|Year : 2020 | Volume
| Issue : 2 | Page : 176-181
Prevalence of dengue vectors, larval breeding habitats, Stegomyia indices and their correlation with dengue cases in urban and rural areas of Punjab, India
Seema Devi1, Taruna Kaura2, Jaspreet Kaur2, Lovleen3, Jaspreet Takkar4, SK Sharma2, GS Grover1
1 National Vector Borne Disease Control Programme, 34-A Chandigarh, Punjab-160022, India
2 National Institute of Malaria Research, Dhakoli, S.A.S. Nagar, Punjab -160104, India
3 Department of Zoology, Lovely Professional University, Jalandhar, Punjab, India
4 Gian Sagar Medical College, Banur, Punjab, India
|Date of Submission||04-Nov-2019|
|Date of Acceptance||07-Jan-2020|
|Date of Web Publication||14-Jul-2021|
Dr G S Grover
Department of Health & Family Welfare, Parivar Kalyan Bhavan, Sector 34-A, Chandigarh
Source of Support: None, Conflict of Interest: None
Background & objectives: The state of Punjab, India is highly endemic for dengue fever as high number of confirmed dengue cases have been reported since 2013. A better understanding of vectors distribution and their seasonal variation is necessary to control the disease. Therefore, the present study was conducted in both rural and urban areas of 11 out of 22 districts of Punjab to highlight seasonal prevalence of Aedes vector mosquitoes.
Methods: Entomological surveys were carried out in different seasons and all kinds of indoor and outdoor breeding habitats were examined and Aedes immatures were collected. The Stegomyia indices were calculated and compared from urban and rural areas in different seasons.
Results: Both vectors of dengue, i.e. Aedes aegypti and Ae. albopictus were recorded to be prevalent. Ae. aegypti mosquitoes were observed in all districts surveyed while Ae. albopictus were found only in seven districts of Punjab. The Stegomyia indices were significantly high during monsoon as compared to pre- and post- monsoon periods. Occurrence of dengue cases were found to be correlated with the Stegomyia indices.
Interpretation & conclusion: This is the first detailed study of prevalence of dengue mosquito vectors in Punjab showing the presence of Ae. aegypti and Ae. albopictus in both urban and rural areas of the state, thereby demonstrating wide distribution of this vector. Different breeding habitats identified in the study should be subjected to targeted intervention such as source reduction in order to achieve effective control of dengue cases.
Keywords: Aedes aegypti, Aedes albopictus, breeding habitats, dengue cases, Stegomyia indices, Punjab, India
|How to cite this article:|
Devi S, Kaura T, Kaur J, Lovleen, Takkar J, Sharma S K, Grover G S. Prevalence of dengue vectors, larval breeding habitats, Stegomyia indices and their correlation with dengue cases in urban and rural areas of Punjab, India. J Vector Borne Dis 2020;57:176-81
|How to cite this URL:|
Devi S, Kaura T, Kaur J, Lovleen, Takkar J, Sharma S K, Grover G S. Prevalence of dengue vectors, larval breeding habitats, Stegomyia indices and their correlation with dengue cases in urban and rural areas of Punjab, India. J Vector Borne Dis [serial online] 2020 [cited 2021 Aug 5];57:176-81. Available from: https://www.jvbd.org/text.asp?2020/57/2/176/313966
Seema Devi, Taruna Kaura, Jaspreet Kaur. Equal contribution
| Introduction|| |
Dengue fever is one of the major mosquito-borne viral infections in tropical and sub-tropical regions and is considered as the leading cause of illness and death,. It is the most common and widespread arboviral infection in the world today with significant morbidity and mortality. The incidence of the disease has increased over the last 50 years with 2.5 billion population living in areas at risk where dengue is endemic affecting 100 million people each year,, and has emerged a major public health concern at international level. Dengue infection has been known to be endemic in India for over two centuries. Increasing urbanization, unplanned growth of cities, mushrooming of urban slums, unsafe water storage practices have contributed to rise of dengue. The mosquitoes Aedes aegypti and Aedes albopictus, the established vectors of dengue and chikungunya are widespread throughout the tropical, subtropical and temperate areas of the world. Primarily, Ae. aegypti is reported to be native to Africa and now known to spread to tropical and sub-tropical areas in six continents. Ae. albopictus mosquito has spread from Asia (where it originated), to Africa, North and South America, Europe, the Caribbean and also some parts of Pacific islands. Ae. albopictus has been known to be carrier of DEN virus and has been considered as a secondary vector of dengue. It is reported to be adaptable in ecologically diverse conditions. Both species are found in a wide array of water receptacles, including both artificial and natural containers which hold clean water. Ae. aegypti preferentially breeds in artificial containers while Ae. albopictus have a preference for natural water receptacles found outdoors.
