Alphaviruses

The alphaviruses are principally mosquito-borne, positive-strand RNA viruses in the family Togaviridae that exhibit a broad range of pathogenicity in humans and animals (1, 2). Members of the genus are distributed worldwide in diverse ecological niches, where they are usually maintained in cycles between mosquitoes and birds or mammals. While human infections generally are incidental to the transmission cycles, in some instances human-mosquito-human cycles can maintain transmission and lead to large outbreaks and epidemics. Among the 24 alphaviruses listed in Table 1, 16 have been associated with human illness. Clinically, these manifest most commonly as polyarthralgia, often accompanied by fever and/or rash, or as central nervous system (CNS) infections. In addition to the alphaviruses circulating between mosquitoes and vertebrate hosts, a single example of an alphavirus, restricted to mosquitoes, has recently been described, Eilat virus (EILV) (3), and there are two known aquatic species, southern elephant seal virus (SESV) and salmon pancreatic disease virus (SPDV), that are likely to have lice as vectors (4, 5). This chapter reviews general aspects of the virology, pathogenesis, laboratory diagnosis, and prevention of the alphavirus infections, followed by more detailed discussion of those that cause human disease.