|Year : 2013 | Volume
| Issue : 3 | Page : 283-286
Assessing effect of climate on the incidence of dengue in Tamil Nadu
S Chandy1, K Ramanathan2, A Manoharan1, D Mathai3, K Baruah4
1 Department of Medicine, Unit 1 and Infectious Diseases, Christian Medical College, Vellore, India
2 Department of Biostatistics, Christian Medical College, Vellore, India
3 Apollo Institute of Medical Sciences and Research, Hyderabad, India
4 National Vector Borne Diseases Control Program, New Delhi, India
|Date of Submission||19-Feb-2013|
|Date of Acceptance||12-Jun-2013|
|Date of Web Publication||25-Jul-2013|
Department of Medicine, Unit 1 and Infectious Diseases, Christian Medical College, Vellore
Source of Support: None, Conflict of Interest: None
Incidence of dengue is reported to be influenced by climatic factors. The objective of this study is to assess the association of local climate with dengue incidence, in two geographically distinct districts in Tamil Nadu. The study uses climate data, rainfall and mean maximum and minimum temperature to assess its association if any, with dengue incidence in two districts of Tamil Nadu, South India. According to this study while precipitation levels have an effect on dengue incidence in Tamil Nadu, non-climatic factors such as presence of breeding sites, vector control and surveillance are important issues that need to be addressed.
Keywords: Dengue, rainfall, Tamil Nadu, temperature
|How to cite this article:|
Chandy S, Ramanathan K, Manoharan A, Mathai D, Baruah K. Assessing effect of climate on the incidence of dengue in Tamil Nadu. Indian J Med Microbiol 2013;31:283-6
|How to cite this URL:|
Chandy S, Ramanathan K, Manoharan A, Mathai D, Baruah K. Assessing effect of climate on the incidence of dengue in Tamil Nadu. Indian J Med Microbiol [serial online] 2013 [cited 2017 Dec 11];31:283-6. Available from: http://www.ijmm.org/text.asp?2013/31/3/283/115640
| ~ Introduction|| |
Dengue fever (DF), dengue haemorrhagic fever and dengue shock syndrome are public health problems in Tamil Nadu, South India.  They are caused by four dengue virus serotypes, which are transmitted to humans by female Aedes mosquito (Aedes aegypti and Aedes albopictus). Aedes species are endophagic and a daytime biter; its biology, geographical range and transmissibility are dependent upon climatic variables, temperature, humidity and wind velocity. Increased precipitation levels provide greater habitat area for proliferation of the vector larvae thereby increasing vector population. , Higher temperature results in miniature adults, which feed more frequently to produce eggs; increased biting rates leads to increased dengue incidence.
The life cycle of Aedes spp has two important phases: the aquatic (egg, larva and pupa stages) and the winged adult phase. The development and survival of the two phases and oviposition of the adult depends upon temperature and rainfall (humidity). Under experimental conditions, the optimal survival temperature range of aquatic stages is >15°C < T <35°C (optimum temperature is 26°C) while that of adult is >15°C < T <30°C. At temperatures greater than 39.95°C, eggs do not transform into adults. 
Studies from India have correlated increased dengue incidence with the monsoon and post-monsoon season. , There is scientific evidence that temperature and rainfall influence dengue incidence. However, complex interactions of ecology, environment, vector and virus serotypes are crucial factors driving dengue outbreaks.
This study attempts to assess the association of local climate with dengue incidence in two different geographical regions in Tamil Nadu.
Tamil Nadu constitutes the south-eastern extremity of the Indian peninsula and has two distinct periods of rainfall, the southwest monsoon, from June to September and the northeast monsoon from October to December.
| ~ Materials and Methods|| |
Data and analysis
Dengue incidence and climatic data was analysed for two geographically distinct districts; Chennai (Nungambakkam) in the Northeast and Tiruchirappali (Trichy) located at the centre of Tamil Nadu. Chennai is a coastal city while Trichy is land-locked. The population density of Chennai and Trichy is 26,702/km 2 and 5100/km 2 respectively.
For ease of analysis, each year was subdivided into three periods; pre-monsoon (March-May), monsoon (June-November) and post-monsoon (December). Year wise (2000-2008) data on the incidence of dengue was obtained from the Directorate of Public Health and Preventive Medicine, Chennai. The Meteorological Department, Chennai provided mean monthly maximum (max) temperatures, minimum (min) temperatures and rainfall for the districts in Tamil Nadu during the period 2000-2008. Dengue incidence in Tamil Nadu during 2000-2006 was sourced from a previous publication. 
For Chennai, 2001, 2003 and 2005 were grouped as high incidence (HI) years and 2002, 2006 and 2008 as low incidence (LI) years. For Trichy, 2003 and 2007 were HI and 2004 and 2005, LI years.
Non-parametric tests (analysis of variance) was used to compare rainfall, max and min temp among pre-monsoon, monsoon and post-monsoon periods in HI and LI groups. Non-parametric t-test was used to comparison rainfall, max temp and min temp between HI and LI groups during pre-monsoon, monsoon and post-monsoon periods.
