Arbovirus is an acronym for arthropod-borne virus and highlights the fact that these viruses are transmitted by arthropods, primarily mosquitoes and ticks. It is a collective name for a large group of diverse viruses, more than 600 at last count. In general, they are named either for the diseases they cause (e.g., yellow fever virus) or for the place where they were first isolated (e.g., St. Louis encephalitis virus).
A new group of viruses called roboviruses has recently emerged. The term robo refers to the fact that these viruses are rodent-borne (i.e., they are transmitted directly from rodents to humans without an arthropod vector). Transmission occurs when dried rodent excrement is inhaled into the human lung, as when sweeping the floor of a cabin. Two roboviruses cause a respiratory distress syndrome that is often fatal: Sin Nombre virus (a hantavirus) and Whitewater Arroyo virus (an arenavirus). These viruses are described in Chapter 46.
Most arboviruses are classified in three families,1 namely, togaviruses, flaviviruses, and bunyaviruses (Table 42–1).
TABLE 42–1Classification of Major Arboviruses |Favorite Table|Download (.pdf) TABLE 42–1 Classification of Major Arboviruses
|Family ||Genus ||Viruses of Medical Interest in the Americas |
|Togavirus ||Alphavirus1 ||Eastern equine encephalitis virus, western equine encephalitis virus, chikungunya virus |
|Flavivirus ||Flavivirus2 ||St. Louis encephalitis virus, yellow fever virus, dengue virus, West Nile virus |
|Bunyavirus ||Bunyavirus3 ||California encephalitis virus |
|Reovirus ||Orbivirus ||Colorado tick fever virus |
Togaviruses2 are characterized by an icosahedral nucleocapsid surrounded by an envelope and a single-stranded, positive-polarity RNA genome. They are 70 nm in diameter, in contrast to the flaviviruses, which are 40 to 50 nm in diameter (see later). Togaviruses are divided into two families, alphaviruses and rubiviruses. Only alphaviruses are considered here. The only rubivirus is rubella virus, which is discussed in Chapter 39.
Flaviviruses3 are similar to togaviruses in that they also have an icosahedral nucleocapsid surrounded by an envelope and a single-stranded, positive-polarity RNA genome, but the flaviviruses are only 40 to 50 nm in diameter, whereas the togaviruses have a diameter of 70 nm.
Bunyaviruses4 have a helical nucleocapsid surrounded by an envelope and a genome consisting of three segments of negative-polarity RNA that are hydrogen-bonded together.