Since there is no specific vaccine and/or treatment available for dengue, for vector control it has become imperative to generate data on vector density, population at infection risk and the sensitivity of mosquitoes to the insecticides,. Monitoring of dengue vector population in each region, such as the Stegomyia indices [Container Index (CI), House Index (HI), Breteau Index (BI) and Pupal Index (PI)] have become indispensable for entomologist to plan disease surveillance and control programmes. In India, dengue has recently dramatically spread and has been reported from all over the country including Punjab where it remains a major public health problem. During 1996, an outbreak of dengue with 720 confirmed cases and 19 deaths were reported from Ludhiana district in the state. In 2015, the state has reported 14,149 confirmed dengue cases and there has been a steady increase in confirmed cases of dengue in the last five years [Table 1]. The current study was designed to assess the prevalence of dengue vector in relation to Stegomyia indices during different seasons in urban and rural areas along with their correlation with dengue cases in 11 districts of Punjab. Based on the results, appropriate control measures can be planned to contain rise of dengue cases in the state.
| Material and Methods|| |
Description of study area
The state of Punjab is located in north western India (latitude 30° 4’ North and longitude 75° 5’ East) and has an area of 50,362 km2. It is bounded on the west by Pakistan, north by Jammu and Kashmir, northeast by Himachal Pradesh and south by Haryana and Rajasthan. Most of the area in this state comprises of fertile plains, alluvial plain with three rivers and an extensive canal system for irrigation.
Selection of study area
The state of Punjab comprises of 22 revenue districts of which 11 districts were selected and based upon the retrospective analysis of last 5 years’ (2012–2016) dengue cases, stratification was done into high, medium, and low risk areas. A cut off value of <250, >250, and >500 dengue cases was used to stratify the districts into low, medium and high-risk areas respectively. These districts were - Ludhiana, Patiala, Mohali, Amritsar (high risk); Hoshiarpur, Jalandhar, Fatehgarh Sahib, Bathinda (medium risk); Sangrur, Shaheed Bhagat Singh Nagar (S.B.S) and Muktsar (low risk).
Study/survey design and period
Cross-sectional entomological surveys were carried out in localities of 11 districts from where dengue cases were reported. The surveys were carried out in houses and peri-domestic areas during pre-monsoon (May–June), monsoon (July–September) and post-monsoon (October–December) seasons in 2017 to determine the preferred breeding sources and distribution of dengue vectors.
Entomological/Aedes infestation survey
A house-to-house entomological survey was carried out in the study areas. The houses included in the study were selected by a systematic random sampling technique. From all the study areas, the first house was randomly included in the study. Thereafter, every 30th house was inspected for mosquito breeding in potential breeding habitats. All kinds of indoor and outdoor breeding habitats were examined to collect the Aedes immatures by following the dipping method. A container containing any amount of water was considered as wet container and the wet container containing any number of immatures (larvae, pupae or both) was considered as positive container. The larvae were collected by using ladle, dipper, pipette depending upon the type and size of breeding source. The collected immatures were kept in plastic containers labelled coded for type of breeding source, locality code, house and date of collection.
Rearing and identification of mosquitoes
Mosquito larvae were reared in trays for each container type in laboratory and pupae were transferred to individual adult cages for emergence. The number of adults emerged out of total number of larvae were recorded and adult emergence rate was calculated. Every day the emerged adult mosquitoes were morphologically identified to species level by following the pictorial taxonomic keys,.
The classical Stegomyia indices were calculated by following the guidelines of World Health Organisation (WHO). Based on estimated indices, we classified the areas/sites in terms of epidemic risk levels for DEN as low, medium or high with reference to established epidemic thresholds.
