Feeding behaviour of potential vectors of West Nile virus in Senegal

Fall et al. Parasites & Vectors 2011, 4:99 http://www.parasitesandvectors.com/content/4/1/99 RESEARCH Open Access Feeding behaviour of potential vectors of West Nile virus in Senegal Assane G Fall1*, Amadou Diaïté1, Renaud Lancelot2, Annelise Tran3,4, Valérie Soti3,4, Eric Etter4,6, Lassana Konaté5, Ousmane Faye5 and Jérémy Bouyer1,2 Abstract Background: West Nile virus (WNV) is a widespread pathogen maintained in an enzootic cycle between mosquitoes and birds with occasional spill-over into dead-end hosts such as horses and humans. Migratory birds are believed to play an important role in its dissemination from and to the Palaearctic area, as well as its local dispersion between wintering sites. The Djoudj Park, located in Senegal, is a major wintering site for birds migrating from Europe during the study period (Sept. 2008- Jan. 2009). In this work, we studied the seasonal feeding behaviour dynamics of the potential WNV mosquito vectors at the border of the Djoudj Park, using a reference trapping method (CDC light CO2-baited traps) and two host-specific methods (horse- and pigeon-baited traps). Blood meals of engorged females were analysed to determine their origin. Results: Results indicated that Culex tritaeniorhynchus and Cx. neavei may play a key role in the WNV transmission dynamics, the latter being the best candidate bridging-vector species between mammals and birds. Moreover, the attractiveness of pigeon- and horse-baited traps for Cx. neavei and Cx. tritaeniorhynchus varied with time. Finally, Cx. tritaeniorhynchus was only active when the night temperature was above 20°C, whereas Cx. neavei was active throughout the observation period. Conclusions: Cx. neavei and Cx. tritaeniorhynchus are the main candidate vectors for the transmission of WNV in the area. The changes in host attractiveness might be related to variable densities of the migratory birds during the trapping period. We discuss the importance of these results on the risk of WNV transmission in horses and humans. Background West Nile fever (WNF) is an arthropod-borne disease caused by a Flavivirus (Flaviviridae) belonging to the Japanese encephalitis antigenic complex [1]. Birds are involved in its pathosystem. Horses and humans are dead-end hosts: their infection often remains unapparent but they can suffer febrile or even fatal illness with neural symptoms [2]. The West Nile virus (WNV) is highly endemic in Africa in general, and particularly in Senegal [3-5]. Migratory birds may be involved in spreading the virus in Africa, Europe, the Middle East and south-western Asia, especially through the Palaearctic migration routes where major flyways are crossing each other [6]. Such transcontinental introduction must * Correspondence: 1 Institut Sénégalais de Recherches Agricoles/Laboratoire National de l’Elevage et de Recherches Vétérinaires BP 2057 Dakar-Hann, Sénégal Full list of author information is available at the end of the article be anchored to local (African) spreading mechanism especially at birds’ nesting, feeding, or resting sites where vector feeding behaviour probably plays a critical role [7]. The Senegal River delta (northern Senegal and southern Mauritania) is characterized by a mixture of natural wetlands and extensive irrigated agricultural activity. It is one of the major wintering sites for birds migrating between Europe and Africa that benefit from abundant food resources in the Djoudj National Park, where this study was conducted. The majority of the bird species migrating from Europe arrive there during the month of October and start their return flight during March/April [8]. A number of mosquito candidate vectors for the transmission of WNV have been identified in Senegal: Culex poicilipes, Cx. naevei, Mymomia spp., Mymomia hispida, M. lacustris, M. splendens, Aedomya africana [5], A. vexans and Mansonia uniformis [9]. All these © 2011 Fall et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Fall et al. Parasites & Vectors 2011, 4:99 http://www.parasitesandvectors.com/content/4/1/99 species have a nocturnal feeding behaviour. However, little is known regarding their host preferences, especially regarding species feeding both on birds and mammals. The main objective of this study was to assess their feeding behaviour and its seasonality during the period of high risk of transmission of WNV in the area [3], to identify potential factors increasing the transmission between birds, and more importantly, from birds to mammals. The study was conducted in Ross Bethio, a small town located 10 km south from the Djoudj National Park, Senegal (Figure 1). In 2005, a serological survey carried out on horses in this region highlighted high WN prevalence rates (0.85; n = 367; 95% CI 0.81-0.89) [10]. The landscape surrounding Ross-Bethio is characterized by grasslands, shrublands and dry saline flats ("tans”). This area is also one of the main agricultural centres in the Senegal River delta, rice and sugar cane representing the main crops. During the dry season (from November to May), green vegetation is limited to some scattered trees and shrubs. During the rainy season (June to October: mean annual rainfall ca. 250 mm), an herbaceous layer is available for domestic ruminants, donkeys and horses. The latter is used for ploughing, transporting goods and humans, and social distinction. The Senegal River valley, including the delta, is flooded each year either by rainfall water naturally collected in the upper Senegal River basin, or by planned water releases from the Manantali dam (upper Senegal River, Page 2 of 7 Mali) while retaining water with the Diama dam located near Ross Bethio. In Ross Bethio, the maximum flooding level is usually observed in early November. Results In total, 28,965 female mosquitoes, representing 12 species in 5 genera, were captured in all traps over 79 nights from September 2008 to January 2009. Mosquitoes from the Culex genus represented 95% (27,443) of the total captures. The predominant species were Cx. tritaeniorhynchus, Cx. neavei and Cx. poicilipes, representing 69.4% (20,109), 19.7% (5,702) and 5.4% (1,575) of the total captures respectively. CDC light CO2-baited traps were the most efficient with 20,382 female mosquitoes (apparent nightly density per trap - ANT of the total collection = 566.2) in 11 species belonging to 5 genera closely followed by the horse-baited trap with 7402 female mosquitoes (ANT of the total collection = 493.5) in 8 species and 4 genera. Pigeon-baited traps collected 1,181 (ANT of the total collection = 42.2) mosquitoes in 6 species and 2 genera (Table 1). The predominance of Cx. tritaeniorhynchus and Cx. neavei over the other species was also clearly showed on the t (...truncated)