La Crosse virus particles. Negatively-stained transmission electron micrograph (TEM) of a section through a number of La Crosse (LAC) encephalitis virus particles (virions). Each particle consists of an icosahedral protein coat (capsid) surrounding single-stranded ribonucleic acid (RNA), the virus' genetic material. This virus is an arbovirus (arthropod-borne virus), being transmitted to humans by the treehole mosquito (Aedes triseriatus), but it can also spread through bites from infected mammals. It causes La Crosse encephalitis. In humans, symptoms can be severe, the worst being seizures, coma, paralysis and permanent brain damage from encephalitis (brain inflammation). The virus was first discovered in 1963 in La Crosse, Wisconsin,