Characterization of Schmallenberg virus-induced pathology in aborted and neonatal ruminants

Schmallenberg virus (SBV) is a novel Orthobunyavirus causing mild clinical signs in cows and is responsible for malformations in aborted and neonatal ruminants in Europe. SBV belongs to the family Bunyaviridae and is transmitted by biting midges. This new virus was identified for the first time by metagenomic analysis in blood samples of cows next to the German city Schmallenberg in North-Rhine Westphalia in November 2011. Since then the virus spread to several European countries. The aim of the present thesis was to characterize the pathology of this new disease in naturally infected sheep lambs, goat kids and calves with special focus on the central nervous system (CNS). Therefore, gross findings of SBV-positive, stillborn, aborted neonates or animals dying in the perinatal period originating from North-Rhine Westphalia were determined. In addition, histopathological CNS lesions were investigated, immunophenotyped and virus distribution was studied. Subsequently, results were compared to lesions described for closely related viruses, like AKV. Gross examination of SBV-infected aborted and neonatal ruminants frequently revealed arthrogryposis, brachygnathia inferior and deformities of the vertebral column. The CNS of affected animals often showed malformations like internal hydrocephalus, por- and hydranencephaly and cerebellar hypoplasia. Calves and sheep lambs also displayed micromyelia. The prevalence of CNS inflammation in naturally infected animals was low, namely 2.9% in bovine and 28.3% in ovine cases. The inflammation was characterized by a lymphohistiocytic, perivascular accentuated meningoencephalomyelitis. Immunophenotyping revealed that CD3-positive T cells outnumbered CD68-positive microglia/macrophages and CD79α-positive B cells in brain and spinal cord. Mesencephalon, temporal and parietal lobes were the most frequently affected brain regions by inflammation indicating that these are suitable areas for diagnostic purposes. In addition, neuronal necrosis, gliosis and glial nodules were diffusely present in naturally SBV-infected aborted and neonatal ruminants. Typical histopathological findings adjacent to porencephaly were cortical atrophy with demyelination, axonal loss, astrogliosis, Gitter cell formation, mineralization and hemosiderosis. These findings were interpreted as secondary events due to a SBV-induced cell injury. Iron deposition in areas of tissue destruction was interpreted as remnants of hemorrhages and von Kossa-positive material putatively represents dystrophic mineralization as a consequence of virus-induced tissue destruction. Inflamed areas in the mesencephalon and malformed temporal and parietal lobes contained the highest SBV protein levels as shown by immunohistochemistry. These findings indicate that viral antigen may trigger the inflammation in the mesencephalon leading to tissue loss and porencephaly in the cerebral cortex. Interestingly, the development of inflammation seemed to develop independently from formation of deformities in the CNS. However, its mechanism needs to be investigated in further studies. It has to be considered, that the time point of infection and the immune status of the fetus play important roles in the development of inflammation in the CNS. Summarized, pathological findings of the naturally-occurring SBV-infection in aborted and neonatal ruminants showed an analogy to lesions caused by other viruses of the Bunyaviridae family in terms of gross findings like arthrogryposis, por- and hydranencephaly, cerebellar hypoplasia and deformities of the vertebral column. Similarly to Akabane and Aino virus, SBV is transmitted by insects, affects sheep, cattle and goats and caused mainly pathology in offspring. Therefore, the term ‘arthrogryposis and hydranencephaly syndrome’ is also appropriate for SBV-infected animals with malformations and this virus has to be added to the list of teratogenic viruses in Europe