Clinical cases of Bluetongue serotype 8 in calves in France in the 2018-2019 winter

Bluetongue virus serotype 8 (BTV-8) caused an epizootic in Europe in 2006/09. Transplacental transmission of BTV-8 was demonstrated leading to abortions, congenital malformations or nervous clinical signs in newborn calves. BTV-8 re-emerged in France in 2015. Although the re-emergent strain is nearly genetically identical to the one that had circulated in 2006/2009, it has caused very few clinical cases. However, from mid-December 2018 to April 2019, cases of calves with congenital malformations or displaying nervous clinical signs occurred in some departments (French administrative unit) in mainland France. Blood samples from these animals were sent to local laboratories, and the positive ones were confirmed at the French Bluetongue reference laboratory (BT-NRL). Out of 580 samples found positive at the local laboratories, 544 were confirmed as RT-PCR BTV-8 positive. The 36 samples found positive in the local laboratories and negative in the BT-NRL were all at the limit of RT-PCR detection. Hundred eighty-eight of the confirmed samples were also tested for the presence of Schmallenberg virus (SBV) and bovine virus diarrhoea virus (BVDV) infection: 4 were found positive for BVDV and none for SBV. The main clinical signs recorded for 244 calves, for which a reporting form was completed by veterinarians, included nervous clinical signs (81%), amaurosis (72%) and decrease/ no suckling reflex (40%). Hydranencephaly and microphthalmia were reported in 19 calves out of 27 in which a necropsy was practiced after death or euthanasia. These results indicate that the re-emergent strain of BTV-8 can cross the transplacental barrier and cause congenital malformations or nervous clinical signs in calves.