to West Bengal), mathematical analyses did not reveal any association between rainfall and dengue occurrences for the said period. This can be attributed to various factors such as (i) issues related to data collection and reporting, or (ii) local environmental factors affecting Aedes breeding and disease transmission. District-wise analyses might reveal the real scenario, but that analyses can be taken up as a project in future.
For the state of Gujarat (located adjacent to Maharashtra), dengue occurrences have been negatively associated with annual rainfall. This may be attributed to the topography and semi-arid nature of landscape which forces people to store fresh water.
Between 2010 and 2016, majority of the rural outbreaks took place in the peninsular states [Supplementary Figure 2]. The all India number of rural outbreaks shows a negative association with all India ARF. This is because 69% of the rural outbreaks occurred in the peninsular India. These states usually have hilly terrain, undulating landscapes [Supplementary Figure 3], where high rainfall may lead to washout of Aedes breeding habitats. Moderate-to-lower rainfall creates more breeding sites as many areas of the peninsular India are drought prone and people store water in houses and/or farms., Thus, water storage and management practices affect the Aedes mosquitoes abundance.,,
Complex geography of the northeastern states of India made it difficult to conduct meaningful mathematical analysis on the basis of the gross state-wise data available to us. Each NE state has great diversity of landscape and ecology, even adjoining districts within a state vary greatly in terms of altitude, topography, and climate. Although dengue has been reported from all the seven NE states, spatiotemporal analysis could not be performed because of geographical/ecological diversity. District-wise analysis for the northeastern states can be taken as future projects.
| Conclusion|| |
An attempt has been made to study the effect of rainfall on dengue occurrences in India. Meteorological data were analyzed and possible correlations with confirmed cases of dengue were obtained. It was observed that annual rainfall (ARF) modulates dengue incidences differently in the northern and southern (peninsular) states of India. While the northern states experience more cases due to high annual rainfall, the southern states, on the other hand, experience increased occurrences with lesser rainfall. Both this phenomenon can be attributed to geography, land use, and water management practices. Hence, we conclude that rainfall affects the dengue occurrences differently between northern and southern states of India. We hope this work will benefit researchers and authorities in public health, especially for framing future control strategies for dengue.
PS would like to thank Dr. DT Mourya, Director, ICMR-National Institute of Virology, Pune, for meaningful inputs, encouragement, and support throughout the study. The author would like to thank Mr. Avinash A Patil, for his technical help in graphics generation for the manuscript.
Financial support and sponsorship
Institutional intramural funds.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]