Circulation of bluetongue virus 8 in French cattle, before and after the re-emergence in 2015.

Bluetongue virus serotype 8 (BTV-8) re-emerged in Central France in August 2015. The viral strain identified is nearly identical to the one that circulated during the 2006/2009 massive outbreak throughout Europe. To address the question of an undetected BTV-8 circulation on the French territory, a serological study was conducted on young cattle along a transect of seven departments, three of them located in areas where the virus presence had been confirmed by RT-PCR by winter 2015/2016. Sera from 2,565 animals were collected during the winters preceding and following the re-emergence, with 414 animals being sampled in each of the two consecutive years. All samples were tested by competitive ELISA (IDVet) and, when enough serum was available, ELISA-positive samples were confirmed by seroneutralization tests. In areas with infected holdings, seropositive animals were found before the re-emergence (N = 14 of 511), significantly more on the following year (N = 17 of 257), and eight animals (N = 158) seroconverted over 2015. Seropositive animals were also detected as early as winter 2014/2015 in one department without known infected holdings (N = 12 of 150), and in winter 2015/2016 in three of them (N = 21 of 555), where seven animals (N = 154) seroconverted over 2015. These results suggest that BTV-8 may have spread at low levels before the re-emergence, even in areas considered virus-free. Unfortunately, whole blood from the seropositive animals was not available to definitely confirm the virus presence by RT-PCR.