Characteristics of the organism and its antigens

The virus persists in the southern half of Africa, either in wildlife reservoirs - wart hogs and soft ticks (Ornithodoros moubata) - or as an enzootic disease of domestic pigs. After more than 30 years as an enzootic disease in domestic pigs in southern Europe it has been eradicated from Portugal (1993) and

Spain (1995). Because it has icosahedral symmetry and replicates in the cytoplasm of infected cells, ASF virus was originally classified as an iridovirus but the genome structure and replication strategy differ substantially from the Iridoviridae and are more like the Poxviridae. The virus is currently classified as the only member of a family called African swine feverlike viruses. The entire genome of a tissue culture-adapted Spanish isolate has been sequenced; it is 170101 nucleotides long and contains 151 open reading frames which include five multigene families of unknown function.

The only cells in the pig in which ASF virus is known to replicate are those of the mononuclear phagocytic system in which virus produces hemadsorption (HAD) of pig erythrocytes to a CD2 homolog expressed on the cell surface (Figure 1).

Infection occurs by receptor-mediated endocytosis via saturable binding sites; it has been shown that

Figure 1 Hemadsorption to an alveolar macrophage infected with the Malawi isolate of African swine fever virus. The red blood cells (rbc) adhere to the surface of the infected cells producing a classical rosette. The viral structures include the perinuclear virus factory (vf) containing virus particles at various stages of assembly and cytoplasmic virus (arrow heads) prior to leaving the cell by budding. Ceil nucleus (n). The bar represents 5 (¿m. (Provided by S.M. Brookes).

Figure 1 Hemadsorption to an alveolar macrophage infected with the Malawi isolate of African swine fever virus. The red blood cells (rbc) adhere to the surface of the infected cells producing a classical rosette. The viral structures include the perinuclear virus factory (vf) containing virus particles at various stages of assembly and cytoplasmic virus (arrow heads) prior to leaving the cell by budding. Ceil nucleus (n). The bar represents 5 (¿m. (Provided by S.M. Brookes).

one receptor-binding protein of the virus is a low molecular weight protein (12 kDa) expressed as a homodimer (17 kDa).

Virus is released from infected cells by budding and acquires an external lipoprotein envelope containing glycosylated components; extracellular virus does not contain glycoproteins. The virus contains over 50 structural proteins, the most highly conserved of which are good immunogens and are used as antigens for serological tests to detect virus. There is no clear identification of virus strains by traditional serological methods but some differentiation is possible by monoclonal antibodies and antibodies that inhibit HAD. There is evidence that field isolates contain viruses with variable polypeptides, that these may also vary on passage in pig macrophages and that there is a complex variety of virus serotypes. Different and distinct genotypes have been identified by restriction enzyme analysis.

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