African Horse Sickness: A Review of Current Understanding and Vaccine Development

Abstract

African horse sickness is a devastating disease that causes great suffering and many fatalities amongst horses in sub-Saharan Africa. It is caused by nine different serotypes of the orbivirus African horse sickness virus (AHSV) and it is spread by Culicoid midges. The disease has significant economic consequences for the equine industry both in southern Africa and increasingly further afield as the geographic distribution of the midge vector broadens with global warming and climate change. Live attenuated vaccines (LAV) have been used with relative success for many decades but carry the risk of reversion to virulence and/or genetic re-assortment between outbreak and vaccine strains. Furthermore, the vaccines lack DIVA capacity, the ability to distinguish between vaccine-induced immunity and that induced by natural infection. These concerns have motivated interest in the development of new, more favourable recombinant vaccines that utilize viral vectors or are based on reverse genetics or virus-like particle technologies. This review summarizes the current understanding of AHSV structure and the viral replication cycle and also evaluates existing and potential vaccine strategies that may be applied to prevent or control the disease.

Immune response of horses to inactivated African horse sickness vaccines

“…Gene segment reassortment between vaccine and field serotypes and reversion to virulence of live attenuated vaccine viruses account for such shortcomings of live attenuated vaccines (15). Among the alternative vaccine candidates which are sub unit vaccines, DNA vaccines, reverse genetic vaccines, inactivated vaccines are considered safe but are uneconomical and can only induce protective immunity upon multiple administrations (12,16). Therefore, we developed inactivated vaccines from serotypes isolated from horse fatalities in Kenya, where all 9 serotypes circulate (17).…”

Outbreak of African horse sickness in Thailand, 2020

“…While LAVs remain the only commercially available option for long-term control of AHSV, issues including reversion to virulence and reassortment with field strains have been recorded (Weyer et al, 2015). While a wide range of alternative vaccination strategies have been developed (Dennis, Meyers, Hitzeroth, & Rybicki, 2019), the market for these vaccines has not to date been sufficient to drive the extremely rapid commercial availability and deployment required for use in areas experiencing AHSV epidemics. This will only be resolved through partnerships between the public and private sector and an alternative product that is both cost-effective and safe.…”

African horse sickness: Its emergence in Thailand and potential threat to other Asian countries

“…As one of the eleven equine notifiable diseases listed by OIE, AHS is an infectious but non-contagious disease that could cause widespread death in naïve equines (Carpenter, Mellor, Fall, Garros, & Venter, 2017;Dennis, Meyers, Hitzeroth, & Rybicki, 2019;Zientara, Weyer, & Lecollinet, 2015). Currently, there is no specific treatment for this disease.…”

“…However, this vaccine has several disadvantages, such as its re-assortment events with field strains and the possibility of reversion to virulence (Carpenter et al, 2017). In addition, vaccinated animals cannot be differentiated from naturally infected animals (Dennis et al, 2019). Accordingly, this vaccine is not recommended for vaccination in AHS-free countries.…”