Synthetic RNA designs for defective virus vaccines of African horse sickness disease

African horse sickness is a viral disease of horses that results in up to 90% death in naïve but can also infect without severe disease symptom other animals (mules, donkeys, zebras, goats), which act as reservoir. AHSV is endemic to the sub-Saharan Africa but there has been the occasional outbreak of the disease in Spain and Portugal which had significant social and economic impact. The virus is spread by biting midges that are found throughout Europe and the UK, increasing the potential risk of an outbreak. This is becoming a greater concern as climate changes influence both the overall populations of these midges and their dispersal. There are current vaccines available for AHSV but they are considered unsafe, causing adverse side effects and have potential for releasing different type of the viruses into the environment. This proposal will develop vaccines that will be highly safe and will protect the animals from infection without the side effects. The new vaccines are only possible due to recent discoveries made by a research team at the LSHTM, and will use technologies, which are not yet available in other public or commercial organizations.

AHSV is an emerging animal pathogen with the potential to cause severe economic impact in the UK and Europe. It is transmitted by biting midges that are also responsible for the transmission and recent emergence of BTV in Europe and the UK. AHSV infection can cause up to 90% death in a naïve equine population within one week of infection. Currently, no AHSV vaccines are licensed in UK and Europe and the horse importation from enzootic countries is prohibited. The main objective of this project is to develop safe, replication-deficient AHSV strains and evaluate their efficacy in a mouse model system. Once the best candidate vaccine has been determined, it will be tested on 3-4 horses. Since AHSV derived ssRNAs are infectious, it will be possible to obtain AHSV progeny from T7-derived AHSV ssRNAs. To generate replication-deficient AHSV, deletions in the coding region will be made in an essential without disrupting the untranslated regions, which contains cis-acting transcription/replication signals. To ensure complete restriction of virus growth in normal cells, other targeted mutations will be made in a second essential gene. We will also adapt a cell-free core reconstitution system that allows for rapid assessment of mutated RNA segments to be packaged into the particle prior to rescuing potential vaccine candidates by the RG system. In parallel, cell lines that constitutively express the wild-type protein(s) to complement the mutation(s) will be produced. The defective viruses will be characterised and used to generate virus strains for all serotypes. The efficacy of these virus strains, as single or multserotype (cocktail) vaccines will be undertaken in model mouse system in collaboration with FLI (Germany). The best candidate vaccine will then be tested on a minimal number of horses to minimise any suffering. This proposal will take basic virology from the laboratory to translational science with the development of a safe AHSV vaccine.

The threat of orbivirus emergence within animal population of northern EU and UK had largely been considered to be very low risk. This perception and complacency left governments and the agricultural industry on the back foot after the outbreak and over wintering of BTV-8 in the EU and incursion into the UK in 2008. The BTV-8 outbreak had significant economic and social cost, largely due to the ban of animal export and trade from infected area as well as the indirect cost associated with restrictions of public and animal movement. This is highlighted by the emergence of BTV-8 in 2007 in France, costing $1.4 billion in one year (Tabachnick, et al. 2008). Even though there is strong evidence that the distribution of orbiviruses have changed, the potentially more devastating and closely related AHSV is still considered to be low risk despite using the same insect vectors, and research into the development of a safe AHSV vaccine not prioritized. The horse industry in the UK is estimated to contribute £7 billion in direct economic impact (Allison, Taylor et al. 2009). The increase in horse movement for racing carnivals, sporting events (polo, dressage, etc) and importation of semen and embryos has increased the risk associated with the introduction of exotic viruses. The economic and social costs are significantly different between agricultural and equine industry. The control measures including animal slaughter, vaccination, movement restrictions and vector control methods are well understood for most agriculturally important disease whereas outbreaks of equine diseases have a degree of extra complexity, as animal slaughter would be unacceptable and restriction in horse movement could potentially have a more serious impact on the industries than the disease itself. The economic impact of an AHSV outbreak is expected to be ~ £4 billion if it remains uncontrolled (Allison, et al. 2009) and furthermore, failure to control and limit an AHSV outbreak in the UK would cause the collapse of the British racing industry within 18 months (Allison, Taylor et al. 2009). A worst-case scenario is the rapid spread of the virus once introduced, by wind-borne dispersal of midges into a susceptible naïve horse population (>1.3 million) in which 75-95% could die. A similar case fatality for AHSV has been documented for the 1959 epidemic, with more than 300,000 horses died due to AHSV infection (House 1993; Mellor and Hamblin 2004). In the past, whilst vaccination was used to control and eradicate AHSV in Spain and Iberian Peninsula outbreak, currently there are no AHSV vaccines licensed in UK or EU to prevent its spread and minimize economic impact due to safety issues associated with the current vaccines. The development of an AHSV vaccine that is safe, efficacious and affords protection would have a major impact in preparing the UK and EU for any potential outbreak.

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Mellor, P. S. and C. Hamblin (2004). "African horse sickness." Vet Res 35(4): 445-66.
Tabachnick, W., C. Smartt, et al. (2008). Bluetongue, University of Florida: 5.