Seroprevalence of Schmallenberg virus antibodies among dairy cattle, the Netherlands, winter 2011–2012

Infections with Schmallenberg virus (SBV) are associated with congenital malformations in ruminants. Because reporting of suspected cases only could underestimate the true rate of infection, we conducted a seroprevalence study in the Netherlands to detect past exposure to SBV among dairy cattle. A total of 1,123 serum samples collected from cattle during November 2011–January 2012 were tested for antibodies against SBV by using a virus neutralization test; seroprevalence was 72.5%. Seroprevalence was significantly higher in the central-eastern part of the Netherlands than in the northern and southern regions (p<0.001). In addition, high (70%–100%) within-herd seroprevalence was observed in 2 SBV-infected dairy herds and 2 SBV-infected sheep herds. No significant differences were found in age-specific prevalence of antibodies against SBV, which is an indication that SBV is newly arrived in the country. During the last 2 weeks of August and the first 2 weeks of September 2011, dozens of veterinary practitioners in the Netherlands reported to a monitoring help desk (GD Veekijker) that several dairy herds with cows showed a sudden decrease in milk production, watery diarrhea, and occasional fever. The affected animals recovered, and extensive bacteriologic, virologic, and parasitologic testing of feces and blood samples of sick cows did not reveal an infectious cause for the clinical signs. Similar problems were reported at about the same time in Germany, and on November 18, 2011, the Friedrich Loeffler Institute (Greifswald, Germany) reported the detection of a novel orthobunyavirus that could be the cause of the clinical problems. Real-time reverse transcription PCR (RT-PCR), made available by the Friedrich Loeffler Institute, was used to test stored blood samples (N = 50) from the clinically diseased cattle in the Netherlands; 36% had positive test results. Since then, the virus has also been associated with congenital malformations in young animals (lambs, goat kids, and calves). The new virus is provisionally called Schmallenberg virus (SBV), or Shamonda-like virus. It is a RNA virus and shows 97% identity with Shamonda virus (SHAV) (small gene segment), 71% identity with Aino virus (medium gene segment), and 69% identity with Akabane virus (AKAV) (large gene segment). All these viruses are part of the Simbu serogroup of the family Bunyaviridae, genus Orthobunyavirus. The Simbu serogroup is composed of several arthropod-borne viruses (arboviruses, including SHAV, AKAV, and Aino virus) transmitted by Culicoides spp. biting midges and mosquitos. SHAV was initially isolated from cattle and Culicoides spp. biting midges in Nigeria in the 1960s. In 2002, SHAV emerged in Japan and was isolated from Culicoides spp. biting midges and sentinel cattle. Japan has been considered an area to which several Simbu group viruses have been endemic in cattle since the 1970s.

In the Netherlands, reporting of suspected cases of SBV infection in animals (occurrence of arthrogryposis hydranencephaly syndrome in calves, lambs, and goat kids) is obligatory; a report is followed by confirmatory testing of brain tissue samples by RT-PCR. However, the observed suspected cases are likely an underestimation of the true rate of infection; in addition, infected livestock may give birth to healthy young animals, adding to the underestimation of the true rate of infection. Therefore, serodiagnostic studies are needed to detect past exposure to SBV in ruminant populations in the affected countries. Within weeks after the start of the SBV epidemic, a virus neutralization test (VNT) was developed at the Central Veterinary Institute (CVI). This test made it possible to quickly execute a seroprevalence study of antibodies against SBV in dairy cattle in the Netherlands.

Emerging Infectious Diseases
May 29, 2012

Original web page at Emerging Infectious Diseases