Genetic variation of African swine fever virus: variable regions near the ends of the viral DNA.

Restriction endonuclease maps of the variable DNA regions of African swine fever virus field isolates from the Iberian peninsula showed that the changes in length are located in the terminal-inverted repetitions and in unique sequences close to the DNA ends. Analysis of nine clones derived from the spleen of an infected pig revealed the existence of frequent length changes within the inverted terminal repetitions. In each clone, changes occurred symmetrically at both terminal-inverted repetitions, suggesting the existence of a terminal-inverted repetition transposition or correction mechanism. Large deletions in unique sequences were detected more frequently in the region located from 8 to 20 kb from the left DNA end. The analysis of this DNA segment from a virulent African swine fever virus isolated in Lisbon (LIS57) showed that this virus strain contains about 8 kb more DNA sequence than the prototype avirulent virus strain (BA71). Hybridization of the additional sequences from LIS57 virus with DNA from different virus field isolates revealed that this DNA region is highly variable in vivo and that it contains several repeated sequences. DNA sequences present around the deletion end points in the variable regions indicate that the deletion process may take place by both homologous and nonhomologous recombination.