African horse sickness virus (AHSV) is a virus species in the genus Orbivirus of the family Reoviridae causing
African Horse Sickness (AHS) in equids with a mortality of about 95% in naïve horses. AHS causes serious
losses in developing countries where horses play a central role in draft power and transportation.
There are nine AHSV serotypes inducing no or low cross-neutralizing antibodies. AHSV is spread by biting
Culicoides midges. AHS is endemic in sub-Saharan Africa, and a serious threat outside Africa, since
Culicoides species in moderate climate conditions are spreading the closely related bluetongue virus.
AHS outbreaks will be devastating for the equestrian industry in developed countries. Live-attenuated
vaccines (LAVs) are licensed, marketed and in use in Africa. Their application is controversial with regard
to safety issues. LAVs are not allowed in AHS-free countries. We here studied inactivated AHSV with different
adjuvants in guinea pigs and horses. Subcutaneous and intramuscular vaccination were studied in
horses. Local reactions were observed after prime and boost vaccination. In general, neutralizing antibodies
(nAbs) titres were very low after prime vaccination, whereas boost vaccination resulted in high nAb
titres for some adjuvants. Vaccinated horses were selected based on local reactions and nAb titres to
study efficacy. Unfortunately, not all vaccinated horses survived virulent AHSV infection. Further, most
survivors temporarily developed clinical signs and viremia. Further, the current prototype inactivated
AHS vaccine is not suitable as emergency vaccine, because onset of protection is slow and requires boost
vaccinations. On the other hand, inactivated AHS vaccine is completely safe with respect to virus spread,
and incorporation of the DIVA principle based on NS3/NS3a serology and exploring a vaccine production
platform for other serotypes is feasible. A superior adjuvant increasing the protective response without
causing local reactions will be required to develop payable and acceptable inactivated AHS vaccines.