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Table of Contents
RESEARCH ARTICLE
Year : 2022  |  Volume : 59  |  Issue : 3  |  Page : 285-292

Abandoned boats contribute to spread of Aedes and Culex vector mosquitoes in coastal belt, Galle District, Southern Sri Lanka


1 Regional Director of Health Service Office, Unawatuna, Galle, Sri Lanka
2 Department of Zoology, Faculty of Science, University of Ruhuna, Matara, Sri Lanka

Date of Submission08-Jul-2021
Date of Acceptance24-May-2022
Date of Web Publication08-Dec-2022

Correspondence Address:
Chandana Dammika Wijekoon
Department of Zoology, Faculty of Science, University of Ruhuna, Matara
Sri Lanka
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-9062.353252

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  Abstract 

Background & objectives: Although, the number of considerable cases of dengue and lymphatic filariasis have been reported from Galle District, Sri Lanka in the past several years, contribution of abandoned boats to spread vector mosquitoes of Aedes and Culex in the coast is not well studied. Our aim was to assess the positive composition of different types of abandoned boats by larval vector mosquitoes to investigate their occurrence and habitat preference, and the monsoonal and co-existence variation of Aedes mosquitoes.
Methods: The 4th instar larvae of Aedes and Culex species from three different abandoned boat types in nine subsampling sites at three municipal areas in Galle District were collected during 2017–2019. In total, 15 sampling rounds were conducted in each site for three years duration as five sample rounds per year. Larval collections and identifications were carried out using standard techniques and identification keys according to WHO guidelines. The occurrence of mosquito larvae was analysed by boat type, locations within area and year. Further, the average infestation variation of Aedes mosquitoes were compared with monsoonal and co-existence changers.
Results; Out of the total abandoned boats, majority (51%) were engine boats and, 32.7%, 16.35% were troller boats and canoe boats, respectively. Troller boats were highly infested boat type for vector mosquitoes. Aedes albopictus was the dominant vector in abandoned boats other than recorded Ae. aegypti and Culex quinquefasciatus. Culex quinquefasciatus showed comparatively low infestation percentages. The average infestation of Ae. albopictus showed a consistent pattern with monsoon variation. A competitive rapid invasion of Aedes aegypti was observed with the suppressing Ae. albopictus in abandoned boats after 2018.
Interpretation & conclusion: Abandoned boats contribute noteworthy to spread of Aedes and Culex vector mosquitoes in coastal belt, Galle District. These study findings would be helpful for researchers and health authorities to design appropriate vector control measures and to mitigate future dengue and filariasis outbreaks.

Keywords: Abandoned boats; Galle District; Sri Lanka; vector mosquitoes


How to cite this article:
Dissanayake DS, Wijekoon CD, Wegiriya H. Abandoned boats contribute to spread of Aedes and Culex vector mosquitoes in coastal belt, Galle District, Southern Sri Lanka. J Vector Borne Dis 2022;59:285-92

How to cite this URL:
Dissanayake DS, Wijekoon CD, Wegiriya H. Abandoned boats contribute to spread of Aedes and Culex vector mosquitoes in coastal belt, Galle District, Southern Sri Lanka. J Vector Borne Dis [serial online] 2022 [cited 2023 Feb 2];59:285-92. Available from: http://www.jvbd.org//text.asp?2022/59/3/285/353252


  Introduction Top


Mosquitoes are considered as the most important insect vectors which transmit deadly diseases especially in tropical countries[1]. These emerging arthropod-borne pathogenic diseases kill and debilitate millions of humans worldwide. Aedes, Culex, Anopheles, and Mansonia are highly distributed main vector mosquitoes globally and they are responsible for the spread of diseases such as dengue, filariasis, malaria and Chikungunya in South East Asian region[2]. Sri Lanka is one of the countries that faces a serious problem of mosquito-borne diseases. It has a long history with lymphatic filariasis and dengue fever as major public health hazards[3]. Southern Province, which is 117 km away from the capital city, is regularly plagued by dengue and has significantly contributed to the national-level burden of the disease[4]. Besides, the Southern Province has been recognized as one of the endemic areas for the lymphatic filariasis in Sri Lanka[3].

