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Table of Contents
RESEARCH ARTICLE
Year : 2018  |  Volume : 55  |  Issue : 3  |  Page : 189-196

Prevalence of disease vectors in Lakshadweep Islands during post-monsoon season


1 Department of Life Sciences; Department of Epidemiology and Public Health, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, India
2 Department of Life Sciences, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, India

Date of Submission14-Jun-2018
Date of Acceptance07-Jul-2018
Date of Web Publication4-Jan-2019

Correspondence Address:
Jayalakshmi Krishnan
Assistant Professor, Department of Life Sciences, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-9062.249127

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  Abstract 

Background & objectives: Increase of vector-borne diseases (VBDs) in India has posed a question on the situation in Lakshadweep Islands, where VBDs are reported from time-to-time. The present investigation was aimed to assess the faunastic situation of the prevailing vectors along with their breeding sites in different islands of the Lakshadweep.
Methods: Extensive surveys were carried out from November 2017 to January 2018 (post-monsoon season) randomly in the nine inhabited islands of Lakshadweep for conducting faunastic studies on mosquitoes and to know the basic binomics like breeding and resting preference of mosquitoes. The study islands included, Kavaratti, Agatti, Chetlat, Bitra, Amini, Kadmath, Andrott, Kalpeni and Kiltan. Both immature and adult collections were carried out by standard/appropriate sampling techniques. The obtained data were calculated and analysed in terms of different entomological indices
Results: A total of 3356 mosquitoes were collected during the study period which comprised of 16 species from nine genera. Out of the 16 species, six belonged to mosquito vectors. The collection included malaria vector, Anopheles stephensi; Japanese encephalitis vector, Culex tritaeniorhynchus; Bancroftian filariasis vector, Cx. quinquefasciatus; Brugian filariasis vector, Mansonia uniformis; and dengue and chikungunya vectors, Stegomya albopicta and St. aegypti. Stegomya albopicta was the most predominant species observed constituting 54% of the catch, followed by Cx. quinquefasciatus, An. stephensi, Cx. tritaeniorhynchus, and St. aegypti constituting 10.5, 6, 3 and 1.2%, respectively. Apart from vector species many non-vectors such as Heizmannia chandi, An. subpictus, An. varuna, Cx. sitiens, Cx. minutissimus, Cx. rubithoracis, Fredwardsius vittatus, Lutzia fuscana, Malaya genurostris and Armigeres subalbatus were also present in the study area. In Kavaratti Island, the capital of Lakshadweep, a non-vector species of sandfly, Sergentomyia (Parrotomyia) babu was observed during the indoor resting collection. The major breeding sites which supported various mosquito species included, discarded plastic containers, tree holes, open sintex tanks (water storage tanks), unused wells, discarded tyres, discarded iron pots, unused and damaged boats, cement tanks, pleated plastic sheets, coral holes, pits and irrigation canals, discarded washing machines, and Colocasia plant leaf axils. Breteau index ranged between 65.3 and 110, CI ranged between 63.64 and 72.41; and HI ranged between 38.46 and 70 among the various islands.
Interpretation & conclusion: Entomological indices such as house index (HI), breteau index (BI) and pupal index (PI) were high in all the nine islands and exceeded the threshold levels specified by WHO, indicating high risk for dengue virus transmission in case of outbreaks. Occurrence of vector as well as non-vector species indicates that the global change in climate is causing notable changes in terms of breeding of vector and non-vector species in the islands. With the reported cases of VBDs and the presence of vectors species in Lakshadweep Islands, a stringent control measure needs to be implemented at the Lakshadweep Islands.

Keywords: Breeding sites; Lakshadweep Islands; mosquito; vector-borne diseases; vector surveillance


How to cite this article:
Krishnan J, Mathiarasan L. Prevalence of disease vectors in Lakshadweep Islands during post-monsoon season. J Vector Borne Dis 2018;55:189-96

How to cite this URL:
Krishnan J, Mathiarasan L. Prevalence of disease vectors in Lakshadweep Islands during post-monsoon season. J Vector Borne Dis [serial online] 2018 [cited 2019 May 27];55:189-96. Available from: http://www.jvbd.org/text.asp?2018/55/3/189/249127




  Introduction Top


Islands have different geomorphologic and geological settings, diverse biodiversity and vegetation, unique culture and food habitats with limited number of population. The health and welfare of islanders are at risk because of the lack of medical facilities when disease outbreaks occur. Vector-borne diseases (VBDs) commonly affect the people in islands, apart from non-communicable diseases such as hypertension, diabetes, etc. India, a country with many states and union territories, is very rich in culture and heritage. Lakshadweep union territory is a coral island located in the Arabian Sea nearby Kerala state and is known for its pristine beauty. Of total 36 islands, 11 are inhabited by humans. The total land area of these islands is about 32 km2, out of which about 26.32 km2 is in use for living. As per the 2011 census, the total population of the Lakshadweep Islands is 64,473. The main occupations of the islanders include coconut cultivation, production of coir and fishing, etc. Every island has primary health centres, schools (up to senior secondary) and other government services like police station, etc. The Directorate of Health Services is located in Kavaratti Island, the capital of this union territory.

