Risk factors associated with the occurrence of undesired effects in sheep and goats after field vaccination with modified-live vaccine against bluetongue virus serotypes 2, 4 and 16.

In the 2004, the Sardinian bluetongue (BT) vaccination campaign used the combination of monovalent BTV-2, BTV-4 and BTV-16 modified-live vaccines manufactured by the Onderstepoort Biological Products in South Africa. Following vaccination, some herds showed temperature, oedema, lameness, hyperaemia and decrease in milk production, and some others remained perfectly healthy. This study aimed to evaluate whether important factors present in the herd at the time of vaccination could be associated to the occurrence of undesired effects observed after immunisation with BTV modified-live vaccines. A sample of 17 sheep and 4 goat flocks, for a total of 670 animals, were included in the study and risk factors such as presence of most important parasitic, bacterial and viral diseases as well as anomalies of biochemical and haematological parameters were associated to the presence or absence of side effects. For each factor the relative risk and 95% confidence interval were calculated. Following vaccination, bluetongue-like symptoms were observed in 13 flocks. In these flocks, a higher (P<0.05) proportion of animals had viraemia and showed higher titers to BTV-16 after immunisation. Positive association (RR=2.50, 1.17-5.04) was also found between flocks in which undesired effect were observed and positive serology against Maedi-Visna virus. On the contrary, presence of BTV genome fractions in the blood of animals at the time of vaccination was found to be protective (RR=0.7, 0.58-0.84) to the occurrence of undesired effect subsequent to BTV vaccination.