1 Introduction

The occurrence and distribution of dengue-related diseases have grown radically in last few decades (Hati, 2009). It is reported that 50 million dengue cases occur annually and about 2.5 billion people live in the regions with potential risk of dengue transmission (Jatanasen and Thongcharoen, 1993; WHO, 2009). Environmental elements including weather variables may play a key role in the infection and transmission of dengue which is a mosquito-borne disease (Wu et al., 2007). Temperature, rainfall, and relative humidity are major parameters influencing the incidence of dengue fever in Kolkata (Hati et al., 2011; Bhattacharya et al., 2014). The prediction of global climate change and transmission of dengue and its geographic spread have been widely studied (Bi et al., 2007; Wongkoon et al., 2011). Seasonality of dengue infection in Kolkata has several distinct features (Hati et al., 2011). The infection simmers almost silently in the non-transmission season in first six months of the year, with a noticeable outbreak of several cases, either in July or August, which rapidly expands in the next two months, September and October, then starts to decline from November and almost disappears after December to simmer (Hati, 2006). This communication deals with effects of the weather on dengue infection in the city of Kolkata. In this present study, we examined the impact of weather variables on dengue transmission in Kolkata.

2 Materials and Methods

Study area: Kolkata is one of the largest cities of eastern India and the capital of West Bengal. It has a land area of 185 km² and a population of about 4.496 million (2011 census). The summer season in Kolkata is from March to June. The rainy season starts in July and ends in October. The winter season follows from November to February. The annual mean temperature is about 29^°C. The annual rainfall is about 1 598 mm. This longitudinal study was conducted in the year 2014.

Data collection: To study the effects of weather variables on dengue, data on maximum, minimum and mean temperature, the highest and lowest temperature (^°C), monthly rainfall (mm), number of rainy days, total in wetting months (mm), the highest rainfall in 24 hours (mm) and relative humidity (%) were recorded along with month-wise distribution of referred dengue cases diagnosed serologically in the Gautam Laboratories Imaging and Research Centre, Kolkata (computerized data).

Statistical analysis: Two-tailed Chi-square test was used to find the differences between seasonal variables. Spearman's correlation analysis was conducted to examine the relationship between monthly dengue incidence and weather variables with a lag of zero months (Fisher and Yates, 1974).

3 Results

From the Figure 1, it was evident that dengue cases started to disseminate in the month of August and in the next month (September) more than threefold rise of the number of cases was observed. Transmission of dengue infection reached the peak in October; thereafter it began to diminish in November and dropped suddenly in December. In 2014, a total of 179 serologically confirmed cases of dengue were recorded from 611 suspected cases, which included 3, 1, 2, 1, 1, 10, 34, 61, 51 and 15 cases in February, March, April, May July, August, September, October, November and December respectively. It again showed the trend of dengue transmission by moving average.

Figure 2 showed the association between total rainfall (mm), relative humidity (%) and mean environmental temperature (^°C) with reactive dengue cases in 2014.

In the month of August, when dengue transmission just started, the maximum, minimum highest and lowest temperature were 32^°C, 26.1^°C, 36.1^°C and 22.6^°C respectively, total rainfall was 328.8 mm, total rainy days were 17.2 and highest rainfall in 24 hours was 253 mm, the average relative humidity was 81.8%. In the next two months September and October when dengue infection rapidly increased the maximum, minimum, highest and lowest temperature ranged between 32.2^°C and 31.9^°C, 25.8^°C and 24^°C, 36.8^°C and 35.6^°C, 22.2^°C and 17.2^°C respectively, total rainfall ranged between 295.9 mm and 151.3 mm and relative humidity ranged between 80% and 75%. In November all the parameters considerably decreased. The maximum, minimum, highest and lowest temperature were 29.8^°C, 18.9^°C, 34.3^°C and 10.6^°C respectively, total rainfall was 17.2 mm and relative humidity was 69%.

Monthly rainfall, the number of rainy days, total in wetting months, relative humidity, and minimum/mean temperature, at a lag between zero months, were positively associated with dengue infection in Kolkata over the study period, whereas maximum and highest temperature were negatively correlated with occurrence of dengue (Table 1).

