Page 365 - Review of Medical Microbiology and Immunology ( PDFDrive )

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42
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Arboviruses

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CHAPTER C ONTENT S

Colorado Tick Fever Virus
Introduction
West Nile Virus
Important Properties
Transmission
Clinical Findings & Epidemiology
Outside the United States
Important Arboviruses That Cause Disease in the
Yellow Fever Virus
United States
Dengue Virus
Eastern Equine Encephalitis Virus
Chikungunya Virus
Western Equine Encephalitis Virus
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St. Louis Encephalitis Virus
Summaries of Organisms
California Encephalitis Virus
Practice Questions: USMLE & Course Examinations
2
INTRODUCTION
(1) Togaviruses are characterized by an icosahedral
nucleocapsid surrounded by an envelope and a single-
Arbovirus is an acronym for arthropod-borne virus and
stranded, positive-polarity RNA genome. They are 70 nm
highlights the fact that these viruses are transmitted by
arthropods, primarily mosquitoes and ticks. It is a collective
50 nm in diameter (see later). Togaviruses are divided into
name for a large group of diverse viruses, more than 600 at
two families, alphaviruses and rubiviruses. Only alphavi-
last count. In general, they are named either for the diseases in diameter, in contrast to the flaviviruses, which are 40 to
ruses are considered here. The only rubivirus is rubella
they cause (e.g., yellow fever virus) or for the place where
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virus, which is discussed in Chapter 39.
they were first isolated (e.g., St. Louis encephalitis virus).
3
(2) Flaviviruses are similar to togaviruses in that they
A new group of viruses called roboviruses has recently
also have an icosahedral nucleocapsid surrounded by an
emerged. The term robo refers to the fact that these viruses
are rodent-borne (i.e., they are transmitted directly from
genome, but the flaviviruses are only 40 to 50 nm in diam-
rodents to humans without an arthropod vector). Trans-
eter, whereas the togaviruses have a diameter of 70 nm.
mission occurs when dried rodent excrement is inhaled
4
(3) Bunyaviruses have a helical nucleocapsid sur-
into the human lung, as when sweeping the floor of a cabin.
rounded by an envelope and a genome consisting of three
Two roboviruses cause a respiratory distress syndrome that
segments of negative-polarity RNA that are hydrogen-
is often fatal: Sin Nombre virus (a hantavirus) and White-
bonded together.
water Arroyo virus (an arenavirus). These viruses are
described in Chapter 46.
Transmission
Important Properties
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these viruses to multiply in both the vertebrate host and
1
Most arboviruses are classified in three families, namely,
togaviruses, flaviviruses, and bunyaviruses (Table 42–1).
2
Toga means cloak.
3
1
A few arboviruses belong to two other families. For example, Colorado
Flavi means yellow, as in yellow fever.
4
tick virus is a reovirus; Kern Canyon virus and vesicular stomatitis virus
“Bunya” is short for Bunyamwera—the town in Africa where the proto-
type virus was located.
are rhabdoviruses.
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