Western hemisphere isolates of African swine fever virus: asymptomatic carriers and resistance to challenge inoculation.

: Convalescent clinically normal pigs were tested for the persistence of African swine fever (ASF) infection. One group of pigs was examined 135 days after inoculation with a Brazilian isolate and a 2nd group was examined 110 days after inoculation with a Dominican Republic isolate. Susceptible pigs exposed by contact to these groups remained clinically normal. These contact pigs plus 2 more pigs added to each group developed ASF after being fed and inoculated with tissues collected from recovered pigs. African swine fever virus was not isolated in swine buffy coat cultures inoculated with supernatant fluid from the collected tissues. The remaining convalescent Brazilian and Dominican Republic ASF pigs were challenge inoculated with homologous virus and then with Lisbon 60 ASF virus. Pigs in both groups remained clinically normal after homologous virus challenge inoculation. Pigs in the Brazilian group remained clinically normal after inoculation of the lisbon 60 ASF viral isolate. Of 5 pigs in the Dominican Republic group, 3 developed a transient viremia after inoculation of the Lisbon 60 ASF viral isolate.