Dengue cases (r=0.702; p=0.01) showed a temporal increase from January to December in 2014. Dengue infection was found to be positively correlated with monthly rainfall (r=0.573; p<0.001), number of rainy days (r=0.624; p<0.001), total in wetting month (r=0.669; p<0.001) and daily maximum rainfall (r=0.578; p<0.001). Rainfall showed its peak in July, whereas dengue cases reached its peak in October. This trend showed maximum rainfall during the month of July might trigger the dissemination of dengue leading its peak in October. Similarly, relative humidity (%) showed a statistically significant (p<0.001) positive correlation with dengue cases in the year 2014, i.e. with increased humidity dengue cases increased in 2014. Both humidity (r=0.636; p=0.026) and dengue cases (r=0.702; p=0.01) in Kolkata showed temporally increasing trend. Consistent with the statistical reports dengue cases reached its peak in October when humidity was also enough high.

Maximum (r=-0.396; p<0.05) and highest (r= -0.473; p<0.001) ambient temperatures (^°C) negatively correlated with dengue cases the year 2014, i.e. with increase of temperature dengue cases declined in 2014. In line with this statistical report maximum environmental temperature showed its peak during April to June when dengue cases were minimum, whereas dengue cases reach its peak in October when ambient temperature is not so high. Again, minimum (r=0.506; p<0.05), mean (r=0.416; p<0.001) and lowest (r=0.518; p<0.001) environmental temperature (^°C) showed a positive correlation with dengue cases in the year 2014 (Table 1).

4 Discussions

During the initial stage of build-up of infection (August) average rainfall was very high (328.8 mm). There were a good number of rainy days (17.2) total in wetting months and highest rainfall in 24 hours were 644.4 mm and 183.6 mm respectively. Relative humidity was also very high 81.80%. These parameters were found to be optimum to nurture the infection in nature. There was rapid spread of infection in next two months (September and October), which period could be regarded as the potential transmission season. The parameters of the weather in these two months were practically the same which favoured transmission and dissemination of dengue. In the month of November with the diminishing of all the parameters of weather, cases began to decline and transmission of infection practically ended in the month of December.

The relationship between weather and dengue was assessed in multiple settings using different statistical methods (Luz et al., 2008; Bhattacharya et al., 2014). Our results indicated that an increase in the number of rainy days and relative humidity were associated with an increase of dengue cases in Kolkata. Rainfall, daily maximum rainfall, and minimum/maximum/mean temperatures were associated with the dengue incidence.

Production of mosquitoes determined by the accessibility of appropriate habitat for the larval stages would depend on rainfall (Russell, 1998). Rainfall was found to correlate with dengue in many countries (Russell, 1998; Gubler et al., 2001). This study also reported positive association of rainfall, rainy days and daily maximum rainfall with the dengue infection. Increased rain would increase larval habitat and vector population size by creating a new habitat or increase adult survival (Russell, 1998). However, in tropical areas in particular, extensive and continuous rainfall could delay the build-up of some mosquito species until late in the season and thus delaying transmission (Russell, 1998; Hati, 2006).

Relative humidity thus would influence longevity, mating, egg production and dengue virus transmission (Russell, 1998; Costa et al., 2010). Adult survival and hatching rate would be affected by the rise in temperature and lower humidity (Costa et al., 2010). Findings of this study suggested that more humidity of the previous month might trigger a higher outbreak of dengue cases, because relatively higher humidity in the surrounding environment would assist mosquitoes in seeking target hosts facilitating disease transmission (Costa et al., 2010).

In conclusion, an increase in the number of rainy days, relative humidity and a decrease in ambient temperature were associated with increase in dengue cases in Kolkata.

Acknowledgements

We are thankful to Mr. Kamal Kumar Mukim, Mr. Dilip Das and Dr. Basab Mandal, Directors of Gautam Laboratories Imaging and Research Centre, India for various helps.

Ethical approval

Ethical approval is not needed for this study. For the study has been designed from the data collected from Goutam Laboratories based on referred dengue cases.

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