Significance of difference in Stegomyia indices in different dengue risk areas and during different seasons in urban and rural areas were compared using ANOVA. Pearson’s correlation test was applied to compare the dengue cases reported in the study area during 2017 with Stegomyia indices. The analysis was carried out using the SPSS version 16.0 software package.
| Results|| |
Relative abundance of Aedes mosquito species and breeding habitats
Overall, 12,412 different types of water-holding containers, 6525 from urban and 5887 from rural areas were inspected during the survey, of which 628 in urban (9.6%) and 428 in rural areas (7.2%) were found to be infested with Aedes mosquito larvae. Some of the common breeding habitats found positive for Aedes larvae during surveys are shown in [Figure 1]. The type of water-holding container in the study with the highest rate of positivity for Aedes mosquito larvae were found to be artificial containers, i.e., earthen pots (14.1%), followed by desert cooler (10.5%), discarded tyres (6.8%), refrigerator trays (6.7%), plastic containers (6.4%), discarded material (6.2%), water tank (5.3%) and miscellaneous (5.1%) in decreasing order. Ae. aegypti was found to be the predominant species and constituted 88.5% of all the emerged adults from all 11 study districts. In comparison, the proportion of Ae. albopictus was only 11.5% in 7 districts of Punjab. Ae. aegypti breeding was highest in desert coolers, plastic containers, earthen pots, refrigerator trays and tyres, whereas the habitats in which both Ae. aegypti and Ae. albopictus were found co-breeding were earthen pots (Ae. aegypti -16.5%; Ae. albopictus - 6.5%), discarded material (Ae. aegypti - 3.4%; Ae. albopictus - 2.3%), discarded tyres (Ae. aegypti - 3.2%; Ae. albopictus - 2.2%) and other miscellaneous material (Ae. aegypti - 4.0%: Ae. albopictus - 1.6%).
|Figure 1: Breeding habitats of Aedes aegypti and Aedes albopictus in study area in the state of Punjab, India. [Upper left: an earthen pot containing water kept for birds; upper right: a water storage plastic container; middle left: discarded tyres; middle right: fire buckets; lower left: desert cooler; lower right: refrigerator tray].|
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A higher dengue vector density was recorded during monsoon when compared with pre- monsoon and post-monsoon period. Ae. aegypti was found to be significantly high in both urban and rural areas whereas Ae. albopictus was found to be significantly high in urban as compared to rural areas [Table 2].
|Table 2: The prevalence of Aedes aegypti and Aedes albopictus adult females in urban and rural areas in study districts of Punjab in year 2017|
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Stegomyia indices and correlation with dengue cases
A total of 3054 and 2828 houses were searched from urban and rural areas, respectively, for Aedes breeding. Out of the total houses surveyed, 565 houses from urban and 390 houses from rural areas were found positive for Aedes breeding. Stegomyia indices (House index, Container index, Breteau index and Pupal index) for Aedes mosquito larvae from the study areas were calculated and all indices were found to be higher in urban as compared to rural areas [Table 3] indicating high breeding potential of Aedes in urban areas. Moreover, the difference in Stegomyia indices between high, medium and low dengue risk areas were found to be significant (P <0.001). The analysis of seasonal variations in Stegomyia indices during pre-monsoon, monsoon and post-monsoon periods showed that breeding was found to be significantly high during monsoon as compared to pre- and post-monsoon (P< 0.001), indicating that rain water collection in containers support maximum breeding of Aedes species during the monsoon [Table 4]. The Pearson correlation analysis between dengue cases for data based in 2017 and Stegomyia indices in both urban and rural areas [Table 5] was found to be positively correlated (P<0.005). The correlation was much more significant with Breteau Index and Pupal Index as compared to House Index and Container Index [Table 5].