A study conducted in 1958 has indicated that Lakshadweep Islands was endemic to filariasis[1]. Sporadic malaria was reported in 1978 from the island[2]. In 2000, presence of An. stephensi, a vector of urban malaria was reported from Agatti and Kavaratti Islands[3]. Similarly, in 2009, presence of St. albopicta was reported from Kalpeni and Andrott Islands of Lakshadweep[4]. A total of 1.39 million chikungunya cases have been reported from India spreading across 35 states and UTs, including Lakshadweep Islands in 2009. Upon ecoentomological investigations, it was observed that St. albopicta was the predominant species in Lakshadweep Islands[5]. There is no any study after 2009 which investigated the prevailing vectors in Lakshadweep Islands.

Albeit, VBDs such as dengue, chikungunya, malaria and filariasis have been reported from the Lakshadweep island from time-to-time in the past, no data is available about vectors responsible for disease transmission and no vector bionomics and systematic faunastic studies have been carried out for most of the islands. Hence, in the present investigation, a vector surveillance was carried out in nine islands of Lakshadweep during the post-monsoon season. In addition, faunastic situation of mosquitoes and their basic bionomics like breeding habitats, resting behaviour, etc. were also explored to add the present knowledge which will be useful for the national vector control programmes.


  Material & Methods Top


Study area

Extensive surveys were carried out from November 2017 to January 2018 (post-monsoon season) randomly in the nine inhabited islands of Lakshadweep for conducting faunastic studies on mosquitoes, to know the breeding and resting preference of mosquitoes. Both immature and adult collections were carried out by standard/appropriate sampling techniques as per WHO methods. The study islands included, Kavaratti, Agatti, Chetlat, Bitra, Amini, Kadmath, Andrott, Kalpeni and Kiltan. The GPS coordinates of all the nine islands are indicated in [Table 1].
Table 1: GPS coordinates of the nine study Islands of Lakshadweep

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Adult mosquito collection

Adult resting mosquitoes were collected using oral aspirator from both indoor as well as outdoor sites (bushes, tree holes etc.) at day-time and CDC light-traps were used in human dwelling, cattlesheds as well as outdoors during night to collect data on distribution of different mosquito species.

Immature collection

Both natural as well artificial water holding habitats (discarded tyres, containers, tree holes, etc.) were thoroughly checked for mosquito immatures using appropriate sampling method (e.g. dipping, pipetting, uprooting and washing, etc.), and when found positive some of larvae and pupae were collected for rearing. Immature collection was made not only from the common container habitats, but also from various lesser examined habitats such as the leaf axils, bamboo cut stumps, fallen coconut spathe, coconut shells, tree holes, discarded shoes, etc. During the collection other parameters like GPS coordinates, pH, presence of algae, pollution level and other vegetation were also noted. Maximum recorded pH of the immature breeding habitat was nine and minimum pH was 7.4. The average temperature recorded during the collection period was 29.4°C. The adult mosquitoes were identified using standard taxonomic keys[6] and the immatures were reared in plastic bottles covered with insect net, and after emergence they were also identified/classified using standard taxonomic keys. The entomological indices like breteau index (BI), container index (CI) and house index (HI) were calculated to understand the risk of Stegomyia transmitting diseases such as dengue, chikungunya and Zika.

Data analysis

The larval survey data were calculated and analysed in terms of different larval survey techniques, viz. HI, CI, and BI. The calculation of larval indices is based on the following mathematical formulae:




  Results Top


In total 3356 mosquitoes were collected from the nine islands which included 16 species from nine genera. The predominant breeding sites are displayed in [Table 2]; and the list of mosquito species (vector as well as non-vector species) collected and their distribution island-wise is given in [Table 3] and [Table 4], respectively. Out of the 16 species collected, six species are known vector mosquitoes in India. The different type of vectors found in the islands included, malaria vector, An. (Cellia) stephensi; Japanese encephalitis vector, Cx. (Cx.) tritaeniorhynchus; Bancroftian filariasis vector, Cx. (Cx.) quinquefasciatus; Brugian filariasis vector, Ma. (Mansonioides) uniformis; dengue, Zika and chikungunya vectors, St. albopicta and St. (St.) aegypti. Stegomya albopicta, is the predominant species which constituted around 54% of the total collected mosquitoes. Other vectors like Cx. quinquefasciatus, An. stephensi, Cx. tritaeniorhynchus, St. aegypti constituted 10.5, 6, 3 and 1.2%, respectively of the catch as shown in [Figure 1]. The total number of mosquitoes from various breeding habitats in nine islands are shown in [Figure 2] and breeding habitats are shown in [Figure 3], and [Table 5]. Individual island-wise comparison of the mosquito species revealed that St. aegypti is present only in Kavaratti and Andrott Islands, whereas St. albopicta and Cx. quinquefasciatus are present in all the islands.
Figure 1: Percentage of collected mosquito vectors in nine islands of Lakshadweep Islands.

