DEVELOPMENT OF A MOUSE MODEL SYSTEM, CODING ASSIGNMENTS AND IDENTIFICATION OF THE GENOME SEGMENTS CONTROLLING VIRULENCE OF AFRICAN HORSE SICKNESS VIRUS SEROTYPE-3 AND SEROTYPE-8

Attenuated ((att)) and wild type ((wt)) strains of the nine AHSV serot ypes were evaluated for virulence in adult Balb C mice. Alt:hough most were avirulent in this system, isolates of AHSV 1(att), 3(wt), 3(att) , 4(wt), 4(att), 5(att), 7(att) and 8(att) caused some mortality when administered via an intranasal route. After plaque cloning, only the a ttenuated vaccine strain of AHSV 7(att) caused any mortality via an in travenous route. AHSV 3(att) and AHSV 8(wt) were virulent (V) and avir ulent (AV) (respectively) in the mouse model and were selected as pare ntal strains for production of genome segment reassortants. These prog eny virus strains were plaque cloned, then characterised to identify t he genome segments that influence virulence of AHSV in the mouse model . Three virulence phenotypes were observed: fully virulent (V); fully avirulent (A); and a novel intermediate virulence (N) not expressed by either parental strain. Genome segment 2 (encoding outer capsid prote in VP2) from the avirulent parent appeared to have a controlling influ ence in production of the A phenotype. Reasortants with the V phenotyp e all contained segment 2 from the virulent parent, however in each ca se they also contained genome segments 5 and 10, also from AHSV 3 (V). Genome segments 5 and 10 encode the smaller outer capsid protein VP5 and the non structural proteins NS3/NS3a, respectively. A combination of genome segments 2, 5 and 6 from the avirulent parent and segment 10 from the virulent parent were found in each of the virus strains with the N phenotype. However, comparison of two reassortants (A79 and A79 0), which differ only in a single segment, showed that replacement of genome segment 10 from the avirulent parent with that from the virulen t parent, conferred the N phenotype on A790.