Page 328 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 39 RNA Enveloped Viruses
given to health care personnel who are likely to transmit
the virus to those at high risk.
features of the H5N1 strain, namely, relative resistance to
interferon and increased induction of cytokines, especially
One side effect of the influenza vaccine used in the
1970s containing the swine influenza strain that caused
tumor necrosis factor. The increase in cytokines is thought
to mediate the pathogenesis of the pneumonia and acute
influenza in humans was an increased risk of Guillain-
Barré syndrome, which is characterized by an ascending
paralysis. Analysis of the side effects of the influenza vac-
infection.
The H5N1 strain is sensitive to the neuraminidase
cines in use during the last 10 years has shown no increased
inhibitors, oseltamivir (Tamiflu) and zanamivir (Relenza),
risk of Guillain-Barré syndrome. respiratory distress syndrome (ARDS) seen in H5N1
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In addition to the vaccine, influenza can be prevented by
but not to amantadine and rimantadine. Tamiflu is the
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using oseltamivir (Tamiflu), which is described in the treat-
drug of choice for both treatment and prevention. There is
no human vaccine available against the H5N1 strain, but
ment section earlier. Oseltamivir is particularly useful in
elderly people who have not been immunized and who may
have been exposed. Note that this drug should not be
FDA approved an inactivated vaccine against H5N1 influ-
thought of as a substitute for the vaccine. Immunization is
enza virus, but as of this writing, it is not available to the
the most reliable mode of prevention.
public. The vaccine is being stockpiled in the National
Emergency Reserve.
2. Avian Influenza Virus
H7N9 Influenza Virus
Infection in Humans
H5N1 Influenza Virus
strain of influenza virus occurred. Prior to this time, the
In 1997, the H5N1 strain of influenza A virus that causes
H7N9 strain affected only birds, especially chickens. As
avian influenza, primarily in chickens, caused an aggres- In 2013, an outbreak of influenza caused by an H7N9
of July 2013, 133 people have been diagnosed with influ-
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sive form of human influenza with high mortality in Hong
enza caused by this virus, 43 of whom have died (32%
Kong. In the winter of 2003–2004, an outbreak of avian
mortality rate). Cases have been limited to China and
influenza caused by H5N1 strain killed thousands of
chickens in several Asian countries. Millions of chickens
spread.
were killed in an effort to stop the spread of the disease.
All of the genes of this virus are of avian origin. It
Four hundred eight human cases of H5N1 influenza
acquired its H7 gene from ducks and its N9 gene from wild
occurred between 2003 and February 2009, resulting in
birds, and all the other genes are from an influenza strain
254 deaths (a mortality rate of 62%). Note that these 408
that infects bramblings, a bird common in Asia and
people were infected directly from chickens. Both the
Europe. This H7N9 strain is susceptible to the neuramini-
respiratory secretions and the chicken guano contain
infectious virus.
vaccine.
The spread of the H5N1 strain from person to person
occurs rarely but remains a major concern because it could dase inhibitors, oseltamivir and zanamivir. There is no
3. Swine Influenza Virus
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increase dramatically if reassortment with the human-
Infection in Humans
adapted strains occurs. In 2005, the H5N1 virus spread
In April 2009, a novel swine origin strain of influenza A
from Asia to Siberia and into eastern Europe, where it
killed thousands of birds but has not caused human dis-
an outbreak of human influenza, which appeared first in
ease. As of this writing (February 2016), there have been no
Mexico, then in the United States, followed by spread to
cases of human influenza caused by an H5N1 virus in the
208 countries by December 2009. The Centers for Disease
United States. However, there have been two cases of
Control and Prevention (CDC) uses the name “novel influ-
human influenza caused by an H7N2 strain of avian influ-
enza A (H1N1)” for this virus.
enza virus.
As of December 2009, millions of cases have occurred
The ability of the H5N1 strain to infect chickens (and
other birds) more effectively than humans is due to the
tries have stopped documenting the number of cases.
presence of a certain type of viral receptor throughout the
Worldwide there have been 9596 deaths, of which 1445
mucosa of the chicken respiratory tract. In contrast, worldwide. There have been so many cases that most coun-
have occurred in the United States. On June 11, 2009, the
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humans have this type of receptor only in the alveoli, not in
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World Health Organization (WHO) declared a level 6 pan-
the upper respiratory tract. This explains why humans are
demic (the highest level alert). By August 2010, the number
rarely infected with the H5N1 strain. However, when the
exposure is intense, the virus is able to reach the alveoli and
ing was rescinded. As of this writing in February 2016, the
causes severe pneumonia.
number of cases in the United States and worldwide have
The virulence of the H5N1 strain is significantly greater
than the H1N1 and H3N2 strains that have been causing
significantly declined.
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