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Figure 2: Total number of vector mosquitoes emerged from collected immatures in different habitats.

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Figure 3: Common breeding habitats of mosquito immatures in Lakshadweep Islands.

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Table 2: Predominant breeding sites of mosquito immatures in Lakshadweep Islands

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Table 3: Island-wise distribution of mosquito species

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Table 4: List of mosquito species collected from nine islands of Lakshadweep

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Table 5: Different breeding habitats of collected mosquito species in Lakshadweep Islands

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The entomological indices are summarised in [Table 6]. The result of the indices indicated that BI (110) and HI (70) were very high in Bitra Island in comparison with other islands. Similarly, the CI (72.41) was very high in Andrott in comparison with other islands. Japanese encephalitis vector, Cx. tritaeniorhynchus was observed only in five islands, namely Chetlat, Amini, Kiltan, Andrott and Kalpeni. Surprisingly, An. stephensi was seen in all the nine islands surveyed. Further, Ma. uniformis the vector of Brugian filariasis was observed in Kalpeni Island only. Among the 16 mosquito species, 12 were present in Amini, Andrott and Kavaratti Islands and nine in the Kadamat and Kalpeni Islands. The presence of same type of mosquito species in these islands, reiterates uniform breeding sites, suitable climatic conditions and similar geographical locations among these islands. In Chetlat and Kiltan Islands, total seven species of mosquitos were observed. In Agatti, seven species of mosquitoes were seen, while in Bitra only three species were observed which was lowest in comparison to other islands.
Table 6: Entomological indices for Stegomyia species in nine islands of Lakshadweep

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  Discussion Top


Worldwide, the epidemics of VBDs such as malaria, dengue, chikungunya are increasing day-by-day. Lakshadweep Islands are more vulnerable to VBDs, as experienced by an outbreak of dengue and chikungunya a decade ago[4]. At present, the only method by which the outbreak of VBDs can be controlled is reduction of vector populations. Data on the biodiversity of mosquitoes, breeding habitats are very limited from Lakshadweep with a notably few publications. Earlier investigations from Lakshadweep reported the presence of An. stephensi (type form), An. varuna, Cx. quinquefasciatus, St. aegypti and St. albopicta species from Kavaratti and Agatti[3] in 2001. The efforts undertaken in this study to understand the prevalence of vector/mosquito biodiversity showed the presence of various types of disease vectors, non-vectors in the Lakshadweep Islands. For example, An. stephensi was collected from unused wells, ponds, pleated plastic sheets, open sintex tanks, discarded plastic containers and tree holes across the study area. The presence of potent malaria vectors predisposes a greater risk of transmitting malaria to the islands. The VBDs that are reported in Kerala could be easily transported to Lakshadweep because of high rate of tour and travel between the island and mainland. The occurrence of An. stephensi in the current study across all the nine islands (intra-island transmission) indicates that the stagnant water in boats or ships during movement of people in-between islands would have carried the vector to all the islands studied.

Recently, the presence of An. stephensi has been reported from Mannar Island, Sri Lanka[7] which is in line with our observation. Anopheles stephensi invasion to Lakshadweep would have occurred from mainland Kerala. There are 23 anopheline species reported from Andaman and Nicobar Islands, and amongst these An. sundaicus is the incriminated species of malaria vector in Andaman[8]. The availability of potential breeding sites for An. stephensi in Lakshadweep islands marks the highest risk of spread of malaria in future. Three imported malaria cases have been reported in Kiltan Island of Lakshadweep in the year 2017 (data collected from PHC, Kiltan). Presence of parasite and local malaria vector, as well as favourable breeding sites can lead to outbreak.

In 2009, a study by Ali et al[4] conducted in Kalpeni and Andrott Islands in Lakshadweep reported St. albopicta as the common vector for both the islands similar to earlier published reports[5]. Interestingly and contrastingly, in this study St. aegypti was recorded from Andrott Island, which was not observed and reported by Ali et al[4] in 2001. In mainland India, St. albopicta is the primary vector of chikungunya, as its high density was observed during mosquito surveillance which might have caused the outbreak of chikungunya in 2006 as well.

