Schmallenberg virus neutralising antibody responses in sheep

Abstract

BackgroundSchmallenberg virus (SBV) is a midge borne virus of cattle and sheep. Infection is typically asymptomatic in adult sheep but fetal infection during pregnancy can result in abortion, stillbirth, neurological disorders and malformations of variable severity in newborn animals. It was first identified in Germany and the Netherlands in 2011 and then circulated throughout Europe in 2012 and 2013. Circulation in subsequent years was low or non-existent until summer and autumn 2016, leading to an increased incidence of deformed newborn lambs and calves in 2016–17. This study reports SBV circulation in October 2016 within a group of 24 ewes and 13 rams. The ewes were monitored at 3 times points over an 11 week period (September to December 2016).ResultsMost ewes displayed an increase in SBV VNT with antibody titre increases greater in older, previously exposed ewes. Two ewes had SBV RNA detectable by RT-qPCR, one on 30/09/16 and one on 04/11/16. Of these ewes, one had detectable serum SBV RNA (indicating viraemia) despite pre-existing antibody. The rams had been previously vaccinated with a commercial inactivated SBV vaccine, they showed minimal neutralising antibody titres against SBV 8 months post-vaccination and all displayed increased titre in October 2016.ConclusionThis data suggests that SBV circulated for a minimum period of 5 weeks in September to October 2016 in central England. Ewes previously exposed to virus showed an enhanced antibody response compared to naïve animals. Pre-existing antibody titre did not prevent re-infection in at least one animal, implying immunity to SBV upon natural exposure may not be life-long. In addition, data suggests that immunity provided by killed adjuvanted SBV vaccines only provides short term protection (< 8 months) from virus.

Failure to detect Schmallenberg virus RNA in ram semen in the UK (2016–2018)

“…Despite collection occurring during and after the 2016/2017 disease outbreak over a wide geographical area of Great Britain, 31 no SBV RNA was detected in ram semen at any stage. This does not completely eliminate the possibility that a small number of rams may shed viral RNA in semen.…”

“…These rams were also part of an SBV viral neutralising antibody study that found blood samples taken from 13 rams (seven semen samples in this study) displayed increases in antibody titres during October 2016. This suggested potential recent exposure to the virus during the 2016 breeding season, 31 yet no SBV RNA was detected in the semen of those rams. SBV has a relatively short viraemic period (2–6 days) 19 ; however, bulls that displayed seroconversion (2–4 weeks postinfection) also had consecutive positive SBV RNA in their seminal cell fraction, 20 eluding the testis as a potential privilege site for SBV infection and transmission.…”

“…However, there does not appear to have been significant variation in the S gene sequence (which the qPCR assay used here targets) that would have resulted in assay failure at this time; indeed, the virus seems remarkably stable and we used the same assay to detect SBV viraemia in sheep in 2016. 6 , 31 Alternative assays have been proposed to detect the virus (particularly in the M segment), 35 which is where the major hypervariable region in SBV is reported. Such variation is, however, likely due to within animal tissue specificity/tropism and mutation; it is not reflected in circulating viruses where selection pressure to maintain transmissibility in multiple mammalian and insect hosts is high.…”

“…Two rams (both Aberfield) sampled in 2016 were participating in an artificial insemination trial in a flock that seroconverted during summer 2016, the results of which have been reported. 31 …”

“…To confirm that the extraction method used was adequate for the detection of SBV RNA in ram semen with RNA later preservative. A pooled semen sample (three rams) was spiked with cultured SBV (2.67 × 10 6 TCID 50 /ml) as previously described; 31 then, RNA extraction in triplicate and qPCR (in triplicate on each extraction) were performed as described above. To compare extraction methods, pooled spiked semen in triplicate was also extracted with TRIzol LS (Invitrogen) and either isopropanol precipitation as per the manufacturer's instructions or magnetic bead precipitation (Agencourt AMPure XP kit, Beckman Coulter) as per the manufacturer's instructions.…”