Galle District is one of the high-risk areas for dengue fever and filariasis in Southern Province. The Galle District contributes to 12-18% dengue cases out of the total cases reported in Sri Lanka[5]. Although Sri Lanka was recognized by the World Health Organization (WHO) for having been eliminated lymphatic filariasis in 2016, recent studies have been documented that there are potential pockets with persistent lymphatic filariasis in coastal region, especially in Galle District[6]. The district has a highly populated coastal belt which in turn supports a remarkable tourism industry in the region. Hence, thousands of local and foreign tourists frequently visit popular coastal localities in Galle District and they play a major role in the transmission of mosquito-borne diseases.

The coastal zone of Galle is abundant with fishing communities and, hence, rich in fishing areas where there are a lot of abandoned and derelict boats. These boats have been left idle for the long time without any attention by the owners and they have created a serious health hazard[7]. There are many compartments on boats in which water can collect and become a perfect breeding ground for mosquitoes[8] compared to other potential artificial breeding sites in the coast. Fishermen, fish buyers and beach visitors are a high-risk population of getting infected with mosquito-borne diseases in the area. Little is known about the infestation of mosquito vectors in coastal habitats of Sri Lanka, plus specific habitats such as abandoned boats in the coast.

The mosquito species in fishing boats in two pocket areas in Galle have been reported by two previous studies. The occurrence of Culex sitiens, Cx. quinquefasciatus, Armigeres subalbatus and Aedes albopictus in fishing boats in Rumassala, Galle, has been reported by Heenatigala et al.,[9]. In contrast to this study, Seneviratne[10] reported high abundance of Aedes albopictus in fishing boats in Galle municipal council area. However, both studies did not specifically consider abandoned boats which infest with vector mosquitoes in the coastal belt in Galle District.

We initiated this study with the intention to investigate the positive composition of abandoned boats for Aedes and Culex larval vector mosquitoes in the coastal belt, Galle District. We compared the positive composition of three types of abandoned boats in different localities for larval vector mosquitoes. In addition, the habitat preference, monsoonal variation and coexistence competition of Aedes larval vectors were compared.

We assume these study findings will be very useful to the society because the ecology of coastal habitats is specific when compared to other breeding habitats of mosquitoes. Further, it is important to implement vector monitoring program and to plan vector control strategies in the area and in the coastal zone of Sri Lanka, thus aiding in our ability to ultimately predict Aedes and Culex spp. range expansion in abandoned boats so as to mitigate future outbreaks.


  Material & Methods Top


Study area

Galle District is located in the Southern Province of Sri Lanka. It has an area of 1,652km[2] with 1,058,771 human population. The average annual temperature is 26.6°C and mean annual rainfall in Galle is between 2000–2500mm. The district has bounded Indian Ocean from West including 73 km lengthy coastal belt from Bentota to Goviyapana.

Study design

The longitudinal-prospective study was conducted along the coastal belt, Galle District which lies between Habaraduwa (6° 00’ 0.00” N, 80° 17› 60.00 E) and Ambalangoda (6° 13’ 18.00” N, 80° 03› 14.40 E).

Localities and sub sampling named as sites

Three main localities, Galle, Hikkaduwa and Ambalangoda municipal area were selected for the study [Figure 1]. Preliminary observations revealed that mainly three types of boats (TB: Toller boats, EB; Engine boats, CB; Canoe boats) were common breeding sites of mosquitoes in the area. Thus, three sub-sampling sites in each locality were selected along the coastal line based on the availability of above three types of abandoned boats [Table 1].
Figure 1: Map indicating the selected localities along the coastal belt, Galle District, Sri Lanka.

