Development of African horse sickness disabled infectious single animal (DISA)-DIVA vaccine platform applied for all nine serotypes

Abstract

African Horse Sickness (AHS) is a devastating vector-borne viral disease of equids with a mortality up to 95 % in naïve domestic horses. The causative African horse sickness virus (AHSV) is a distinct species of the genus Orbivirus of the family Sedoreoviridae, consisting of nine serotypes showing limited cross protection. AHSV is transmitted by Culicoides biting midges. Outbreaks cause huge economic losses in developing African countries. AHS has become a serious threat for countries outside Africa, since endemic Culicoides species in moderate climates appear competent vectors of the closely related prototype orbivirus, bluetongue virus. In the developed world, AHS outbreaks will result in losses in the equestrian industry and will have an enormous emotional impact on owners of leisure horses. Live-attenuated vaccine viruses (LAVs) are unsafe and differential detection of infected equids in LAV-vaccinated populations is not possible. Reverse genetics has paved the way to develop improved AHS vaccines, particularly with regard to vaccine safety and the DIVA principle (DIVA = Differentiating Infected from Vaccinated). Here, we developed an AHS Disabled Infectious Single Animal (DISA)-DIVA vaccine platform based on a dispensable deletion in genome segment 10 completely abolishing its virulence. The vaccine platform was applied for all nine AHSV serotypes by exchange of genome segments 2 and 6 encoding serotype specific immunodominant outer shell proteins. These nine promising AHS DISA-DIVA vaccines, named shortly DISA1 to DISA9 after their serotype, were extensively checked by several in vitro methods. The accompanying DIVA PCR-test targeting the deleted region in genome segment 10 was developed and validated. Based on previous research on virulent AHSV, DISA1 to DISA9 are not virulent, are not transmittable by midges, can be distinguished from wildtype AHSV, and are now ready for vaccination-challenge experiments in the equine target host to study efficacy.