According to a surveillance study reported in 2007, Kadamat, Amini and Kavaratti Islands are known to have high larval indices for St. albopicta[9]. Anthropogenic factors like, travel and tourism, population growth, water storing practices, etc. could have contributed to the spread of St. aegypti as vector to Andrott Island. The nearby Minicoy Island[2] has reported the presence of St. aegypti in 1974. The studies carried out in Galapagos Islands have shown that the water reserving habitats of islanders (as like mainlanders) in various containers might have paved the way for the breeding of St. taeniorhynchus[7]. Stegomya aegypti and St. albopicta are widely present in Andaman and Nicobar Islands as well[10].

Irrespective of rainy season, dengue vectors are reported throughout the year in these islands and mainland, mainly because of water storage practices[9],[11],[12],[13],[14]. Entomological indices are very useful in predicting the disease (dengue) outbreak in high risk areas. In the present study, higher levels of HI, BI and CI indices were observed in all the islands even in dry season. Breteau index ranged between 65.3 and 110; CI ranged between 63.64 and 72.41 and HI ranged between 38.46 and 70. These high indices combined with presence of artificial containers once again highlights the risk of outbreak of dengue in Lakshadweep Islands in future[14]. The presence of rat damaged coconuts and their use can cause arboviral diseases. This study reports the presence of Cx. quinquefasciatus in all nine islands. Culex quinquefasciatus is reported to cause Bancroftian filariasis in many parts of world and India, also it is a common species in Andaman and Nicobar Islands[15]. Additionaly, the study observed the presence of Cx. tritaeniorhynchus in Amini, Andrott, Chetlat, Kiltan and Kalpeni Islands of Lakshadweep and Ma. (Mansonioides) uniformis in the Kalpeni Island. Coexistence of Culex and Steogomyia species may be attributed to container breeding and availability of various organic consumption materials for the growth of larvae. This is in line with an earlier report which indicated the evidence of coexistence of Culex and Aedes[16].

It is for the first time that several non-vector mosquito species such as, Heizmannia (Heizmannia) chandi, An. (Cellia) subpictus, Cx. (Culex) sitiens, Cx. (Lophoceraomyia) minutissimus, Cx. (Lophoceraomyia) rubithoracis, Fredwardsius vittatus, Lutzia (Metalutzia) fuscana and Malaya genurostris were recorded from different islands of the Lakshadweep. Interestingly, Sergentomyia (Parrotomyia) babu, a non-vector species of sandfly was also collected from the Kavaratti Island. Out of the four collected sandfly species three were males and one was female. The Hz. chandi, Ar. subalbatus and An. stephensi were collected from tree holes while Ma. uniformis was found in Pistia plants near irrigation canals. There were 12 different kind of breeding habitats for St. albopicta including tree hole, cement tanks, discarded tyres and plastic containers, open sintex tanks, unused wells, discarded tyres, discarded iron pots, unused boats, cement tanks, pleated plastic sheets, coral holes, Colocasia plant leaf axil and discarded washing machine. This explains the high abundance of St. albopicta in Lakshadweep.


  Conclusion Top


The study recorded the presence of different disease vectors in the studied islands, for example, malaria vector, Anopheles stephensi; Japanese encephalitis vector, Cx. tritaeniorhynchus; Brugian filariasis vector Ma. uniformis; Bancroftian filariasis vector, Cx. quinquefasciatus; and dengue and chikungunya vectors, St. albopicta and St. aegypti. Apart from this, seven non-vector mosquito species were also recorded from the islands namely, An. subpictus, An. varuna, Cx. sitiens, Hz. chandi, Cx. minutissimus, Cx. rubithoracis, Fr. vittatus, Ml. genurostris and Lt. fuscana. The most common vector species recorded across all the islands are St. albopicta, the vector of dengue and chikungunya which breeds in both natural and artificial habitats, and Cx. quinquefasciatus, the vector of lymphatic filariasis which breeds in polluted habitats, such as wells (not in use), pits, etc. High entomological indices such as house index, breteau index and pupal index, exceeding the threshold levels specified by the WHO, indicates high risk for dengue virus transmission in case of outbreaks. The marked presence of these vector species in Lakshadweep Islands, suggests implementation of stringent vector control measures to prevent future their outbreak.

Conflict of interest

The authors declare that they have no competing interests.


  Acknowledgements Top


The authors acknowledge and thank to the Director of Medical and Health Services, Lakshadweep Islands for their valuable cooperation in proving the entry pass and accommodation to the team. We thank Dr Vasuki from ICMR-Vector Control Research Centre, Puducherry for helping in the identification of sandfly species and acknowledge Mr Ganesamurthy S, Mr Sivaraj and Mr Ramesh, Field assistants for helping in field studies. The authors gratefully thank the Central University of Tamil Nadu, Tamil Nadu for providing the facilities and the Indian Council of Medical Research (ICMR), New Delhi for financial grant to this study.



 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]



 

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