Anopheles mosquitoes, which transmit the malaria parasite by their bite are present almost throughout the world. Only five areas are exceptions: Antarctic and Iceland, where there are no mosquitoes at all, New-Caledonia, the Central Pacific islands, like French Polynesia, and the Seychelles in the Indian Ocean.
A favourable environment
The case of the Seychelles has been troubling researchers for several decades. The archipelago appears to have all the factors appropriate for anophelines to be present. And most of the other islands in the same area, like Madagascar, the Comores or La Réunion, are affected by malaria. The prevailing conditions are indeed favourable for its vectors. The climate is tropical oceanic, with temperatures of between 24°C in July-August and 30°C in April-May. Moreover, considering the history of the Seychelles, endemic anophelines could have lived there in geological time. The granitic islands in the north of the archipelago were formerly attached to the Gondwana supercontinent, before plate tectonics separated them from the rest of Africa and the Indian Peninsula 65 million years ago.
This absence is even more puzzling in that the Seychelles are known worldwide as a biodiversity hotspot, with a high rate of endemism.
No mammals, no anophelines
An IRD scientist and his research partners1 recently lifted some of the mystery. They put their finger on the common point between some of the areas of the world spared by the disease. That is the absence of native mammals, apart from bats. The scientists conclude that the presence of Anopheles depends directly on that of terrestrial mammals. For the first time, they showed that the mosquitoes feed exclusively on mammalian blood. If there are no terrestrial mammals, the malaria vector does not turn to other substitute sources of nutrition such as birds, reptiles or bats. This new observation could prove important for the control of malaria transmission.
Some of the mystery remains
However, at the end of the 18th Century, livestock, dogs, cats, rats, mice and other mammals arrived in the Seychelles with humans. Since then, anophelines could have conquered the archipelago, like some other mosquitoes of the genus Aedes2. Nevertheless, out of 6 islands thoroughly searched in December 2008, the researchers did not find a single larva or adult individual of Anopheles, including in the islands of Mahé, Praslin and La Digue, where 99 % of the Seychelles population live. In spite of sustained high volumes of air and maritime traffic generated by intense tourism, with over 150 000 tourists per year, the mosquito has not been imported. However, as the researchers emphasize, international flights and cruise ships arrive essentially from Europe or the Middle East, where malaria has been eradicated. Furthermore, the Seychelles authorities have set up draconian prevention measures, including systematic spraying with insecticide inside every aircraft cabin and inspection of each vessel coming from overseas.
In historic time, anophelines have been introduced into the Seychelles. In 1908 Anopheles gambiae was brought on to the coralline atoll Aldabra, probably on a ship from Madagascar. However, the species disappeared spontaneously. A second epidemic arose in 1930, also on Aldabra. The last case was reported in 1931. Since then, the Seychelles have been completely free of the disease.
The difficulty anophelines have in surviving in the Seychelles appear be due to the long dry season on the coralline atolls, in the west of the archipelago, and to the steep topography of the granitic islands in the north-east. Both these factors prevent the anophelines from reproducing, as the larvae need patches of stagnant water for their development. But in Madagascar, as this study observes, in sites where the topography, altitude and ecology are similar, anophelines are present. Could the Seychelles' exemption be only provisional? The researchers conclude that the authorities must remain extremely vigilant to keep their archipelago out of reach of the world pandemic.
Malaria is the leading human parasitic disease in both its frequency and its gravity. On the African continent above all, it is one of the first causes of child mortality. As well as its public health benefits, the absence of the disease in the Seychelles, the unique case in Africa, is a boon for tourism, one of the country's prime economic activities, and the largest employment sector in the archipelago.
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