Bluetongue disease in Germany (2007–2008): monitoring of entomological aspects

Abstract

In the summer of 2006, a bluetongue epidemic started in the border area of Belgium, The Netherlands, and Germany, spread within 2 years over large areas of Western and Central Europe, and caused substantial losses in farm ruminants. Especially sheep and cattle were severely affected, leading to a case–fatality ratio of nearly 40% in sheep (Conraths et al., Emerg Inf Dis 15(3):433–435, 2009). The German federal ministry of food, agriculture, and consumer protection (BMELV) established a countrywide monitoring on the occurrence of the vectors of this virus, i.e., midges (family Ceratopogonidae) of the genus Culicoides. The monitoring was done on 91 sites, most of which were localized in the 150-km restriction zone that existed in December 2006. A grid consisting of 45 × 45 km² cells was formed that covered the monitoring area. As a rule, one trap was placed into each grid cell. The monitoring program started at the end of March 2007 and lasted until May 2008. It included the catching of midges by ultraviolet light traps—done each month from days 1 until 8, the selection of midges of the Culicoides obsoletus, Culicoides pulicaris group, and other Culicoides spp., the testing of midges for bluetongue virus (BTV) by polymerase chain reaction (PCR), and the daily registration of weather data at each trap site for the whole monitoring period. The following main results were obtained: (1) Members of the C. obsoletus group were most commonly found in the traps, reaching often 3/4 of the catches. The African and South European vector of BTV—the species Culicoides imicola—was never found. (2) Members of the C. obsoletus group were most frequently found infected with BTV besides a few cases in the C. pulicaris group and other species. (3) Members of the C. obsoletus group were also found in winter. Their numbers were reduced, however, and they were caught mostly close to stables. Therefore, a true midge-free period does not exist during the year in Germany. (4) The amounts of midges caught daily depended on the weather conditions. If it was cold and/or windy, the traps contained only a few specimens. Since the months from January to May 2008 were considerably colder (at all farms) than their correspondents in 2007, the growing of the population of midges started 2–3 months later in 2008 than in 2007. (5) The highest populations of midges occurred in both years (2007 and 2008) during the months September and October. This corresponded significantly to the finding of highest numbers of infected midges and to the number of diseased cattle and sheep during these 2 months. (6) It is noteworthy that in general, the first virus-positive midges of the species C. obsoletus were found about 1 1/2 months later than the first clinical cases had occurred or later than the first PCR-proven virus-positive sentinel animals had been documented. In 2007, the first BTV-positive cattle were detected in May in North Rhine-Westphalia, while the first positive Culicoides specimens were only found in August on the same farm. Evaluating these main results of the entomological monitoring and the fact that many wild ruminants have also been infected with BTV, it becomes evident that bluetongue disease has become endemic in Central Europe, and that only constant effort including vaccination and perhaps also insecticidal protection of cattle and sheep will keep the economical losses at a reasonable level. The following papers (1–10) in this journal will contribute more details obtained from this worldwide unique entomological monitoring: Bartsch et al. 2009; Bauer et al. 2009; Stephan et al. 2009; Clausen et al. 2009; Hörbrand and Geier 2009; Kiehl et al. 2009; Mehlhorn et al. 2009; Kiel et al. 2009; Vorsprach et al. 2009; Balczun et al. 2009.