Virulence in African swine fever: its measurement and implications.

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 (HA50) or TCID50 required to produce 1 LD50 for swine, the number of HA50 or TCID50 required to produce one 50% pig infectious dose (PID50), and the number of PID50 required for each LD50. The virulent virus (group 1) required less than or equal to 10 virus units (HA50 or TCID50) for 1 PID50 and LD50 (highly infectious and highly lethal), respectively, and had a ratio of 1.0 for PID50/LD50, 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 less than 10 virus units for 1 PID50 and 20 to 562 virus units for 1 LD50 (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 PID50 and 56,200 to 3,120,000 virus units for 1 LD50 (slightly infectious and less lethal).(ABSTRACT TRUNCATED AT 250 WORDS)