polyproteins form the core shell between the internal membrane and the nucleoprotein core. An additional outer membrane is gained as particles bud from the plasma membrane. The virus encodes proteins that inhibit signalling pathways in infected macrophages and thus modulate transcriptional activation of immune response genes. In addition, the virus encodes proteins which inhibit apoptosis of infected cells to facilitate production of progeny virions. Viral membrane proteins with similarity to cellular adhesion proteins modulate interaction of virus-infected cells and extracellular virions with host components. Genotypes Based on sequence variation in the C-terminal region of the B646L gene encoding the major capsid protein p72, 22 ASFV genotypes (I–XXIII) have been identified. All ASFV p72 genotypes have been circulating in eastern and southern Africa. Genotype I has been circulating in Europe, South America, the Caribbean, and western Africa. Genotype VIII is confined to four East African countries. Evolution The virus is thought to be derived from a virus of soft tick (genus Ornithodoros) that infects wild swine, including giant forest hogs (Hylochoerus meinertzhageni), warthogs (Phacochoerus africanus), and bushpigs (Potamochoerus porcus). In these wild hosts, infection is generally asymptomatic. This virus appears to have evolved around 1700 AD. This date is corroborated by the historical record. Pigs were initially domesticated in North Africa and Eurasia. They were introduced into southern Africa from Europe and the Far East by the Portuguese (300 years ago) and Chinese (600 years ago), respectively. At the end of the 19th century, the extensive pig industry in the native region of ASFV (Kenya) started after massive losses of cattle due to a rinderpest outbreak. Pigs were imported on a massive scale for breeding by colonizers from Seychelles in 1904 and from England in 1905. Pig farming was free-range at that time. The first outbreak of ASF was reported in 1907. Taxonomy ASFV has no known close relatives. It is the only species in the genus Asfivirus, family Asfarviridae, and order Asfuvirales. Each of these three taxa is at least partly named after ASFV. Signs and symptoms In the acute form of the disease caused by highly virulent strains, pigs may develop a high fever, but show no other noticeable symptoms for the first few days. They then gradually lose their appetites and become depressed. In white-skinned pigs, the extremities turn blueish-purple and hemorrhages become apparent on the ears and abdomen. Groups of infected pigs lie huddled together shivering, breathing