African swine fever virus lived in warthogs and ticks before infecting domestic pigs

African swine fever began as a virus circulating between warthogs and ticks, where it caused neither species any apparent harm. After jumping to domestic pigs, it became a major threat to economies and a difficult target for vaccine researchers.

The first large outbreak in domestic pigs occurred in Kenya in 1907. It took researchers 14 years to establish that the disease came from a new virus and that warthogs had transmitted it to domestic pigs. In the 1960s, scientists found the virus in blood-feeding ticks living in warthog burrows, completing the outline of its cycle between the two species.

The virus’s own origins remain unclear. It has no close relatives. Its nearest known relative is considered a virus found on the ocean floor that infects abalone, large marine snails that can grow to half a metre. Genetic evidence also suggests an evolutionary connection to giant viruses that infect amoebas, although the relatives are so distant that firm conclusions are difficult.

One theory holds that ancestral forms lived in soil and infected amoebas or other single-celled organisms before moving into ticks. Signs of the virus’s presence long ago have also been found in tick genomes.

The sharp contrast between warthogs and domestic pigs is now central to research. In domestic pigs, the virus hides from the immune system while continuing to replicate. By the time the immune response recognizes the threat, the virus has spread throughout the body, triggering a massive cytokine storm and Ebola-like hemorrhagic fever. Warthogs appear to avoid this destructive overreaction, but researchers have not yet found a way to transfer their resistance to domestic pigs.