The role of dipterous insects in the mechanical transmission of animal viruses

Animal viruses may be transmitted by arthropods in two ways, either biologically or mechanically. Many different species of Diptera are implicated in mechanical transmission, but haematophagous species are the most important. The insects become contaminated with virus during normal feeding behaviour, and virus persists on their mouthparts or body until the next feed. Some viruses are inactivated rapidly on mouthparts, whereas others survive for many days or weeks, prolonging the potential period of transmission. Some viruses produce high titres in the skin of the infected vertebrate host, which facilitates transmission, whereas other viruses are transmitted even during relatively low levels of viraemia. Mechanical transmission by arthropods is important in the epidemiology of many animal diseases, and may be the major mode of horizontal transmission. In other instances vector spread is merely incidental.