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Table 1: Localities and sub-sampling sites selected for the study

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Sample collection

Larval surveys were conducted from March 2017 to December 2019. In total 15 sampling rounds were conducted in each site for a duration of three years as five sample rounds (in March, May, July, September and November) per year. Three different abandoned boat types were surveyed [Figure 2]. Larval mosquito samples were collected from all abandoned boats available at, each site at the time. During the first round of sampling all abandoned boats were labelled. Samples were collected by standard pipettes (10 ml capacity), ladles (120ml capacity) and dippers (250ml capacity) based on the amount of water availability as per WHO guidelines. Ten larvae were collected from the breeding site if more than ten larvae were present while all were collected with the presence of larvae less than ten (Guidelines for Aedes vector surveillance and control in Sri Lanka). The collected larvae were placed in labelled plastic vials at the laboratory for species identification.
Figure 2: Boat types surveyed; (A) Troller boat, (B) Engine boats, (C) Canoe boats.

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Sample identification

The collected 4th instar mosquito larvae were identified by the standard mosquito larval identification keys. Collected stages III and IV larvae were placed individually in a depression microscopic slide with a minimum amount of water and identified under a light microscope with an objective (×10) using standard mosquito larval identification keys. Stages I and II instar larvae were reared to reach III and IV instar larvae and identified in species level.

Data analysis

Data were coded and entered into a database created using the Statistical Package for the Social Sciences (SPSS data editor 20.0) software. Data checking and cleaning were done. The data set was tested to the normality. The percentage positive composition of each boat type in each locality for larval vector mosquitoes were calculated using the number of positive boats in each type for a particular vector species in the year dividing by the total number of each boat type counted in each site. The average mosquito infestation was calculated for each boat type and each species of mosquito. The significance in the variation of each larval vector mosquitoes per location and per type of boats were compared using one-way ANOVA analysis at α=0.05 significance level. The average boat infestation in each month per year was calculated to analyse the monsoonal variation of Ae. albopictus. The relationship between the average positive boats for Ae. albopictus and monthly rainfall were calculated by Pearson’s correlation analysis at α=0.01 significance level. In addition, the average boat infestation for each year by two Aedes species was used to study the co-existent competition pattern.

Ethical statement

The permission for this study was granted by the Regional Director, Regional Director’s office of Health Service, Unawatuna, Galle, Sri Lanka.


  Results Top


Abandoned boats

In total 52-TB, 81-EB and 26-CB were identified as abandoned boats in nine sub-sampling sites in three localities along the coastal belt, Galle. Among the total, 30-TB, 25-EB and 15-CB in Galle MA, 7-TB, 16-EB and 6-CB in Ambalangoda MA and 15-TB, 40-EB and 5-CB in Hik-kaduwa MA are located. The recorded majority (51%) of abandoned boats are engine boats and, 32.7%, 16.35% percentages of TB and CB recorded respectively in the coastal line of Galle.

Occurrence of larval vector mosquitoes in abandoned boats

The overall results of occurrence of larval mosquitoes of genus Aedes and Culex recorded from three different abandoned boat types showed that two Aedes species, Ae. aegypti and Ae. albopictus, and a Culex species, Cx. quinquefasciatus were recorded during the survey. All species of larval vector mosquitoes were recorded from all types of abandoned boats.

Percentage positive composition of abandoned boats for larval vector mosquito of species were calculated

The percentage of positive composition of abandoned boats for vector mosquitoes were calculated by the type of boat, locality and year are shown in [Table 2]. According to [Table 2], the highest percentage infestation (96%) across study years, boat types and localities is shown by Troller boats infested with Ae. aegypti in Galle Municipal area in 2019. Among the three types of boats, troller boats recorded the statistically significant high infestation percentages compared to the other types of boats (ANOVA, P<0.05). From overall results, Ae. albopictus showed high infestation percentages in all studied municipal areas in Galle during the consecutive three years of the study. No Ae. aegypti vector mosquitoes were found in any types of abandoned boats from all municipal areas, Galle in 2017. Whereas, Ae. aegypti was recorded from all types of boats in all municipal areas in Galle after 2018. The recorded infestation of Cx. quinquefasciatus in three types of boats from three municipal areas in Galle shows slightly high percentages in CB across all years in the study.
Table 2: Percentage of boats positive for mosquito larvae in each site during 2017 to 2019 (%)

