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Dry season ecology of Anopheles gambiae complex mosquitoes in The Gambia.

Jawara M, Pinder M, Drakeley CJ, Nwakanma DC, Jallow E, Bogh C, Lindsay SW, Conway DJ - Malar. J. (2008)

Bottom Line: These two villages had the highest proportions of An. gambiae s.s. adults, and experienced the most substantial increase in proportions of An. gambiae s.s. after the onset of rains.During the dry season population minimum, An. melas was the predominant vector species, but differences among villages in availability of fresh-water breeding sites correlate with egg laying activity and relative numbers of An. gambiae s.s. adults, and with the increase in this species immediately after the beginning of the rains.Local variation in dry season vector persistence is thus likely to influence spatial heterogeneity of transmission intensity in the early part of the rainy season.

View Article: PubMed Central - HTML - PubMed

Affiliation: Medical Research Council Laboratories, Fajara, PO Box 273, Banjul, The Gambia. mjawara@mrc.gm

ABSTRACT

Background: Malaria in The Gambia is highly seasonal, with transmission occurring as Anopheles gambiae s.l. populations expand during and immediately after a single annual rainy season that lasts from June to October. There has been very limited investigation of the ecology of vectors during the dry season, when numbers are very limited and distributions may be restricted.

Methods: Weekly adult mosquito collections (pyrethrum spray, light trap, and search collections from rooms, as well as light trap collections from animal shelters, abandoned wells and grain stores), and artificial sentinel breeding site surveys were performed in four villages near the upper tidal and partially saline part of the Gambia River in the last four months of an annual dry season (March to June). Mosquito species were identified by morphological and DNA analysis, and ELISA assays were performed to test for Plasmodium falciparum sporozoites and human blood meal components.

Results: Adults of An. gambiae s.l. were collected throughout the period, numbers increasing towards the end of the dry season when humidity was increasing. Adult collections were dominated by An. melas (86%), with An. gambiae s.s. (10%) and An. arabiensis (3%) also present throughout. Most females collected in room search and spray collections contained blood meals, but most from light traps were unfed. None of the females tested (n = 1709) contained sporozoites. Larvae (mostly An. gambiae s.s.) were recovered from artificial sentinel breeding sites in the two villages that had freshwater pools. These two villages had the highest proportions of An. gambiae s.s. adults, and experienced the most substantial increase in proportions of An. gambiae s.s. after the onset of rains.

Conclusion: During the dry season population minimum, An. melas was the predominant vector species, but differences among villages in availability of fresh-water breeding sites correlate with egg laying activity and relative numbers of An. gambiae s.s. adults, and with the increase in this species immediately after the beginning of the rains. Local variation in dry season vector persistence is thus likely to influence spatial heterogeneity of transmission intensity in the early part of the rainy season.

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Map of the study area in the North Bank District of The Gambia, showing the four study villages located west of Farafenni town. The large water body at the bottom of the Figure is the Gambia River, and that at the top left surrounded by floodplains is the Bao Bolong tributary.
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Figure 1: Map of the study area in the North Bank District of The Gambia, showing the four study villages located west of Farafenni town. The large water body at the bottom of the Figure is the Gambia River, and that at the top left surrounded by floodplains is the Bao Bolong tributary.

Mentions: Mosquito sampling was conducted in the last 15 weeks of the dry season from March 13th to June 21st 2000 in four villages located west of Farafenni town in the North Bank Division of The Gambia (Figure 1). The last week of sampling followed the first rainfall (9.5 mm on June 13th) but was considered as a dry season sample of adult mosquitoes as these could not have emerged from rain-fed breeding sites (a minimum of 8 days is needed for development from eggs to adults). The villages were Yallal (population of 495 in 52 compounds), Alkali Kunda (abbreviated here as Alkali, population of 889 in 58 compounds), Jajari (population of 752 in 32 compounds), and Dai Mandinka (abbreviated here as Dai, population of 246 in 9 compounds). Two of the villages (Dai and Jajari) are less than 1 km from the Bao Bolong (a tributary of river Gambia with marshland along its banks that remain flooded for most of the dry season). The other two are over 4 km away from the Bao Bolong, and all are several kilometres away from the main River Gambia that is mostly bordered by Rhizophora sp. and Avicennia sp. mangrove in this tidal region approximately 100 km upstream from the river mouth. Data on maximum and minimum daily temperature and humidity were collected at the government meteorological station at Yallal village, which is approximately 3 km from Alkali, 6 km from Jajari and 10 km from Dai.


