Page 366 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com 1 mebooksfree.com blood ingested during an insect bite. After ingestion, the 355 mebooksfree.com
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CHAPTER 42 Arboviruses
TABLE 42–1 Classification of Major Arboviruses
virus replicates in the gut of the arthropod and then
Viruses of Medical Interest
spreads to other organs, including the salivary glands.
Genus
Family
in the Americas
Only the female of the species serves as the vector of the
Togavirus
Eastern equine encephalitis virus,
Alphavirus
virus, because only she requires a blood meal in order for
western equine encephalitis
virus, chikungunya virus
5
called the extrinsic incubation period, must pass before
2
Flavivirus
Flavivirus
St. Louis encephalitis virus, yellow
the virus has replicated sufficiently for the saliva of the
fever virus, dengue virus, West
vector to contain enough virus to transmit an infectious
Nile virus progeny to be produced. An obligatory length of time,
dose. For most viruses, the extrinsic incubation period
mebooksfree.com mebooksfree.com mebooksfree.com arboviruses are transmitted by vertical “transovarian” pas- mebooksfree.com
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California encephalitis virus
Bunyavirus
Bunyavirus
ranges from 7 to 14 days.
Reovirus
Colorado tick fever virus
Orbivirus
In addition to transmission through vertebrates, some
1
Alphaviruses of other regions include Chikungunya, Mayaro, O’Nyong-Nyong, Ross
River, and Semliki Forest viruses.
sage from the mother tick to her offspring. Vertical trans-
2
Flaviviruses of other regions include Japanese encephalitis, Kyasanur Forest, Murray
mission has important survival value for the virus if a
Valley encephalitis, Omsk hemorrhagic fever, Powassan encephalitis viruses, and
vertebrate host is unavailable.
West Nile viruses.
Humans are involved in the transmission cycle of arbo-
3
Bunyaviruses of other regions include the Bunyamwera complex of viruses and
Oropouche virus.
viruses in two different ways. Usually, humans are dead-
blood is too low and the duration of viremia is too brief for
the bloodsucking vector (Figure 42–1). For effective
transmission to occur, the virus must be present in the
the next bite to transmit the virus. However, in some dis-
eases (e.g., yellow fever and dengue), humans have a high-
bloodstream of the vertebrate host (viremia) in suffi- end hosts, because the concentration of virus in human
level viremia and act as reservoirs of the virus.
ciently high titer to be taken up in the small volume of
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Incidental hosts
(humans and horses)
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FIGURE 42–1
Arbovirus transmission cycle. Arboviruses typically cycle between the vertebrate reservoir host, often a bird, and the
vector, often a mosquito. The infected vector can also bite other hosts, such as humans and horses, which are “dead-end” hosts because their
viremia is too low to provide the vector with an infectious dose. (Source: Centers for Disease Control and Prevention.)

5
The intrinsic incubation period is the interval between the time of the bite and the appearance of symptoms in the human host.

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