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Average infestation of types of boats by vector mosquito species

The average number of boats infested by each species of vector mosquitoes are significantly different (ANOVA, P<0.05). On an average, Ae. albopictus showed the statistically high infestation (63.47%) (ANOVA, P<0.05) than Ae. aegypti (22.95%), and Cx. quinquefasciatus (13.57%). According to [Figure 3], both TB and EB were most commonly infested by Aedes vector mosquitoes. Of the Aedes species, Ae. albopictus was dominant in all three types of boats (EB-27.32, TB-17.6 and CB-7.77). The average proportion of boats positive for Ae. aegypti was slightly high in TB (9.37) than EB (7.91), and low in CB (1.76). For all boat types, the average infestation by Cx. quinquefasciatus was low during the study. Out of the total infestation of different boats by vector mosquitoes, EB was the highest positive, TB moderate and CB low.
Figure 3: The average number of boats infested by larval vector mosquitoes. TB: Trollar boat, EB: Engine boat; CB: Canoe boat

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Since Ae. albopictus was the most heavily infested vector throughout the study [Figure 3], the analysis was focused to study the monsoonal variation of positive abandoned boats with Ae. albopictus during three consecutive years of the study.

Monsoonal variation of abandoned boats infestation by Aedes albopictus

The average proportion of abandoned boats positive for Ae. albopictus was highest in September in every year of the study. Although the pattern of infestation was consistent across study years, the average abundance of positive boats increased gradually during 2017–2019. The number of boats infested by Ae. albopictus were high in 2019. The infestation percentages were increasing by May and the highest peak is in September in every year of the study [Figure 4]. There was a positive correlation between average infested boats by Ae. albopictus with rainfall of the current month in 2017 (Pearson, r =0.08; P = 0.88), in 2018 (Pearson, r =0.24; P = 0.69) and in 2019 (Pearson, r =0.11; P = 0.85), though not significant.
Figure 4: The average variation of Ae. albopictus infested boats by year.

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Coexistence of Ae. aegypti and Ae. albopictus in different types of boats

Interestingly, the proportion of abandoned boats positive for two Aedes species has significantly changed after 2018. For all types, the average positive boats for Ae. aegypti have increased from 2017–2019. Although, the proportion of infestation for Ae, aegypti showed a consistent pattern in all boat types from 2017–2018, except for TB, for which the proportion of infested TB in 2018 was slightly more than 2017. The proportion of positive boats has increased, with greater numbers from 2018–2019.

The increasing percentage of positive boats for Ae. aegypti from 2017–2019 was high in CB (100%), EB (95.78%) and in TB (83.98%). The average number of positive abandoned boats for Ae. albopietus has reduced gradually by the year (from 2017–2019). It is common for all types of studied boats in the area. The average positive boats for Ae. albopietus became low in 2019 when compared with Ae, aegypti in same year. The decreasing percentage of positive boats for Ae. albopietus from 2017–2019 was high in EB (18.54%) and notably low in both TB (0.71%) and CB (0.54%).


