The study was conducted on foetuses aborted by animals following the 2003 Italian BT vaccination campaign, which used the combination of the monovalent BTV-2 and BTV-9 MLV produced by OBP. A total of 1272 foetuses and malformed offspring from sheep (n = 669), cattle (n = 431), goats (n = 155) and buffaloes (n = 17) were tested and processed for the most common abortion pathogens such as BVDV, BoHV-1, BoHV-5, Serratia marcescens, Campylobacter spp., Brucella spp., Toxoplasma gondii, Neospora caninum, Chlamydophila abortus, Listeria monocytogenes, Leptospira spp., Salmonella enterica Serovar abortusovis, Coxiella burnetii as well as the presence of BTV. The animals originated from 812 farms distributed in 11 different regions of Italy (Table 1). The farms reported reproductive failures following the BT vaccination. Spleen and/or brain from each foetus or malformed offspring was/were collected and stored at −80°C until testing was performed. The isolation or detection methods of the most common abortion-related pathogens of domestic ruminants are listed in Table 2. The presence of BTV was detected as described by the OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals (Manual of Diagnostic Tests and Vaccines for Terrestrial Animals, OIE, 2008). Briefly, spleen/brain homogenates were inoculated intravascularly in 10-day-old embryonated chicken eggs (ECE). The eggs were incubated in a humid chamber at 32–33 °C and candled daily. Embryos that died between days 2 and 7 were stored at 4°C, and live embryos were killed at day 7. All dead embryos were analysed as two separate pools; the first pool consisted of embryos that died from 2 to 7 days, the other pool consisted of embryos that were killed at day 7. Livers, hearts, spleens, lungs and kidneys of the embryos were pooled and homogenised after homogenization, they were inoculated on confluent baby hamster kidney (BHK21) cells lines. Presence of BTV in cell culture was confirmed by RT-PCR as described by Polci et al. (2007). The BTV serotype involved was identified by virus neutralization according to the methods described in the Manual of Diagnostic Tests and Vaccines for Terrestrial Animals (Manual of Diagnostic Tests and Vaccines for Terrestrial Animals, OIE, 2008). Conversely, whether the BTV-2/9 strains had a vaccine or field origin was determined by using the method described by Elia et al. (2008). The influence of variables like species, period of gestation at the time of vaccination, BTV serotype, foetal organs and the time elapsed between vaccination/abortion on the occurrence of BTV in foetal tissues was also investigated. The average duration of the gestation period for cattle and buffaloes was considered as 39 weeks; while, for sheep and goats, the gestation period was considered to be 22 weeks. According to the period of gestation at the time of vaccination, two groups were considered, the first group included animals vaccinated during the first half of gestation (within 19 weeks for buffalo and cattle, within 11 weeks for sheep and goats); the second group included animals vaccinated during the second half of pregnancy. The maximum duration of viraemia following vaccination with MLV was considered to be 60 days (at least in cattle) (OIE, 2011); dams vaccinated 60 days before mating were also included in the study. For each percentage value, its 95% confidence interval was calculated. The virological data were estimated through a Bayesian approach using the Beta (s + 1, n−s + 1) distribution where s is the total number of positives and n is the total number of tested animals (Sivia, 1996). The peak of the distribution represents the most probable value of the percentage of positive animals, and the wideness gives information about the uncertainty of the estimates due to sample size.