**1. Introduction**

Arboviruses (arthropod-borne viruses) are a group of pathogens that are transmitted by hematophagous arthropods, mainly mosquitoes and ticks, between susceptible vertebrates [1]; many of them are also characterized by their movement through arthropod communi‐ ties: vertical (or transovarial) [2] and venereal transmission [3]. Thus far, more than 500 ar‐ boviruses have been identified worldwide, particularly in tropical and subtropical areas [4-6]. Of these, some 80 species can cause human diseases with a broad spectrum of symp‐ toms, including encephalitis, fever, and hemorrhaging [7]. Most arboviruses are classified into three families (the Togaviridae, Flaviviridae, and Bunyaviridae) in the current viral clas‐ sification system. Minor groups of arboviruses include those belonging to the Rhabdoviri‐ dae (vesicular stomatitis Indian and bovine ephemeral fever viruses), Reoviridae (bluetongue virus and Colorado tick fever), and Asfarviridae (African swine fever virus; ASFV); all of which have trivial or no roles in causing human diseases.

Viruses belonging to the Togaviridae are enveloped and spherical with a size of 65~70 nm in diameter; they contain an icosahedral nucleocapsid within which is included single-strand‐ ed positive-sense RNA [8]. Viral RNA serves as both the genome and viral messenger (m)RNA. The entire genome encodes a non-structural polyprotein which is processed by host and viral proteases, while a structural polyprotein is expressed by subgenomic mRNA [9]. The genus *Alphavirus* in the family Togaviridae includes 29 virus species, all of which are transmitted by mosquitoes [10]. The Flaviviridae is composed of viruses that also con‐ tain single-stranded positive-sense RNA; however, their virions are smaller in size than Al‐ phaviruses, usually 45~50 nm in diameter [11]. The genus *Flavivirus* contains about 70 members; a number of them are infectious to humans, *e.g.*, dengue virus and West Nile (WN) virus. Flaviviral RNA possesses a single open reading frame, encoding a polyprotein, which is then processed to three structural proteins (C, M, and E) and seven non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5) by host and viral proteases [11].

© 2013 Perng and Chen; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The Bunyaviridae is one of the largest groupings of animal viruses, containing more than 300 viruses [12]. Except for the genus *Hantavirus*, all of them are transmitted by arthropods [12]. Viral particles are spherical with a size >100 nm in diameter, and are composed of four structural proteins encoded on its tripartite single-stranded negative-sense RNA genome consisting of the L, M, and S segments [13].

window of time between 1993 and 2006 in the Czech Republic, indicating a steep rise in this region [25]. Rodents are the primary reservoir hosts of this virus, which is transmitted by the

Arboviral Encephalitis

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http://dx.doi.org/10.5772/52327

**Figure 1.** Transmission cycles of arboviruses in nature. Two major cycles cover the transmission of most arboviruses,

The JE virus is mainly amplified in pigs and birds and are transmitted by *Culex* mosquitoes (primarily *Cx. tritaeneorhunchus*) between vertebrates [26]; it causes a significant number of human encephalitis cases in most areas of Asia, especially eastern, southern, and southeast‐ ern Asia, as well as the South Pacific regions [27]. It recently expanded to the Torres Strait of northern Australia in 1999, and has now become endemic in Australia [28, 29]. JE virus is estimated to cause about 30,000~50,000 cases each year worldwide [15, 30]; of which,

WN virus was first isolated from a febrile patient in the West Nile region of Uganda in 1937 [32]. It has caused epidemics in Africa, Europe, the Middle East, Asia, and, more recently, in North America [33]. Since the emergence of WN virus in the United States in 1999, it has

one is mam-to-man and the other usually involves non-human mammals and birds.

10,000~15,000 may be fatal [31].

bites of hard ticks (*Ixodes*) in nature [24].

Various arboviruses belonging to those three major families can specifically cause encephali‐ tis. Of these, Eastern equine encephalitis (EEE) virus, Western equine encephalitis (WEE) vi‐ rus, and Venezuelan encephalitis (VEE) virus belong to the Togaviridae [14], Japanese encephalitis (JE) virus, St. Louis encephalitis (SLE) virus, WN virus, and tick-borne encepha‐ litis (TBE) virus are from the Flaviviridae [15, 16], while California encephalitis (CE) virus and La Crosse (LAC) virus are members of the Bunyaviridae [7]. Recently, increasing evi‐ dence has shown that certain arboviruses such as dengue (DENV) and chikungunya viruses (CHIKV) may occasionally cause encephalitis in addition to their conventional symp‐ toms,which usually involves headaches, muscle and joint pain, and rashes [17-19].
