Serological Evidence on the Role of the Onderstepoort Attenuated African Horse Sickness Vaccine by Induction of Antibody Responses to a Nine Serotype Containing CVRL Inactivated African Horse Sickness Vaccine

Abstract

Although the frequency and severity of African horse sickness (AHS) outbreaks have declined significantly in Southern Africa, studies have shown that vaccinating horses only with the Onderstepoort attenuated vaccine did not always give full protection against AHS. The objective of this study was, therefore, to investigate whether an inactivated AHS vaccine containing all 9 serotypes would increase the serological response of horses that had been previously immunized with the Ondestepoort attenuated AHS vaccine. For this experiment, twenty-seven horses were selected from AHS endemic area in northern Kenya. The study horses were simultaneously vaccinated against all 9 AHS serotypes in 2 injections and their seroconversion was recorded. Sixteen horses, which were regularly immunized for many years with the Onderstepoort attenuated vaccine (Onderstepoort Biological Products, OBP) until 2013 and then annually 6 times until 2019 with the inactivated CVRL vaccine, developed high ELISA and virus neutralising antibodies (VN). Eleven horses which never received the OBP vaccine, but since 2015 six times the CVRL vaccine also developed ELISA and VN antibodies, which however were significantly lower than in the first group. The result of the current experiment confirmed the already known fact that multiple vaccinations against all 9 serotypes are important for the development of high antibody levels. Furthermore, it suggests the possible induction of higher levels of antibodies by the horses vaccinated with an inactivated vaccine which had been previously immunized with an attenuated vaccine compared to the ones which were not immunized with the latter vaccine. In conclusion, alternate immunization with an inactivated and an attenuated vaccine may provide better protection against AHS.