  Discussion Top


The present study revealed the potential infestation of vector mosquitos in available derelict boats across the coastal area in Galle District, Sri Lanka. We compared infestation across different boat types by year from 2017–2019. Among the three types of boats in selected locations for the study, engine boats were the most common abandoned boat type in the coastal zone, Galle, indicating engine boats are the main operated boat type in the coastal fishery industry in Sri Lanka[11]. We identified that abandoned boats are one of the significant sources of Aedes and Culex vector mosquito spread in the coast, because these boats were left by their owners without any long-term attention. Thus, they might be perfect breeding sites for frequent and heavy infesting with mosquitoes. Abandoned boats usually are located 50–100m away from the sea, and we confirm that rain water is the main source of water in the breeding sites of boats as it has less possibility to get contaminated with sea water. The major health hazard of theses abandoned boats is that they are not removed or discarded. Hence, our understanding is that persistent mosquito breeding sites in abandoned boats contribute to spread of vector mosquitos significantly compared with other artificial breeding sites in the coast.

According to past few studies by Galapatthi et al.,[7], Heenatigala et al.,[9], one Aedes species, Ae. albopietus and two Culex species, Culex sitiens and Culex quinquefasciatus have been recorded as the mosquitoes in fishing boats in two selected areas in Galle District. In the present study, we focused only on vector mosquitoes found in abandoned boats and it shows that there are some significant changes in the species composition of vector mosquitoes found in the coast, with the newly recorded Ae. aegypti. The previously recorded Culex sitiens is a non-vector mosquito species[9]. Hence, our study clearly shows that Ae. aegypti was recorded from boats in the Galle coast after 2018 with a few positives in 2017 [Table 2]. During the present study, three vector mosquito species were encountered viz., Ae. aegypti, Ae. albopictus and Cx. quinquefaseiatus.

The selected three types of abandoned boats were positive with all species of vector mosquitos recorded. It is not surprising that vector mosquito colonization of abandoned boats is extremely common, given that boats provide all of the mosquito life cycle needs; abundant oviposition sites, dark and cool resting places for adults and surrounding human hosts for blood meals[12]. Among three types of boats, TB recorded the high infestation percentages in every year compare with engine boats and canoe boats [Table 2] because large surface area with many compartments of TB provides good breeding sites for mosquitos compared to EB and CB. The slightly similar infestation percentages were recorded from EB and CB. Generally, engine boats have moderate surface area with several separations in the bottom floor by ribs and CB has no compartments or ribs separation at its bottom surface.

Aedes albopietus showed the high infestation percentages for all types of boats in the three municipal areas studied in Galle in each year [Table 2] because Ae. albopietus is the commonly abundant Aedes vector species in Sri Lanka and the species originated in South East Asia[10].

The types of average infested boats for each species of larval vector mosquitoes reveals that both Aedes species were highly positive for both TB and EB than CB [Figure 3]. Generally, TB and EB have comparatively large surface area than CB and the results of Sarah et al.,[12] showed that large boats are heavily infested with Aedes species. Usually, CB is considered as traditional boat in Sri Lanka, and low number of CB are comparatively used for the fishery industry in Sri Lanka[11].

Average infested boats by Cx. quinquefaseiatus was significantly low for all types of boats [Figure 3]. The larvae of Culex species are common in breeding sites with polluted water[13]. The reason to observe low infestation of Culex species in study sites might be that they have more suitable oviposition habitats in coast with rich polluted water other than abandoned boats. Although, there are lot of abandoned boats in urban than rural areas in Galle, several cleaning programmes are continuously been conducted in urban areas in the district and which might be one of the reasons to have low infestation of boats by Culex species. Hence there may be less preference to breed in boats by Cx. quinquefaseiatus, though their typical breeding sites are abundant in the coast. However, the infestation percentages for Cx. quinquefaseiatus are slightly high in CB in all studied years [Table 2]. Generally, the curved bottom shape of CB supports to fill deep water layer inside than other boat types and Kabirul et al. [13] mentioned that Culex species prefers high density water, while Aedes species select shallow water depth for oviposition.