Dry season ecology of Anopheles gambiae complex mosquitoes in The Gambia.

Jawara M, Pinder M, Drakeley CJ, Nwakanma DC, Jallow E, Bogh C, Lindsay SW, Conway DJ - Malar. J. (2008)

Map of the study area in the North Bank District of The Gambia, showing the four study villages located west of Farafenni town. The large water body at the bottom of the Figure is the Gambia River, and that at the top left surrounded by floodplains is the Bao Bolong tributary.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2533673&req=5

Figure 1: Map of the study area in the North Bank District of The Gambia, showing the four study villages located west of Farafenni town. The large water body at the bottom of the Figure is the Gambia River, and that at the top left surrounded by floodplains is the Bao Bolong tributary.
Mentions: Mosquito sampling was conducted in the last 15 weeks of the dry season from March 13th to June 21st 2000 in four villages located west of Farafenni town in the North Bank Division of The Gambia (Figure 1). The last week of sampling followed the first rainfall (9.5 mm on June 13th) but was considered as a dry season sample of adult mosquitoes as these could not have emerged from rain-fed breeding sites (a minimum of 8 days is needed for development from eggs to adults). The villages were Yallal (population of 495 in 52 compounds), Alkali Kunda (abbreviated here as Alkali, population of 889 in 58 compounds), Jajari (population of 752 in 32 compounds), and Dai Mandinka (abbreviated here as Dai, population of 246 in 9 compounds). Two of the villages (Dai and Jajari) are less than 1 km from the Bao Bolong (a tributary of river Gambia with marshland along its banks that remain flooded for most of the dry season). The other two are over 4 km away from the Bao Bolong, and all are several kilometres away from the main River Gambia that is mostly bordered by Rhizophora sp. and Avicennia sp. mangrove in this tidal region approximately 100 km upstream from the river mouth. Data on maximum and minimum daily temperature and humidity were collected at the government meteorological station at Yallal village, which is approximately 3 km from Alkali, 6 km from Jajari and 10 km from Dai.

Bottom Line: These two villages had the highest proportions of An. gambiae s.s. adults, and experienced the most substantial increase in proportions of An. gambiae s.s. after the onset of rains.During the dry season population minimum, An. melas was the predominant vector species, but differences among villages in availability of fresh-water breeding sites correlate with egg laying activity and relative numbers of An. gambiae s.s. adults, and with the increase in this species immediately after the beginning of the rains.Local variation in dry season vector persistence is thus likely to influence spatial heterogeneity of transmission intensity in the early part of the rainy season.

View Article: PubMed Central - HTML - PubMed

Affiliation: Medical Research Council Laboratories, Fajara, PO Box 273, Banjul, The Gambia. mjawara@mrc.gm

ABSTRACT

Background: Malaria in The Gambia is highly seasonal, with transmission occurring as Anopheles gambiae s.l. populations expand during and immediately after a single annual rainy season that lasts from June to October. There has been very limited investigation of the ecology of vectors during the dry season, when numbers are very limited and distributions may be restricted.

Methods: Weekly adult mosquito collections (pyrethrum spray, light trap, and search collections from rooms, as well as light trap collections from animal shelters, abandoned wells and grain stores), and artificial sentinel breeding site surveys were performed in four villages near the upper tidal and partially saline part of the Gambia River in the last four months of an annual dry season (March to June). Mosquito species were identified by morphological and DNA analysis, and ELISA assays were performed to test for Plasmodium falciparum sporozoites and human blood meal components.

Results: Adults of An. gambiae s.l. were collected throughout the period, numbers increasing towards the end of the dry season when humidity was increasing. Adult collections were dominated by An. melas (86%), with An. gambiae s.s. (10%) and An. arabiensis (3%) also present throughout. Most females collected in room search and spray collections contained blood meals, but most from light traps were unfed. None of the females tested (n = 1709) contained sporozoites. Larvae (mostly An. gambiae s.s.) were recovered from artificial sentinel breeding sites in the two villages that had freshwater pools. These two villages had the highest proportions of An. gambiae s.s. adults, and experienced the most substantial increase in proportions of An. gambiae s.s. after the onset of rains.

Conclusion: During the dry season population minimum, An. melas was the predominant vector species, but differences among villages in availability of fresh-water breeding sites correlate with egg laying activity and relative numbers of An. gambiae s.s. adults, and with the increase in this species immediately after the beginning of the rains. Local variation in dry season vector persistence is thus likely to influence spatial heterogeneity of transmission intensity in the early part of the rainy season.

Show MeSH
Related in: MedlinePlus