Chapter 18 - African Swine Fever Virus: A Missing Link Between Poxviruses and Iridoviruses?

Abstract

Cytoplasmic DNA viruses infecting animals are classified into three families: the poxviridae, the iridoviridae, and the asfarviridae family where the African swine fever virus (ASFV) is currently included. A comprehensive comparison of the DNA sequences for ASFV, several poxviruses, and the iridovirus lymphocystis disease virus may allow us to establish the evolutionary relationships among these viruses. One of the analyses reveals that the lymphocystis disease virus gene, coding for the thymidine kinase, diverged before those of ASFV and the poxviruses, while the ASFV enzyme is more distantly related than the poxvirus thymidine kinase to their cellular homologs. These data are in accordance with the proposed placing of the iridoviruses at the bottom of the evolutionary tree of cytoplasmic DNA viruses as well as with the intermediate position of ASFV in this tree. Although these viruses may have a common ancestor and share certain properties related to the replication cycle, they have further diverged to acquire different sets of genes for the modulation of the host response to infection, developing different mechanisms to avoid the host immune response, and to prevent the induction of apoptosis during infection. The morphological resemblance between ASFV and the iridoviruses and the fact that the capsid proteins of both viruses are homologous strongly suggests the existence of an evolutionary relationship among these viruses.