Diversity of African swine fever virus.

An African swine fever virus is an heterogeneous population, consisting of clones having different biological characteristics in respect to hemadsorption, virulence, infectivity, plaque size, and antigenic determinants. The following observations were made: Nonhemadsorbing virus (NHV) have been segregated from field isolates from Haiti (HT-1) and a bone marrow- and buffy coat-passaged Portuguese isolate (L'60BM89BC1) and appear as a major, minor, or equal mixture with hemadsorbing viruses in the virus population. Biological characteristics of the virus inoculated into pigs often differed from viruses isolated later from the same pigs. Virulence and nonhemadsorbing characteristics of isolated clones were genetically stable. The lethal effect of 2 NHV clones of L'60BM89BC1 virus was dose-dependent; small doses of virus induced immunologic deaths or recoveries from the clinical disease in pigs, and large doses induced acute deaths. The NHV of Lisbon isolate of 1960 (L'60) and HT-1 isolate share the same antigenic determinants for inducing protection. Tengani isolate contained clones of distinctly different antigenic determinants, not shared by L'60 or HT-1 isolate that enabled it to overcome the protection induced by the other clones. Passaging of an African swine fever virus isolate in pigs or cell cultures may readily alter the proportions of the different clones in the population and thereby change its overall characteristics. A new virus population with atypical hemadsorption was found in HT-1 field isolate and L'60BM89BC1 virus.