The virulence of 2 non-haemadsorbing African swine fever virus isolates were compared with 2 haemadsorbing viruses. While 3 of these isolates usually produced acute death in pigs, 1 non-haemadsorbing virus caused either a fatal infection with an extended course, or few or no obvious signs of infection. Pigs that survived infection with the latter virus were resistant to the lethal effects of the other 3 strains as well as to a pool of 7 isolates made from Ornithodorus porcinus porcinus (sensu Walton, 1964) and warthog obtained in the Northern Transvaal.
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