Why are anopheline mosquitoes not present in the Seychelles?

<p>Abstract</p> <p>Background</p> <p>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, <it>Anopheles gambiae s.l. </it>was introduced, leading to massive proliferation and then elimination, with the most recent autochthonous malaria cases recorded in 1931.</p> <p>Methods</p> <p>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.</p> <p>Results</p> <p>No evidence of larval or adult anophelines was found at the surveyed sites, which supports their absence in the Seychelles.</p> <p>Conclusions</p> <p>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.</p

Skewed distributions - new outcome measures

The traditional measure of caries, the DMF index, either as prevalence or incidence of disease, has become highly positively skewed among children and young adults. Most discussion of skewed distributions has focused on the properties of statistical analyses using such data or the implications for sample sizes and subject selection in clinical trials. This paper examines the full range of epidemiologic studies, their aims and constitutive interest in order to identify the measurement problems associated with skewed DMF index data. Constitutive interests include: description; documentation; explanation and prediction; evaluation; advocacy; and, experimentation. 'New' outcome measures that would assist in reaching the aims and constitutive interests of the epidemiology of caries include caries severity grading, variants of prevalence, extent and severity and their combination into case definitions, and weighting of the components of the DMF index. Research questions for each area of 'new' outcome measures are identified as steps in the codifying of their use in the epidemiology of caries