|Table 3: Stegomyia indices of Aedes mosquito breeding in urban and rural areas along with number of dengue cases in study districts of Punjab during 2017|
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|Table 4: Comparison of Stegomyia indices of Aedes mosquito breeding in different seasons|
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|Table 5: Correlation of dengue cases# with Stegomyia indices for data based on 2017*|
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| Discussion|| |
Dengue fever has emerged as the most important mosquito-borne viral disease and its vectors are spreading in areas hitherto free from the disease in India,. In Punjab, dengue fever puts a high load of morbidity not only on the poor but also among socio-economically better communities of the state. The high incidence of the disease every year is regarded as a negative reward of the rapid urbanization and changed social behaviour such as water storage practices and improper disposal of solid waste materials in and around house dwellings thus resulting in the creation of more favourable breeding habitats for Aedes mosquitoes. The present study has focused on the identification of different type of dengue vectors and their preferential breeding habitats in some dengue endemic districts of Punjab so that targeted interventions may be planned to curb the disease. The study has also determined the Stegomyia indices during different seasons in high, medium and low dengue risk areas and worked out their correlations with the dengue cases reported from the study area. The Stegomyia indices were observed to be significantly high during monsoon period and suggest high-risk of dengue transmission and positive correlation with the recorded dengue cases in the state. This study showed that Aedes mosquitoes seemed to preferentially breed in earthen pots followed by desert coolers and discarded tyres. The possible reason behind this is that artificial containers were abundantly located close to human habitation and they can also hold water for extended periods of time making these suitable breeding habitats for mosquitoes,,. The present results are in conformity with the earlier studies where earthen/mud pots, desert coolers and tyres were reported to be the prominent habitats for Aedes breeding,. These findings may enable a more focused approach to vector control in which specific types of water-holding containers would be targeted. In such an approach, source reduction would have the greatest impact to control disease transmission.
In this study, the dominant Aedes mosquito species was found to be Ae. aegypti which appeared to be playing a major role in dengue virus transmission; further studies are underway to isolate DEN virus from different Aedes species to ascertain their role in disease transmission. Similarly, Ae. albopictus was also recorded in low proportions from the study areas but its role in DEN virus transmission is yet to be established in Punjab. There is a need to identify circulating dengue virus strains and incriminating vectors specific to an area for formulating intervention strategies. The relative abundance of both species, Ae. aegypti and Ae. albopictus, was higher during monsoon period than pre-monsoon and post-monsoon periods in agreement with previous studies in different parts of India,. Another important finding from the present study shows that Ae. aegypti was found to be high in both rural as well as urban areas whereas Ae. albopictus was abundant in urban than rural areas. The underlying reasons could be rapid urbanization in rural areas and introduction of piped water supply leading to water storage practices resulting in shift of breeding of Ae. aegypti in rural areas too. Ae. albopictus, which is believed to be predominantly a rural species and prefers to breed outdoors, was found more in abundance in urban areas, which indicates a shift in the breeding preferences from outdoors to peri/intradomestic containers in urban areas. However, it was suggested that urbanization has substantially resulted in its increased density which in turn is potentially increasing its vectorial capacity in urban areas. In another study, it was found that Ae. albopictus has higher blood feeding rate in urban areas compared to rural areas attributed to more host availability.
This study was the first attempt to characterise the presence of Aedes mosquitoes and their preferred breeding habitats in Punjab. There are only few study reports on number of dengue and chikungunya cases from few districts but that lacked data on the dengue vector prevalence and breeding characteristics,,. An outbreak of dengue was reported in Ludhiana district of the state in 1996. Both epidemiological and entomological surveillance was carried out for the investigation of outbreak-related factors. Entomological surveillance revealed high House index and Container indices in different localities of the city. Breeding was mostly in desert coolers, tin/plastic containers, earthen pots and plastic buckets. Mosquito adults Ae. aegypti were collected from rooms and water receptacles.
The present results are consistent with the number of dengue cases indicating the potential of spreading dengue viral infection. Further investigations on arboviral transmission by Aedes mosquitoes are warranted. The study further showed that entomological indices were directly correlated with the number of reported dengue cases in the study area. The districts were stratified into high, medium and low risk status for dengue based on the retrospective data of five years before the start of the study. However, during 2017, dengue cases increased substantially in Hoshiarpur and Sangrur districts and decreased in Amritsar district, thereby indicating that the stratification of districts on the basis of dengue cases in the past years does not hold well because of year-to-year variations in the cases. Despite this limitation, this study provides the baseline data on the presence of the arbovirus vector Aedes mosquitoes in the state of Punjab.
| Conclusion|| |
Lack of awareness results in the creation of man-made breeding habitats for Aedes proliferation. Such breeding habitats should be subjected to targeted interventions such as source reduction with the removal of water-holding containers around household premises and proper disposal of discarded tyres so as to achieve sustainable dengue vector control.
Conflict of interest: None
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]