Virological and serological response of cattle following field vaccination with bivalent modified-live vaccine against bluetongue virus serotypes 2 and 9.

Following the bluetongue (BT) epidemic in Italy, the government initiated a vaccination campaign involving all domestic ruminants (cattle, sheep and goats) in the affected and adjacent areas to create a resistant population and to reduce virus circulation. Based on the serotypes circulating in the affected areas, monovalent BT virus (BTV) serotype 2 (BTV-2), or bivalent BTV-2 and BTV-9, modified-live vaccines were used. These are manufactured by Onderstepoort Biological Products in South Africa and, because they are recommended for use in sheep only, very little data exists on their use in cattle under field conditions. To evaluate duration and levels of viraemia and the antibody response following vaccination, 30 cattle in various stages of pregnancy were selected and vaccinated with a bivalent BTV-2/BTV-9 vaccine. Blood samples were taken from the animals three times a week for two months and screened using the competitive enzyme-linked immunosorbent assay (c-ELISA) and the virus neutralisation (VN) test. Intravenous egg inoculation, followed by two blind passages in Vero cells, was used to isolate BTV-2 and BTV-9 from ethylene-diaminetetra-acetic acid (EDTA) blood samples, and virus titres determined in viraemic animals. Titres against BTV were detected in 27 animals between days 4 and 35 post vaccination (pv). Viraemia peaked on day 9 pv with average viral titres of 10(4.5)TCID50/ml. From day 9 pv, the c-ELISA detected antibodies in all animals while low VN titres were observed commencing on day 18 pv. Furthermore, VN antibody to BTV-2 was detected in only 17 of the animals vaccinated and to BTV-9 in 27 animals.