INTRODUCTION

The word vector is derived from the Latin and means ‘bearer’ or ‘one who does something’, but for the purposes of this chapter, we can use the term as defined by Gordh & Headrick (2001) as an arthropod that carries disease-producing organisms to a vertebrate host. A wide variety of arthropods are involved as vectors in pathogen transmission cycles but the focus of this chapter is the mosquito. To date, there are at least 537 named arthropod-borne viruses (arboviruses) (Karabatsos, 1985), encompassing species from many genera, examples of which are given in Table 1. With one exception (African swine fever virus, transmitted by ticks) all of the viruses regarded as true arboviruses have an RNA genome. The vast majority of arboviruses (e.g. Togaviridae, Flaviviridae, Bunyaviridae) are enveloped, i.e. the nucleocapsid is surrounded by a lipid-containing envelope. A few (e.g. Reoviridae) are non-enveloped. Most of these (e.g. bluetongue virus and Colorado tick fever virus) are transmitted by midges and ticks. Fewer than 20 have been isolated from mosquitoes and only lebombo and Orungo have been associated with human infections. As discussed by Mitchell (1983), because arboviruses are typically not pathogenic to their insect hosts, it has been hypothesized that they may have evolved in insects and later became infectious to vertebrates. Tick-borne arboviruses (see review by Nuttall & Labuda, 2003) and the true insect viruses (e.g. nuclear polyhedrosis viruses) that infect and replicate in mosquitoes, but are not transmitted to vertebrate hosts (Miller & Ball, 1998), are not considered in this chapter. Given that almost all mosquito-borne viruses are enveloped and have an RNA genome, unless specified otherwise, this review therefore focuses on this predominant group of arboviruses.