Related strains of African swine fever virus with different virulence: genome comparison and analysis

Two strains of African swine fever virus (ASFV), the high virulence Lisboa60 (L60) and the low virulence NH/P68 (NHV), which have been used previously in effective immunization/protection studies, were sequenced. Both were isolated in Portugal in the 11-year period after the introduction of ASFV into the European continent in 1957. The predicted proteins coded by both strains were compared, and where differences were found, these were also compared to other strains of known virulence. This highlighted several genes with significant alterations in low virulence strains of ASFV that may constitute virulence factors, several of which still lacking knowledge regarding their function. Phylogenetic analysis grouped L60 and NHV closest with other P72 genotype I ASFV strains from Europe and West Africa, consistent with the assumed West-African origin of all European strains. Interestingly, a relatively lower genomic identity exists between L60 and NHV, both isolated in a similar geographical location 8-years apart, than with other European and West-African strains isolated later in time and in more distant locations. This may reflect the intensive passage in tissue culture during the early 60’s of a Portuguese isolate to obtain an attenuated vaccine, which may have led to NHV. This study contributes to a better understanding of the evolution of ASFV and defines additional potential virulence genes for future studies of pathogenesis towards the development of effective vaccines.