Institutional Review Board (IRB) approval was obtained for the study (IRB Number: 7242 dated 11.08.2010).
| ~ Results|| |
During the study period, (2000-2008) 6892 dengue cases were reported from the state, by public health authorities. Dengue activity increased from 81 cases in 2000 to 1610 cases in 2003. More than half the total dengue cases (52%) seen from 2000 to 2008 were reported during 2001, 2003 and 2005. During the study years, 45% of the dengue burden was reported from Chennai and 10.6% from Trichy [Figure 1]. The number of dengue cases was few during the pre-monsoon period and increase in cases coincided with the monsoon and post-monsoon months.
|Figure 1: Dengue incidence from 2000 to 2008 for Tamil Nadu, Chennai and Tirichurapalli|
Click here to view
In Chennai, the mean monthly rainfall during the monsoon period in 2001, 2003 and 2005 was 212.83 mm, 107.33 mm and 337.17 mm respectively. Compared with the LI years, there was heavy rainfall during December in 2001 and 2005, but the difference was not statistically significant. Among all the years under study, Chennai district received lowest mean monthly rainfall in 2003 yet had high dengue incidence during that year. In Chennai, the max and min temperature range from June to December in HI years ranged from 28.6°C to 34.6°C and 21.3°C to 25.9°C respectively.
In Trichy, the max- and min-temperature range from June to December in HI years ranged from 29.4°C to 34.5°C and 21.2°C to 25.5°C respectively. Rainfall levels between HI and LI years were no statistically significant [Table 1].
|Table 1: Climatic parameters for Chennai and Trichy during HI and LI years |
Click here to view
Rainfall, max-temp and min-temp were significantly different among pre-monsoon, monsoon and post-monsoon periods in the low-incidence group of Nungambakkam. Rainfall was significantly different among pre-monsoon, monsoon and post-monsoon periods in high-incidence group of Nungambakkam, but max-temp and min-temp were not. Rainfall, max temp and min temp were not significantly different among pre-monsoon, monsoon and post-monsoon periods during LI years in Trichy. Max temp was significantly different among pre-monsoon, monsoon and post-monsoon periods during HI years in Trichy, but rainfall and min temp were not. For Chennai and Trichy, no significant difference was found with the respect to the climatic factors (rainfall, max temp and min temp) in LI and HI groups between the pre-monsoon period, monsoon post-monsoon periods.
| ~ Discussion|| |
Incidence of dengue is believed to be influenced by climatic factors.  Published reports from Tamil Nadu have linked increased dengue virus activity during the monsoon and post-monsoon periods of the year. No data is available on the effect of other climatic variables. We attempted to compare the effect of rainfall, max and min temperature during LI and HI years in two districts in Tamil Nadu.
Our results indicate that in Tamil Nadu, rainfall and temperatures influence vector breeding and thereby dengue incidence. However, analysis of climatic variables during HI and LI years does not show any significant difference. Temperatures were conducive to vector proliferation year round. Chennai received excess rainfall during the monsoons of 2001 and 2005, which explained high dengue incidence during those years. There was poor rainfall but high dengue incidence in 2003, which indicates that both excessive rainfall and drought-like-conditions are conducive to surge in dengue incidence. Low precipitation increases ambient temperature, usage of water and air coolers and improper storage of water that may serve as breeding sites.
Climatic factors in isolation cannot dictate rate of dengue incidence. Other important factors influencing vector biology are the presence of breeding sites, pattern of rainfall, number of rainy days and relative humidity. Construction sites, clogged drains, irregular piped water supplies and accumulated garbage have a significant association with high vector (A. aegypti) density. The intensity of dengue transmission depends upon the population density of the host, vector density and the numbers of non-immunised people in any particular community. Chennai, a densely populated city, has a higher chance of experiencing a dengue outbreak even if the mosquito house index in the area is low. Distances between houses may also be of epidemiological significance; in densely packed housing areas the vector (A. aegypti) doesn't have to travel far to encounter its host. An outbreak of DF will be able to spread rapidly in such an area. ,,
Our findings though preliminary in nature reiterate that in Tamil Nadu, rainfall and non-climate variables influence dengue transmission. A similar study from Southern Thailand asserts that unique local climatic factors limit generalisation of association of climate and dengue incidence.  While climatic factors can't be controlled or modified, monitoring them will help understand their effect on mosquito-transmitted diseases.
There are a few limitations in the study. Only annual incidence data for the different districts was available. Weekly incidence data when analysed with weekly weather conditions helps explore the true impact of climate variability on dengue incidence. Data on other climatic factors such as humidity, wind velocity known to have a positive effect on dengue incidence were not available.
It is widely reported that climate change propels increased emergence and transmission of vector-borne diseases. The role of non-climatic and social factors needs to be assessed to make our vector control program more effective. More detailed studies are needed to be able to use climate variables to predict any disease outbreak well in advance.
| ~ Acknowledgment|| |
We acknowledge the help received from the Directorate of Public Health and Preventive Medicine, Chennai and Meteorological Department, Chennai.
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