Identification and characterization of larval and adult anopheline mosquito habitats in the Republic of Korea: potential use of remotely sensed data to estimate mosquito distributions

Abstract

Background: Plasmodium vivax malaria reemerged in the Republic of Korea in 1993, with more than 2,000 cases reported in the northwestern part of the country over the last 10 years. To better assess the risk of malaria transmission we conducted a surveillance study to identify and characterize the habitats that produce potential Anopheles vector mosquitoes. Immature and adult mosquito collection data were incorporated into a Geographic Information System (GIS) along with remotely sensed satellite imagery, and imagery classified to land use to determine if remote sensing data could be used to estimate mosquito habitats.

Results: More than 2,100 anopheline larvae were collected and mapped from 186 immature habitats, which were categorized into 9 types. Anopheles sinensis was the most commonly collected species, representing more than 97% of the specimens, followed by Anopheles pullus 1.0%, Anopheles lesteri 1.0%, and Anopheles sineroides 0.8%. Anopheles sinensis, pullus, and lesteri were found most frequently in rice paddies followed by: ditches, flooded areas, ground pools, wheel tracks, swamps, irrigation canals, and stream margins. Anopheles sineroides was found most commonly in flooded areas. Supervised classification was conducted on a LANDSAT 7 ETM+ image to identify 5-6 land-use classes that were considered to be of interest. The spatial distribution of the larval mosquito collections was overlaid on the land-use image derived from the LANDSAT image, and the number of mosquitoes habitats in each class calculated. On Ganghwa Island 67% of the immature habitats containing Anopheles sinensis were in the Paddy land-use class, although the class only represented 17% of the land area. In Paju District 53% of the immature habitats containing Anopheles sinensis were in the Paddy land-use class which represented only 9 % of the area of the district. There was significant (p < 0.05) correlation between habitats containing all four species and land use on Ganghwa Island; however, only Anopheles sinensis was significantly correlated with land-use in Paju District. In Ganghwa Island almost 50% of the Anopheles sinensis adults were collected in the paddy land-use class, and there was a positive correlation between larval and adult distributions. However, in Paju District adult Anopheles sinensis adults were most commonly collected in the Bare land-use class and only 17% collected in the Paddy class. There was a negative correlation between larval and adult distributions.

Conclusion: Immature habitats most commonly associated with Anopheles sinensis, Anopheles pullus, Anopheles sineroides and Anopheles lesteri were flooded rice paddies. Adult Anopheles sinensis, the most commonly collected anopheline, were found closely associated with rice habitats on Ganghwa Island but not in Paju District. Both QuickBird and LANDSAT satellite data were used to display spatial data in the form of geographic coverage and descriptive information in the form of relational databases associated with the mapped features. Supervised classification of LANDSAT imagery permitted good separation between Paddy, Forest, and Water land-use classes. The immature collections of Anopheles sinensis were significantly correlated with land-use as determined in the land-use classification in both Ganghwa Island and Paju District. These data suggest that classified remotely sensed data could potentially be used to estimate the distribution of immature and adult mosquito populations in the Republic of Korea.

Figure 2

Adult females collected in light traps. Thematic map shows total numbers of adult anophelines collected in light traps during 32 nights during August-September 2003 overlaid on LANDSAT 7 satellite data (dated 10 September 2002, spatial resolution 30 × 30 meters) displayed in true color (band combination 3,2,1).

Figure 1

Thematic map showing numbers of anopheline larvae collected at various habitats; Shown are Anopheles sinensis in A, Anopheles pullus in B, Anopheles sineroides in C, and Anopheles lesteri in D, surveyed during August-September 2003, and LANDSAT 7 satellite data (dated 10 September 2002, spatial resolution 30 × 30 meters) displayed in true color (band combination 3, 2,1) in E.

Figure 3

Locations of larval habitats of North Ganghwa Island. Collection sites are displayed on a true color of QuickBird multispectral satellite image (resolution 1 × 1 m.) overlaid on the Digital Elevation Model to depict 3-dimensions. Collection sites are defined by various shades of green which depict a range of elevations in meters above mean sea level (pale yellow = 0–5 meters, lime green 5–10 meters, florescent green 11–15 meters, and dark green >15 meters. Colored areas represent habitats that contained anopheline larvae during 2–14 August 2003.

Figure 4

Land-use/land-cover classification of Ganghwa Island. Image was acquired on 10 September 2002. Rice paddy, forest, urban, bare land, seawater, and inland water land-use are depicted by yellow, green, red, gray, light blue, and dark blue, respectively.

Figure 5

Land-use/land-cover classification of Paju District. Image was acquired on 10 September 2002. Rice paddy, forest, urban, bare land, and water are depicted by yellow, green, red, gray, and blue, respectively.