PAPERS & ARTICLES_ Prediction of areas around the Mediterranean at risk of bluetongue by modelling the distribution of its vector using satellite 0 0 maging M. BAYLIS, P. S. MELLOR, E. J. WITTMANN, D. J. ROGERS Bluetongue is an infectious disease of ruminants caused by a virus transmitted by biting midges, one species of which, Culicoides imicola, is the major vector in the Old World. Following an epizootic of African horse sickness, a related disease, in Iberia and Morocco between 1987 and 1991, C imicola was trapped for two years at 44 sites in the affected region and models were developed for predicting the abundance of C imicola at these sites. Discriminant analysis was applied to identify the best model of three levels of abundance from 40 Fourier-processed remotely sensed variables and a digital elevation model. The best model correctly predicted the abundance level at 41 of the 44 sites. The single most important variable was the phase of the annual cycle of the normalised difference vegetation index. The model was used to predict the abundances of C imicola elsewhere around the Mediterranean and predicted high levels of abundance in many areas recently affected by bluetongue, including the Balearics, Sardinia, Sicily, eastern Greece, western Turkey, Tunisia and northern Algeria. The model suggests that eastern Spain, the island of Ibiza, the provinces of Lazio and Puglia in Italy, the Peloponnese and parts of northern Algeria and Libya may be at risk of bluetongue in 2001. Veterinary Record (2001) 149, 639-643 M. Baylis, BA, DPhil, Institute for Animal Health, Compton Laboratory, Compton, Newbury, Berkshire RG20 7NN P. S. Mellor, BSc, PhD, DSc, E. J. Wittmann, BSc, MSc, PhD, Institute for Animal Health, Pirbright Laboratory, Pirbright, Surrey GU24 ONF D. J. Rogers, BA, DPhil, Department of Zoology, University of Oxford, South Parks Road, Oxford OXI 3PS BLUETONGUE is an infectious disease of ruminants caused by the bluetongue viruses, of which there are 24 known serotypes (Mellor and others 2000). The viruses are found in the Americas, Africa, the Middle East, southern Asia and Australia, and there are occasional incursions into Europe. The first recorded incursion was into Spain and Portugal between 1956 and 1960 when serotype 10 caused the deaths of approximately 180,000 sheep (Mellor and Boorman 1995). Approximately 20 years later, in 1977, serotype 4 appeared in Cyprus, (Mellor and Pitzolis 1979) and in western Turkey from 1977 to 1981 (Jennings and others 1983), and spread to the Greek islands of Lesbos in 1979 and Rhodes in 1980 (Mellor and Boorman 1995). More recently there have been two independent incursions of several serotypes of the virus into Europe. In the first, serotype 9 was reported from several Greek islands in 1998 and from southern Bulgaria, northern and central Greece, and European Turkey in 1999, serotypes 4 and 16 were reported from Greece in 1999, and serotype 16 from Anatolian Turkey in 2000. In the second incursion, which began in December 1999, serotype 2 was initially reported from Tunisia and then, in 2000, from Tunisia, Algeria, sev- eral of the larger islands of the Mediterranean (Sicily, Sardinia, Corsica, Majorca and Menorca) and mainland Italy. The virus is transmitted between ruminants by biting midges of the genus Culicoides (Diptera: Ceratopogonidae) and, in the Old World, including the Mediterranean region, a single species has been implicated as the major vector (Mellor and others 2000). This species, Culicoides imicola, also transmits several other viruses which cause severe disease in livestock, including African horse sickness. Following an epi- zootic of African horse sickness in Iberia and Morocco between 1987 and 1991, C imicola was trapped at weekly intervals for two years at 49 sites across the region (Baylis and others 1997, Rawlings and others 1997). These studies revealed the close similarities between the temporal and spatial distributions of the vector and the disease during the epizootic, and confirmed C imicola as the major vector. Most of the areas affected by the recent outbreaks of blue- tongue (Bulgaria, mainland Greece, European Turkey, Tunisia, mainland Italy, Sicily, Sardinia, Corsica, Majorca and Menorca) have never previously reported any of the viruses known to be transmitted by C imicola. Furthermore, most of them have recorded the presence of C imicola only after the outbreaks of bluetongue in the past three years had alerted the authorities to the probable occurrence of the vector in the region. In some of the affected areas, surveys for Culicoides species had been conducted at earlier times but C imicola was not reported. These observations suggest that the distribution of C imicola may be expanding within Europe, possibly in response to global warming (Wittmann and Baylis 2000) and that new areas may be exposed to the diseases carried by this vector in the future. With this possibility in mind, the trapping data, together with several remotely sensed data sets, have been used to develop a model for predicting the abundance of C imicola in Iberia and Morocco. The predictions of the model have then been extrapolated to other areas of the Mediterranean region, to identify areas that may be suitable for C imicola and where it may be likely to occur in the near future. An attempt to val- idate the model has been made by comparing these predic- tions with the reported distribution of bluetongue and African horse sickness from 1950 to the present, and further areas that may be at risk of bluetongue or other diseases borne by C imicola have been identified. MATERIALS AND METHODS The abundances of C imicola at sites in Iberia and Morocco were obtained from published surveys undertaken in the 1990s and divided into three classes: zero to low, intermediate and high levels of abundance. Discriminant analysis was then used to identify the best multivariate model of the allocations to these classes from combinations of 41 remotely sensed vari- ables. The model was then used to predict the abundances of C imicola throughout the Mediterranean region. Abundances of C Imicola Following the 1987 to 1991 epizootic of African horse sick- ness in Iberia and Morocco, between 1993 and 1995, C imi- cola were trapped at weekly intervals in Pirbright-type light The Veterlnary Record, November 24, 2001 639 group.bmj.com on February 27, 2014 - Published by veterinaryrecord.bmj.com Downloaded from