Bluetongue (BT) is a non-contagious disease of sheep and other domestic and wild
ruminants caused by the bluetongue virus (BTV). Currently 26 serotypes of the virus have
been identified. In South Africa, 22 serotypes have been identified and BT is controlled
mainly by annual vaccinations using a freeze-dried live attenuated polyvalent BTV vaccine.
The vaccine is constituted of 15 BTV serotypes divided into three separate bottles and the
aim is to develop a vaccine using fewer serotypes without compromising the immunity
against the disease. This study is based on previously reported cross-neutralisation of specific
BTV serotypes in in vitro studies. Bluetongue virus serotype 4 was selected for this trial and
was tested for cross-protection against serotype 4 (control), 1 (unrelated serotype), 9, 10 and
11 in sheep using the serum neutralisation test. The purpose of the study was to determine
possible cross-protection of different serotypes in sheep. Of those vaccinated with BTV-4
and challenged with BTV-1, which is not directly related to BTV-4, 20% were completely
protected and 80% showed clinical signs, but the reaction was not as severe as amongst the
unvaccinated animals. In the group challenged with BTV-10, some showed good protection
and some became very sick. Those challenged with BTV-9 and BTV-11 had good protection.
The results showed that BTV-4 does not only elicit a specific immune response but can also
protect against other serotypes.