Oral exposure, reinfection and cellular immunity to Schmallenberg virus in cattle
Introduction
In late 2011, Schmallenberg virus (SBV), an orthobunyavirus related to Akabane virus (AKAV), was discovered in Germany (Hoffmann et al., 2012). Adult cattle infected with AKAV generally do not develop apparent clinical signs. However, if they are pregnant, foetal infection during early and mid- to late gestation can lead to abortion or congenital arthrogryposis-hydranencephaly syndrome (George and Kirkland, 2004). SBV was first discovered in blood samples of adult cows with fever, reduced milk yield and diarrhoea (Hoffmann et al., 2012), but has since been implicated in many cases of severely malformed bovine and ovine offspring (Beer et al., 2012, Dominguez et al., 2012, Garigliany et al., 2012b, Herder et al., 2012).
In a first animal experiment, three heifers were inoculated with blood samples from infected adult cows or virus amplified by one passage in KC cells, derived from Culicoides variipennis midges, L1062, Collection of Cell Lines in veterinary Medicine (CCLV), Insel Riems. All animals became PCR-positive, one developed fever and another came down with diarrhoea (Hoffmann et al., 2012). The further experiment presented here was conceived to address several pressing questions: the possibility of oral infection, immunity in convalescent animals and the cellular immune status after natural infection.
Section snippets
Virus
SBV was isolated from the blood of a cow as previously described (Hoffmann et al., 2012). After initial isolation on KC cells the virus was passaged once in baby hamster kidney (BHK) cells (CCLV L0164) and again in KC cells.
Animals and experimental design
The experimental protocol was reviewed by a state ethics commission and has been approved by the competent authority (State Office for Agriculture, Food Safety and Fisheries of Mecklenburg-Vorpommern, Rostock, Germany, ref. LALLF M-V TSD/7221.3-1.1-004/12).
Two
Results
In three out of five subcutaneously inoculated animals (C05, C06, C07) SBV genome was first detectable 2 days after infection (dpi). C04 scored positive in the RT-qPCR on day 3 for the first time. SBV genome remained detectable until dpi 5 (C07) or 6 (C04, C05, C06). Compared to C04–C08 the first detection of viral RNA in C08 was delayed by 3 days; serum and whole blood samples were positive in the RT-qPCR on days 5 to 8 and 6 to 8, respectively (Fig. 1). No remarkable differences between serum
Discussion
In late 2011, SBV was identified in blood samples of adult cows with fever, reduced milk yield and diarrhoea in Germany. Thereafter the virus was detected in several other European countries (Anonymous, 2012a, Beer et al., 2012). It has again been circulating in the spring and summer of 2012 (Anonymous, 2012b, ProMED-mail, 2012b, ProMED-mail, 2012c). Acute cases in adult cattle have been reported from regions which had remained free during the previous season (ProMED-mail, 2012a, ProMED-mail,
Conclusions
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Immunity to re-infection with SBV persists for at least two months.
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Viral RNA is detectable for less than a week after infection.
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Oral instillation of SBV does not lead to infection, but viral RNA was detected in faecal, oral and nasal swabs taken from subcutaneously infected animals.
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SBV does not replicate in bovine PBL but influences the lymphocyte homeostasis in blood.
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Viral RNA persists in the lymphoreticular system for at least 5 weeks after infection.
Conflict of interest statement
The authors have not any financial, personal, or professional interests that could be construed to have influenced this paper.
Acknowledgements
Patrick Zitzow, Bianka Hillmann, Stefanie Knöfel and Christian Korthase provided excellent technical assistance. The dedicated animal care by the staff of the isolation unit of the FLI is gratefully acknowledged.
This study was supported by the Germany Federal Ministry of Food, Agriculture and consumer protection and the European Union as outlined in Council Decision 2012/349/EU concerning a financial contribution by the Union for studies on Schmallenberg virus.
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Present address: Faculty of Veterinary Medicine, University of Calgary, Canada.
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Present address: Institute of Veterinary Pathology, Department of Veterinary Medicine, Freie Universität Berlin, Oertzenweg 19b, 14163 Berlin, Germany.