Virulence in African swine fever: Its measurement and implications

SUMMARY A method of measuring and expressing the virulence of African swine fever virus in numerical terms was developed. Seventeen viruses (13 hemadsorbing and 4 nonhemadsorbing) were tested and classified into 3 groups: highly infectious and highly virulent, highly infectious and moderately virulent, and slightly infectious and slightly virulent. This classification was based on the number of 50% hemadsorption unit ( ha 50 ) or tcid 50 required to produce 1 ld 50 for swine, the number of ha 50 or tcid 50 required to produce one 50% pig infectious dose ( pid 50 ), and the number of pid 50 required for each ld 50 . The virulent virus (group 1) required ≤ 10 virus units ( ha 50 or tcid 50 ) for 1 pid 50 and ld 50 (highly infectious and highly lethal), respectively, and had a ratio of 1.0 for pid 50 / ld 50 , ie, all infected pigs died from acute African swine fever. Tengani, L'60, and DR-I isolants and nonhemadsorbing viruses of Haiti-1 isolant belong to this group. The moderately virulent virus (group 2) required < 10 virus units for 1 pid 50 and 20 to 562 virus units for 1 ld 50 (highly infectious and moderately lethal), respectively, and recoveries from the clinical disease and immunologic deaths were frequent. Madrid '75, Haiti-1, DR-II, and BR-I isolants belong to this group. The slightly virulent virus (group 3) required 56 to 10,000 virus units for 1 pid 50 and 56,200 to 3,120,000 virus units for 1 ld 50 (slightly infectious and less lethal). The L'60BM89BC1 virus, which originated from Portugal as a vaccinal virus, and L'60BM89BC20-2 virus, which was obtained from L'60BM89BC1 virus by 20 subpassages at limiting dilution, belong to this group. The nonhemadsorbing viruses isolated from the moderately virulent Haiti-1 isolant were highly infectious and highly virulent. Their nonhemadsorbing and virulent characteristics were genetically stable. The virulence of a virus preparation was determined by the proportion of highly virulent and slightly virulent, and possible moderately virulent, clones in the preparation at the time of testing. Observations on the clinical disease caused by viruses of a varying virulence were described and implications were discussed.