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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 Vallée de Mai, Praslin Island. Note the flora, composed largely of palm trees. (Photo V. Robert/IRD).
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Figure 3: Photo of Vallée de Mai, Praslin Island. Note the flora, composed largely of palm trees. (Photo V. Robert/IRD).

Mentions: The archipelago can be divided into two distinct types of islands, based on their geological origin, granitic and coralline. The granitic islands are located in the northeastern part of the Seychelles, about 1,000 km northeast of Madagascar and 1,500 km east of Kenya, and comprise three large islands (Mahé, Praslin and La Digue) on which 99% of the human population lives. The granitic portions of the Seychelles are the only mid-oceanic granitic islands in the world and are remnants of the break-up of ancient Gondwana supercontinent associated with the splitting of Indo-Madagascar [8,9]. These granitic islands have been separated from other emerged land for c.75 million years. Given their elevation, in geological time these islands were never totally submerged associated with rising sea-levels, although in some cases modern separate landmasses, which rest on a submarine microcontinental plateau, would have been connected during periods of lower sea-level (Figure 2). Mahé has the highest peak in the archipelago, Morne Seychellois at 930 m. This mountain mass holds extensive humid tropical forests dominated by exotic plants, but with relict largely primary forests above 600 m. The Vallée de Mai on Praslin has native forest, composed by a variety of endemic palm trees (Figure 3).


Why are anopheline mosquitoes not present in the Seychelles?

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

Photo of Vallée de Mai, Praslin Island. Note the flora, composed largely of palm trees. (Photo V. Robert/IRD).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 3: Photo of Vallée de Mai, Praslin Island. Note the flora, composed largely of palm trees. (Photo V. Robert/IRD).
Mentions: The archipelago can be divided into two distinct types of islands, based on their geological origin, granitic and coralline. The granitic islands are located in the northeastern part of the Seychelles, about 1,000 km northeast of Madagascar and 1,500 km east of Kenya, and comprise three large islands (Mahé, Praslin and La Digue) on which 99% of the human population lives. The granitic portions of the Seychelles are the only mid-oceanic granitic islands in the world and are remnants of the break-up of ancient Gondwana supercontinent associated with the splitting of Indo-Madagascar [8,9]. These granitic islands have been separated from other emerged land for c.75 million years. Given their elevation, in geological time these islands were never totally submerged associated with rising sea-levels, although in some cases modern separate landmasses, which rest on a submarine microcontinental plateau, would have been connected during periods of lower sea-level (Figure 2). Mahé has the highest peak in the archipelago, Morne Seychellois at 930 m. This mountain mass holds extensive humid tropical forests dominated by exotic plants, but with relict largely primary forests above 600 m. The Vallée de Mai on Praslin has native forest, composed by a variety of endemic palm trees (Figure 3).

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