Determination of the minimum protective dose for bluetongue serotype 2, 4 and 8 vaccines in sheep

Abstract

The bluetongue (BT) live attenuated virus vaccine has been used successfully in the control of BT in southern Africa and Europe. However, concerns about the safety, possible development of viraemia and clinical signs post vaccination (p.v.) presented an opportunity to investigate the possibility of reducing the current bluetongue virus (BTV) vaccine titre to below 104PFU/ml. A total of 83 merino sheep were used and vaccinated with BTV monovalent vaccines containing either serotypes 2, 4 or 8 with the following titres: 102, 103 and 104 PFU/ml. Positive and negative control sheep were also included. Animals were bled from Day 0, 3, 6, 9, 12, 15, 18, 21, 25 and 28 p.v and tested for viraemia. Seroconversion was determined on Day 0, 3, 9, 15, 21, 6 weeks, 3 and 4 months p.v. Vaccinated sheep were then challenged at 6 weeks p.v. using BTV infected blood and at 4 months using cell cultured material and evaluated for 14 days using the clinical reaction index. Seroconversion was demonstrated p.v. in more than 70% of sheep vaccinated with a low titre 102 PFU/ml of BTV serotypes 2, 4 and 8 from day 9 and at 4 months. All three serotypes did not demonstrate any viraemia p.v. at the three different titres (102, 103&104PFU/ml). Viraemia was demonstrated p.c. with cell culture material in sheep vaccinated with low titres (102&103 PFU/ml) of BTV serotypes 2 and 4. Viraemia could not be detected in sheep p.v. and p.c. with BTV serotype 8 in all different titres. Sheep challenged with cell culture material of BTV 2 and 4 showed mild clinical signs compare to those challenged with blood culture material that did not respond as expected as positive controls did not demonstrate any clinical signs of BT. It was demonstrated in this study that BTV monovalent vaccines containing serotypes 2, 4 and 8 with titres below 104 PFU/ml can protect more than 90% of vaccinated animals against clinical disease. Although certain serotypes failed to protect against viraemia, all serotypes protected against the development of clinical disease when challenged with either BTV-infected blood or cell cultured material. Copyright