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INTRODUCTION

The viruses described in this chapter are united by having an animal reservoir. This indicates that these viruses can replicate within both the cells of the host animal and within human cells. Most viruses that cause human disease are limited to replicating in human cells as the attachment proteins on the viral surface interact only with receptors on the surface of human cells.

The animal reservoir for arboviruses is quite varied. Birds are the most common, but monkeys and rodents serve as the reservoir for some arboviruses. Many mammals serve as a reservoir for rabies virus. In the United States, bats, skunks, and raccoons are common reservoirs, whereas worldwide, dogs are the most common. The animal reservoir for Ebola virus appears likely to be fruit bats.

Several uncommon viruses with an animal reservoir, such as hantavirus, Lassa fever virus, and Marburg virus are discussed in Chapter 46.

Additional information regarding the clinical aspects of infections caused by the viruses in this chapter is provided in Part IX, titled Infectious Diseases.

ARBOVIRUSES

Arbovirus is an acronym for arthropod-borne virus. This 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 [YFV]) or for the place where they were first isolated (e.g., St. Louis encephalitis [SLE] virus).

As an aside, another group of viruses called roboviruses can be mentioned here for comparison. 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.

Important Properties of Arboviruses

Most arboviruses are classified into three families,1 namely, togaviruses, flaviviruses, and bunyaviruses (Table 42–1).

  1. 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 that causes disease in humans is rubella virus, which was discussed in Chapter 39.

  2. 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 ...

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