Effect of semen processing methods on lumpy skin disease virus status in cryopreserved bull semen

Lumpy skin disease is an economically important disease of cattle, caused by the lumpy skin disease virus (LSDV; Capripoxvirus). It has a variable clinical appearance but, in severely affected animals, is associated with extensive skin damage, pneumonia and death. The LSDV can be found in the semen of infected bulls for prolonged periods of time, from where it can be transmitted by mating or artificial insemination and cause clinical disease in heifers and cows. In this study, an ejaculate was collected from a LSDV seronegative bull and confirmed free from LSDV DNA by PCR. The ejaculate was split into a control sample (C), a sample spiked with a 4 log TCID₅₀ dose of an LSDV isolate (HD) and a 10³ dilution of the virus suspension (ND) and frozen routinely. Two straws from each of the different semen treatment groups (HD, ND and C) were subsequently thawed and subjected to swim-up, single layer centrifugation, Percoll^® density gradient and a Percoll^® density gradient with added trypsin. For one set of straws, semen quality variables were recorded, and viral DNA status determined using PCR; the other set was used for positive staining electron microscopy. Samples determined to be positive for LSDV DNA by PCR were then subjected to virus isolation (VI). Complete elimination of LSDV from semen did not occur with use of any of the processing methods. Trypsin did reduce the viral load, and eliminated LSDV from the ND sample, but severely negatively influenced semen quality. The LSDV virions, as assessed by electron microscopy, were associated with the sperm plasma membrane. Further investigation is needed to establish the efficacy of immuno-extenders for rendering semen free from LSDV.