The monsoonal variation of boats infestation by Ae. albopietus indicates it was increasing from May to September [Figure 4]. In the present study the number of boats infested by Ae. albopietus is positively correlated with rainfall from 2011–2019. It is well known that mosquito larval abundance increases in rainy season. Mainly, the South-west monsoon rainy season is active from May to September and it brings between 1000mm to 1600mm of rain to the South-western coastal belt during these five months (Meteorology Department, 2019). Besides, the number of average infested boats in the study increased by the year [Figure 4] and the reason might be increased suitable outdoor breeding sites with rapid urbanization in the coast. The highest peak of infested boats is seen in September and it has closely followed a similar pattern across all studied years. Although high precipitation is reported at the beginning of South-west monsoon season, greater proportion of boats infested by larval Ae. albopietus are observed at the end of the monsoon period. This pattern may be a result of washing out existing breeding habitats of vector mosquitoes by heavy rainfall with the commencing of monsoon season. It is proofed by Pathirana et al.,[14], that vector abundance is low during heavy rainfall but increases when the rainfall started to decrease.

We studied the monsoonal variation of Ae. albopietus infected boats because Ae. aegypti prefers indoor habitats, hence, it is less affected by rainfall than Ae. albopietus that have outdoor larval habitats[15]. Significantly, our study showed that Ae. aegypti first invaded boats after 2018. The results of two previous studies Heenatigala et al.,[9] and Seneviratne[10] support that Ae. aegypti has not been recorded from boats in coastal zone, Galle before 2018. From zero to the present, number of infested boats by Ae. aegypti have expanded quickly. It may have happened as a result of rapid and extensive urbanization and by the higher fecundity and shorter life cycle of Ae. aegypti[16].

In the present study, the number of positive boats for Ae. aegypti are increased with the notable deduction of infested boats by Ae. albopietus after 2018 [Figure 5]. The increasing percentage of positive boats for Ae. aegypti was high than the decreasing percentage of positive boats for Ae. albopietus from 2017–2019. The coexistence of these two Aedes species has been explained by several studies in other countries with the survey of Ae. aegypti invasion. In 2016, Guo et al.,[17], reported that, Ae. aegypti is the superior competitor since its invasion when sharing the same breeding containers with Ae. albopietus. Two past studies, Gilotra et al.,[18] and Tandon et al.,[19] implied that the invasion of Ae. aegypti had an effect on the populations of Ae. albopietus. Further, studies of Rudnick et al.,[20] and Gilotra et al.,[18] mentioned Ae. aegypti had an overall competitive advantage over Ae. albopietus in urban habitats. As such, Ae. Aegypti is becoming fast as the more dominant vector mosquito in urban areas of Galle District, Sri Lanka than Ae. albopietus and that may cause more epidemiological implications in the region in the near future.
Figure 5: Average number of different types of abandoned boats infested by Aedes vector mosquitos.

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  Conclusion Top


Engine boats are the dominant type of abandoned boat in coastal zone, Galle, Sri Lanka. Aedes aegypti, Ae. albopietus and Cx, quinquefaseiatus breed in all kind of boats studied in the coast. Troller boats showed the highest percentage positive composition for vector mosquitoes. Aedes albopietus is the dominant vector mosquito found in all types of abandoned boats. Culex quinquefasciatus shows comparatively low infestation in all types of boats. The pattern of number of boats infested by Ae. albopietus increases with the South-west monsoon season and hence, rainy season has high risk to spread vector mosquitoes in the coast. There is an emerging trend to rapidly invade outdoor habitats such as abandoned boats by Aedes aegypti since 2017. With the establishing of Ae. aegypti, the positive abandoned boats for Aedes albopictus declined notably due to species competition. The study results emphasized that abandoned boats positively contribute to spread of Aedes and Culex vector mosquitos in the coastal zone. Therefore, the current knowledge would be useful to design appropriate control measures in order to prevent vector borne diseases in the coastal belt.

Conflict of interest: None


  Acknowledgements Top


We would like to thank all field supporters for their kind help to collect various information for the study.

Abbreviations:

TB; Toller boats

EB; Engine boats

CB; Canoe boats

MA; Municipal area





 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2]



 

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