Spatial distribution of Culicoides imicola, the main vector of bluetongue virus, in Spain

BLUETONGUE (BT) is a disease caused by a virus of the genus Orbivirus that is mainly transmitted among vertebrate hosts by several species of biting midges of the genus Culicoides (Diptera: Ceratopogonidae) (Du Toit 1944, Mellor and Pitzolis 1979). BT virus (BTV) can replicate in all species of ruminants and causes severe disease in certain breeds of sheep and some species of deer (Taylor 1986, MacLachlan 1994). In Europe, the main vector for BTV is Culicoides imicola Kieffer (Mellor 1996), although species included within the Culicoides obsoletus group have been proposed as the probable vector of BTV in some European areas (Mellor and Wittmann 2002, Savini and others 2003, De Liberato and others 2005). Historically, BTV has made only brief, periodic incursions into Europe (Manso-Ribeiro and others 1957, Mellor and Pitzolis 1979, Jennings and others 1983). However, a large BT epidemic, which started in 1998, has affected many European countries, including several that had not previously experienced the virus, and is still ongoing in some areas. This extension of BTV into Europe appears to be partially linked to the recent northward expansion of C imicola across the Mediterranean European countries – an expansion that seems to be a consequence of recent changes in the European climate (Purse and others 2005). In Spain, during the last BT epidemic, BTV entered the Balearic Islands from eastern Europe in 2003. In late 2004, further incursions occurred from Morocco into south-west Spain, and in late 2005 BTV was spreading northwards in peninsular Spain. Previous studies suggested that the distribution of C imicola in Spain was mainly restricted to southwest peninsular Spain, and the Toledo province was thought to be the northernmost site for the species (Ortega and Holbroock 1994, Ortega and others 1997, 1998, Rawlings and others 1997). In the past five years, however, local surveys have reported, for the first time, the presence of C imicola in the Balearic Islands (Miranda and others 2003), in coastal areas of Catalonia (Sarto i Monteys and others 2003), in Madrid and in the eastern coastal provinces of Alicante and Murcia (Martinez 2004; F. Collantes, personal communication). A national surveillance programme aimed at estimating the spatial distribution of C imicola in Spain was carried out during 2004. The landmass of peninsular Spain was divided into 202 quadrats of 50 x 50 km; 100 evenly distributed quadrats were selected as sampling units, and within each sampling unit at least two livestock holdings or farms were sampled. In addition, seven farms were sampled in the Balearic Islands (three on Majorca, three on Minorca and one on Ibiza), five in the Canary Islands (two each on Tenerife and Fuerteventura and one on Gran Canaria), and one in the town of Ceuta, a Spanish town in northern Africa. Two samplings were carried out: one in spring/summer (April to June) and another in summer/autumn (September to November). Each farm was sampled for two consecutive nights. A 4 W UV light trap fitted with a suction fan (Miniature Blacklight model 1212; John Hock) was placed outside each selected farm but within 30 m of livestock, at between 1·7 and 2·5 m above ground level. The traps were lit from just before dusk until after dawn over both sampling days. Specimens of C imicola were identified on the basis of their wing pattern, as described by Rawlings (1996). In approximately one-quarter of the sampling units more than two farms were sampled; most extra samplings occurred during the summer/autumn period. In total, 364 farms were sampled; 229 during spring/summer and 292 in summer/ autumn; 159 of the farms were sampled in both periods. A total of 12,878 C imicola were trapped over the course of both sampling periods. The species was found on all the farms sampled in the Balearic Islands, with the exception of the one on Ibiza; however, it is known that C imicola is present on Ibiza, albeit in very low numbers (B. V. Purse, M. A. Miranda, personal communication). No C imicola were captured on the farm sampled in Ceuta, and the species was not detected in the Canary Islands. C imicola was found in all the provinces of peninsular Spain where it had previously been reported (Fig 1) except Sevilla, where it was not detected on any of the farms sampled. In addition, C imicola was found for the first time in nine other provinces, mainly located to the north and east of the previously defined distribution area of the species. The highest numbers of C imicola trapped in one sampling session of two consecutive nights in provinces where the species had previously been reported were in Malaga (6304 specimens), Huelva (973 specimens) and Jaen (348 specimens). Among the nine provinces where C imicola was recorded for the first time, the highest catches were in Albacete (480 specimens), Avila (194 specimens) and Granada (66 specimens). All of the sampling sessions with the highest catches occurred during the summer/autumn sampling period. The altitudes of the sampled farms ranged from 5 to 1400 m above sea level. The highest altitude at which C imicola was Short Communications