Schmallenberg virus RNA in bovine semen

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In semen of Schmallenberg virus (SBV) infected bulls low concentrations of SBV-RNA were found by CVI, but no viable SBV was detected in the semen of these bulls. The risk of SBV infection through insemination of cows and infection of the foetuses of these cows therefore seems to be very low. However this route of transmission cannot be entirely excluded yet.

In order to study SBV excretion in bovine bulls, CVI in cooperation with the Utrecht Univeristy infected two bulls with an infectious SBV isolate. Shortly after infection both SBV inoculated bulls showed clinical symptoms: fever and diarrhoea, and SBV- RNA was detected in semen of both animals. The peaks of excretion were seen in the first week after inoculation. However it must be noted that the concentrations found were always low and viable SBV was not detected in semen. Infectious SBV was only isolated  from blood samples and not from semen or genital tissues. Although infectious virus was not detected in semen in this study, in an earlier study by others, infectivity of semen from SBV RNA-positive straws was found. This was observed after subcutaneous injections in calves with SBV-RNA positive semen, even in case of relatively low concentrations of SBV RNA.

Even in the case of contamination of semen with infectious SBV it still needs to be elucidated whether infectious virus can be transmitted to susceptible cows by insemination. A study on SBV infection after insemination with qRT-PCR-positive semen straws will be needed to answer this question.

Currently, to declare semen free of SBV, it is advised to test samples using an approved RNA extraction and qRT-PCR test; this test is not required when the semen was produced before June 2011 or if the bull was tested SBV-specific antibody negative at least 28 days after semen production.

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