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Why are anopheline mosquitoes not present in the Seychelles?

Robert V, Rocamora G, Julienne S, Goodman SM - Malar. J. (2011)

Bottom Line: No evidence of larval or adult anophelines was found at the surveyed sites, which supports their absence in the Seychelles.In the southwestern atolls (Aldabra and Providence-Farquhar groups), the presence of a long dry season of up to nine months and the total absence of permanent natural freshwater prevents the breeding of anophelines and their successful colonization.The Seychelles does not have any native land mammals and like in other parts of the world (Antarctica, Iceland, New Caledonia, Central Pacific islands) their absence is associated with the lack of anophelines.

View Article: PubMed Central - HTML - PubMed

Affiliation: MIVEGEC Maladies infectieuses et Vecteurs: Ecologie, Génétique, Evolution et Contrôle (IRD 224, CNRS 5290, Université Montpellier 1), Centre IRD France-Sud, BP 64501, 911 Avenue Agropolis, 34394 Montpellier cedex 5, France. vincent.robert@ird.fr

ABSTRACT

Background: Species of anopheline mosquitoes are largely distributed over emerged lands around the world and, within the tropics, few areas are without these insects, which are vectors of malaria parasites. Among the exceptions is the Seychelles archipelago in the western Indian Ocean. However, in the Aldabra island group, located in the extreme western portion of the archipelago, Anopheles gambiae s.l. was introduced, leading to massive proliferation and then elimination, with the most recent autochthonous malaria cases recorded in 1931.

Methods: In order to re-examine the absence of anopheline mosquitoes in the Seychelles, an entomological field survey was conducted in December 2008 at 17 sites on four granitic islands, including Mahé and Praslin, and ten sites on coralline atolls in the extreme west, including Aldabra.

Results: No evidence of larval or adult anophelines was found at the surveyed sites, which supports their absence in the Seychelles.

Conclusions: In the granitic islands of the Seychelles, the climate is favourable for anophelines. However, these islands are protected by their remoteness and prevailing seasonal winds. In addition, stagnant freshwater, required in anopheline larval development, is relatively uncommon on the granitic islands because of the steep slopes. In the southwestern atolls (Aldabra and Providence-Farquhar groups), the presence of a long dry season of up to nine months and the total absence of permanent natural freshwater prevents the breeding of anophelines and their successful colonization. The Seychelles does not have any native land mammals and like in other parts of the world (Antarctica, Iceland, New Caledonia, Central Pacific islands) their absence is associated with the lack of anophelines. This suggests an obligatory relationship for anophelines to feed on terrestrial mammals, without alternative for blood-feeding sources, such as bats, birds and reptiles.

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Photo of Aldabra Atoll, Island of Grande Terre. Note the low elevation of the coralline island rising just above sea level. The local peak is at 18 m a.s.l. (a sand dune) and the average altitude is between 2 and 3 m. (Photo V. Robert/IRD).
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Figure 4: Photo of Aldabra Atoll, Island of Grande Terre. Note the low elevation of the coralline island rising just above sea level. The local peak is at 18 m a.s.l. (a sand dune) and the average altitude is between 2 and 3 m. (Photo V. Robert/IRD).

Mentions: Coralline islands are situated beyond the Seychelles Plateau and grouped into several archipelagos, the Amirantes group, the Farquhar group and the Aldabra group. They present calcareous substratum on volcanic bedrock and have a maximum elevation above sea-level of a few meters. Consequently, they were totally submerged during marine transgressions, periods that would have eliminated the terrestrial biota. For instance, in recent geological history, Aldabra was completely underwater on at least two occasions, with the most recent complete submersion occurring c.125,000 years ago [10-12]. Aldabra is a huge atoll (155 km2 of emerged land) of 34 km long, 14.5 km wide; it consists in raised coral of up to 8 m above sea level (Figure 4), and comprises 4 main islands enclosing a central lagoon of 224 km2 [13]. The greatest colony in the world of giant tortoises (Testudo gigantea) is found at Aldabra, with up to 150,000 individuals. The avifauna is very diversified for both landbirds and seabirds, the latter being most abundant in the mangroves and islets around the lagoon [14].


