Systematic Veterinary Virology

TRANSMISSION

• AHS is not transmitted directly between horses. The infection is transmitted by via the biting Culicoides. C imicola, C obsoletus and C pulicaris are the common biological vectors for AHSV. The virus is also transmitted biologically by midges, and these insects are most active just after sunset and at about sunrise.
• Other insects such as mosquitoes also act as biological vectors, and large biting flies (e.g., Stomoxys, Tabanus) transmit AHS virus mechanically. Wind-borne spread of midges may assist the short-distance spread of the disease but that long-distance jumps of the infection are invariably the result of movement of infected Equidae.
• The spread of disease is influenced by climatic conditions, which favour the spread of carrier insects (vectors) including warm, moist weather and high rainfall, as well as spread by wind dispersal.
• In order for AHSV to become infective, it must be taken up in a blood meal from an infected host.
• After the virus matures within the arthropod vector, it can be introduced into a susceptible host at the following blood meal. Once inside the new host, the virus replicates in the local lymph nodes and disseminated through viraemia.