Schmallenberg Virus in Culicoides spp. Biting Midges, the Netherlands, 2011

Armin R.W. Elbers

, Rudy Meiswinkel, Erik van Weezep, Marianne M. Sloet van Oldruitenborgh-Oosterbaan, and Engbert A. Kooi

Author affiliations: Central Veterinary Institute, Lelystad, the Netherlands (A.R.W. Elbers, E. van Weezep, E.A. Kooi); Rocca di Cave, Rome, Italy (R. Meiswinkel); and Utrecht University, Utrecht, the Netherlands (M.M. Sloet van Oldruitenborgh-Oosterbaan)

Abstract

To determine which species of Culicoides biting midges carry Schmallenberg virus (SBV), we assayed midges collected in the Netherlands during autumn 2011. SBV RNA was found in C. scoticus, C. obsoletus sensu stricto, and C. chiopterus. The high proportion of infected midges might explain the rapid spread of SBV throughout Europe.

During early summer 2011, Schmallenberg virus (SBV), a novel orthobunyavirus of the Simbu serogroup, spread across much of northern Europe, infecting ruminant livestock. The Simbu serogroup (family Bunyaviridae, genus Bunyavirus) includes Shamonda virus, Akabane virus, Sathuperi virus, and Aino virus. These viruses cause teratologic effects in ruminants and are arthropod-borne, and most have been isolated in the Old World from mosquitoes and Culicoides spp. biting midges (1). Recent preliminary studies indicate that ≥1 species of Culicoides midges act as field vectors for SBV in Europe (2). To determine which Culicoides midge species harbor SBV, we analyzed midges collected from 3 livestock holdings in eastern and northeastern parts of the Netherlands.

The Study

Throughout September and early October 2011, Culicoides spp. biting midges were trapped almost daily at a dairy in the municipality of Ermelo (eastern Netherlands) by various methods, including the standard Onderstepoort-type blacklight trap. In addition, during several days in August and September 2011, Culicoides spp. biting midges were trapped near sheep flocks in the municipalities of Bilthoven (central Netherlands) and Midden-Drenthe (northeastern Netherlands) by using the Onderstepoort-type trap and a drop-tent cage. Captured midges were stored in 70% ethanol.
Female midges were categorized as nulliparous, parous, gravid, or freshly blood fed (engorged) (3); only midges belonging to the first 3 categories were assayed. The 6,100 selected midges were divided into 610 species-specific pools, 10 midges per pool. Under a dissecting microscope, the heads were separated from abdomens by use of a scalpel; 10 heads were then pooled and assayed for SBV, whereas the corresponding abdomens (also pooled) were stored in 70% ethanol.
All midges were identified morphologically, but because female C. obsoletus sensu stricto midges cannot be separated with confidence from C. scoticus midges, they were pooled and are referred to jointly as the C. obsoletus complex. The number of pools assayed for each species was as follows: C. obsoletus complex (230), C. chiopterus (144), C. dewulfi (130), C. punctatus (105), and C. pulicaris (1). After assays were conducted, the species identity of each SBV-positive midge pool was established by using molecular techniques.
Only when a pool of 10 heads was found SBV positive was the corresponding pool of dissected abdomens retrieved and assayed. In this instance, the 10 abdomens were assayed singly, so that the individual abdomen that was SBV-positive could be identified molecularly, to establish exactly which of the 2 species of the C. obsoletus complex was involved and to confirm or refute the morphologic identifications that had been made for the remaining Culicoides species.