Why are anopheline mosquitoes not present in the Seychelles?

Robert V, Rocamora G, Julienne S, Goodman SM - Malar. J. (2011)

Photo of Aldabra Atoll, Island of Grande Terre. Note the low elevation of the coralline island rising just above sea level. The local peak is at 18 m a.s.l. (a sand dune) and the average altitude is between 2 and 3 m. (Photo V. Robert/IRD).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3045994&req=5

Figure 4: Photo of Aldabra Atoll, Island of Grande Terre. Note the low elevation of the coralline island rising just above sea level. The local peak is at 18 m a.s.l. (a sand dune) and the average altitude is between 2 and 3 m. (Photo V. Robert/IRD).
Mentions: Coralline islands are situated beyond the Seychelles Plateau and grouped into several archipelagos, the Amirantes group, the Farquhar group and the Aldabra group. They present calcareous substratum on volcanic bedrock and have a maximum elevation above sea-level of a few meters. Consequently, they were totally submerged during marine transgressions, periods that would have eliminated the terrestrial biota. For instance, in recent geological history, Aldabra was completely underwater on at least two occasions, with the most recent complete submersion occurring c.125,000 years ago [10-12]. Aldabra is a huge atoll (155 km2 of emerged land) of 34 km long, 14.5 km wide; it consists in raised coral of up to 8 m above sea level (Figure 4), and comprises 4 main islands enclosing a central lagoon of 224 km2 [13]. The greatest colony in the world of giant tortoises (Testudo gigantea) is found at Aldabra, with up to 150,000 individuals. The avifauna is very diversified for both landbirds and seabirds, the latter being most abundant in the mangroves and islets around the lagoon [14].

Bottom Line: No evidence of larval or adult anophelines was found at the surveyed sites, which supports their absence in the Seychelles.In the southwestern atolls (Aldabra and Providence-Farquhar groups), the presence of a long dry season of up to nine months and the total absence of permanent natural freshwater prevents the breeding of anophelines and their successful colonization.The Seychelles does not have any native land mammals and like in other parts of the world (Antarctica, Iceland, New Caledonia, Central Pacific islands) their absence is associated with the lack of anophelines.

View Article: PubMed Central - HTML - PubMed

Affiliation: MIVEGEC Maladies infectieuses et Vecteurs: Ecologie, Génétique, Evolution et Contrôle (IRD 224, CNRS 5290, Université Montpellier 1), Centre IRD France-Sud, BP 64501, 911 Avenue Agropolis, 34394 Montpellier cedex 5, France. vincent.robert@ird.fr

ABSTRACT

Background: Species of anopheline mosquitoes are largely distributed over emerged lands around the world and, within the tropics, few areas are without these insects, which are vectors of malaria parasites. Among the exceptions is the Seychelles archipelago in the western Indian Ocean. However, in the Aldabra island group, located in the extreme western portion of the archipelago, Anopheles gambiae s.l. was introduced, leading to massive proliferation and then elimination, with the most recent autochthonous malaria cases recorded in 1931.

Methods: In order to re-examine the absence of anopheline mosquitoes in the Seychelles, an entomological field survey was conducted in December 2008 at 17 sites on four granitic islands, including Mahé and Praslin, and ten sites on coralline atolls in the extreme west, including Aldabra.

Results: No evidence of larval or adult anophelines was found at the surveyed sites, which supports their absence in the Seychelles.

Conclusions: In the granitic islands of the Seychelles, the climate is favourable for anophelines. However, these islands are protected by their remoteness and prevailing seasonal winds. In addition, stagnant freshwater, required in anopheline larval development, is relatively uncommon on the granitic islands because of the steep slopes. In the southwestern atolls (Aldabra and Providence-Farquhar groups), the presence of a long dry season of up to nine months and the total absence of permanent natural freshwater prevents the breeding of anophelines and their successful colonization. The Seychelles does not have any native land mammals and like in other parts of the world (Antarctica, Iceland, New Caledonia, Central Pacific islands) their absence is associated with the lack of anophelines. This suggests an obligatory relationship for anophelines to feed on terrestrial mammals, without alternative for blood-feeding sources, such as bats, birds and reptiles.

Show MeSH
Related in: MedlinePlus