Page 329 - Review of Medical Microbiology and Immunology ( PDFDrive )

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318
PART IV Clinical Virology
The disease affected primarily young people (60% of
cases were 18 years old or younger). Symptoms were in
American swine origin and the polymerase genes have the
general mild, with the few fatalities occurring in medically
same origin as the quadruple reassortant. This strain does
not have genes of Eurasian swine origin.
compromised patients. There was no outbreak of swine
influenza in pigs prior to this human outbreak. Eating pork
The key point is that most people worldwide do not
does not transmit the virus.
S-OIV is a quadruple reassortant: The hemagglutinin,
of S-OIV even though they may have antibodies against the
nucleoprotein, and nonstructural protein genes are of
seasonal strain of H1N1 virus acquired either by immuni-
North American swine origin, the neuraminidase and have protective antibodies against the swine hemagglutinin
zation or by exposure to the virus itself. Note also that
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matrix protein genes are of Eurasian swine origin, the genes
S-OIV spreads readily from human to human in contrast to
the avian H5N1 strain that does not.
that encode two subunits of the polymerase are of North
American avian origin, and the gene that encodes the third
A PCR test for the diagnosis of S-OIV infection is avail-
subunit of the polymerase is of human H3N2 origin.
resistant to amantadine and rimantadine. Both an inacti-
A triple reassortant strain circulated in North American
swine for several years prior to 2009 but caused human
vated and a live, attenuated vaccine against S-OIV became
influenza only rarely. In the triple reassortant strain, all five
widely available in November 2009.
PARAMYXOVIRUSES
The paramyxovirus family contains four important human Important Properties
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pathogens: measles virus, mumps virus, respiratory syncy-
The genome RNA and nucleocapsid of measles virus are
tial virus, and parainfluenza viruses. They differ from
those of a typical paramyxovirus (see earlier). The virion
orthomyxoviruses in that their genomes are not seg-
mented, they have a larger diameter, and their surface
ing activity and the other with cell-fusing and hemolytic
spikes are different (see Table 39–1).
activities (see Table 39–4). It has a single serotype, and the
Paramyxoviruses are composed of one piece of single-
hemagglutinin is the antigen against which neutralizing
stranded RNA, a helical nucleocapsid, and an outer lipopro-
antibody is directed. Humans are the natural host.
tein envelope. The virion contains an RNA-dependent RNA
polymerase, which transcribes the negative-polarity
genome into mRNA. The genome is therefore not infectious.
The envelope is covered with spikes, which contain hemag-
After adsorption to the cell surface via its hemaggluti-
glutinin, neuraminidase, or a fusion protein that causes cell
nin, the virus penetrates and uncoats and the virion
fusion and, in some cases, hemolysis (Table 39–4). Summary of Replicative Cycle
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RNA polymerase transcribes the negative-strand genome
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into mRNA. Multiple mRNAs are synthesized, each of
MEASLES VIRUS
which is translated into the specific viral proteins; no
Disease
is made. The helical nucleocapsid is assembled, the
This virus causes measles, a disease characterized by a
matrix protein mediates the interaction with the enve-
lope, and the virus is released by budding from the cell
maculopapular rash. It occurs primarily in childhood. (See
membrane.
“Clinical Findings” section for additional information.)
TABLE 39–4 Envelope Spikes of Paramyxoviruses
Virus 2 Hemagglutinin Neuraminidase Fusion Protein 1
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–
Measles virus
+
+
Mumps virus
+
+
+
–
–
Respiratory syncytial virus
2
Parainfluenza virus
+
+
1
The measles and mumps fusion proteins are hemolysins also.
2
In mumps and parainfluenza viruses, the hemagglutinin and neuraminidase are on the same spike, and the fusion protein is on a different spike.
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