Cerebral Malformations in Calves Presumed to Be Associated with an Outbreak of Bluetongue Virus Serotype 3 Infection

Bluetongue is a vector-borne viral infectious disease primarily affecting ruminants, transmitted by biting midges of the <i>Culicoides</i> species. The first bluetongue virus 3 (BTV-3) cases occurred in Hesse, Germany, in July 2024. From December 2024 onwards, field veterinarians observed calves born with neurological symptoms. A convenience sample of affected calves were admitted to the Veterinary Teaching Hospital at Justus-Liebig-University Giessen, Germany. A total of 13 calves from unvaccinated dams with pronounced neurological symptoms and positive PCR results for BTV-3 were studied. General and neurological examinations were performed and a blood sample was obtained for hematologic, blood biochemical and blood gas analysis. In 11 calves, magnetic resonance imaging (MRI) was performed. Due to the severe neurological lesions, all calves were euthanized and postmortem examinations were performed. The neurological examination of the calves revealed neurological indications consistent with diffuse forebrain disease. MRI revealed that all calves exhibited various stages of cortical parenchyma cell loss and secondary enlargement of the lateral ventricles. Postmortem examination revealed moderate to severe hydrocephalus internus or hydranencephaly. The results of clinical and neurological examinations, MRI, and postmortem examination, indicate severe impairment of brain development presumably associated with transplacental BTV-3 infection. Epidemiologic data and health records from the originating farms suggest that neurologic malformations developed after BTV-3 infection between 95 and 227 of the gestation period.