Abstract:
Recent outbreaks of bluetongue virus (BTV) serotypes 2 and 8 in many European countries
provided an opportunity to investigate the possibility of improving the safety of the modified
live vaccines administered mainly in South Africa. Modified live vaccines (MLV) released at
a titre of 5 x 104 PFU/mL, raised concerns and prompted the need to determine the minimum
titre which will still be protective and also safe. The BTV serotypes 2 and 8 vaccines were
produced at the following titres: 102 PFU/mL, 103 PFU/mL and 104 PFU/mL, and were injected
into 24 sheep which were then monitored. Blood was collected on days 0, 3, 6, 9, 12, 15, 18, 21,
25, 28 and 4 months post vaccination, for seroconversion and viraemia studies. These sheep
were later challenged at 4 months post vaccination using BTV infected cell culture material,
they were then observed and bled and again tested for viraemia. There was no viraemia post
vaccination, however, a febrile reaction did occur and seroconversion was demonstrated at
low titres for both BTV 2 and 8. Although viraemia was demonstrated post challenge, sheep
vaccinated with the low titre BTV 2 vaccine showed more than a 90% protection index at a
lower titre of 103 PFU/mL, compared with BTV 8 that showed a protection index above 90%
at all the titres used. It is recommended that for BTV 2 vaccine, sheep should be vaccinated at
a titre of 103 PFU/mL and at a titre of 102 PFU/mL with BTV 8 vaccine.
