Louping ill virus is a flavivirus (RNA virus), also known as sheep encephalomyelitis virus. Flaviviruses are transmitted by arthropods, and louping ill virus is transmitted by ticks. In red grouse, this virus can cause mortality as high as 78%. The main tick vector is the sheep tick "Ixodes ricinus". Although traditionally tick-borne diseases are thought to be caused when the parasite bites its host, it has been shown that red grouse chicks can be affected when they eat ticks with which they come into contact. This virus may be a significant factor in red grouse populations.