Page 387 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com significant concern in those countries that variant CJD may mebooksfree.com
PART IV Clinical Virology
376
It is unknown how many people harbor the pathogenic
emerge in humans. Two cases of BSE in cattle in the United
prion in a latent (asymptomatic) form. The possibility that
States have been reported.
there may be people who are asymptomatic carriers of the
vCJD prion and who could be a source for infection of oth-
ers (e.g., via blood transfusions) has led blood banks in the
Chronic Wasting Disease
United States to eliminate from the donor pool people who
Chronic wasting disease (CWD) of deer and elk is a
have lived in Great Britain for more than 6 months.
prion-mediated disease that exists in the United States.
Because vCJD is strongly suspected to be transmitted by
SLOW DISEASES OF ANIMALS
SELF-ASSESSMENT QUESTIONS mebooksfree.com
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mebooksfree.com mebooksfree.com mebooksfree.com ingesting meat, there is concern regarding the conse- mebooksfree.com
quences of eating deer and elk meat (venison). In 2002, it
The slow transmissible diseases of animals are important
was reported that neurodegenerative diseases occurred in
models for human diseases. Scrapie and visna are diseases
three men who ate venison in the 1990s. One of these
of sheep, and BSE (mad cow disease) is a disease of cattle
diseases was confirmed as CJD. Whether there is a causal
that appears to have arisen from the ingestion of sheep tis-
relationship is unclear, and surveillance continues. This
sue by the cattle. Chronic wasting disease occurs in deer
concern was heightened in 2006 when prions were
and elk. Visna is caused by a virus, whereas the other three
detected in the muscle of deer with CWD but not in the
are prion-mediated diseases.
muscle of normal deer.
Scrapie
Scrapie is a disease of sheep, characterized by tremors,
ataxia, and itching, in which the sheep scrape off their
wool against fence posts. It has an incubation period of 1. Regarding “slow viruses” and their diseases, which one of the fol-
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mebooksfree.com mebooksfree.com mebooksfree.com (B) PML is caused by a virus that causes widespread inapparent mebooksfree.com
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many months. Spongiform degeneration without inflam-
lowing is the most accurate?
(A) The viruses that cause slow diseases, such as progressive multi-
mation is seen in the brain tissue of affected animals. It has
focal leukoencephalopathy (PML), have a slow rate of replica-
been transmitted to mice and other animals via a brain
tion that accounts for the long latent period and slow
extract that contained no recognizable virus particles.
progression of the disease.
Studies of mice revealed that the infectivity is associated
with a 27,000-molecular-weight protein known as a prion
infections early in life but causes the disease PML primarily in
(see page 229).
people with reduced cell-mediated immunity.
(C) Creutzfeldt-Jakob disease (CJD) is caused by CJ virus, a retro-
Visna
virus that integrates a DNA copy of its genome into the DNA
of brain neurons.
Visna is a disease of sheep that is characterized by pneumo-
(D) CJD occurs primarily in immunocompromised people, but
nia and demyelinating lesions in the brain. It is caused by
infection with the virus that causes CJD is common, as evi-
visna virus, a member of the lentivirus subgroup of retrovi- 2. Regarding prions, which one of the following is the most accurate?
denced by the presence of antibodies.
ruses. As such, it has a single-stranded, diploid RNA
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mebooksfree.com mebooksfree.com mebooksfree.com (B) Prion proteins are characterized by having changes in confor- mebooksfree.com
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genome and an RNA-dependent DNA polymerase in the
(A) The genome of prions consists of a negative-polarity RNA that
virion. It is thought that integration of the DNA provirus
has a defective polymerase gene.
into the host cell DNA may be important in the persistence
of the virus within the host and, consequently, in its long
mation from the alpha-helical form to the beta-pleated sheet
incubation period and prolonged, progressive course.
form.
(C) Prions are very sensitive to ultraviolet (UV) light, which is why
UV light is used in hospital operating rooms to prevent their
Bovine Spongiform Encephalopathy
transmission.
BSE is also known as mad cow disease. The cattle become
(D) The main host defense against prions consists of an inflam-
aggressive, ataxic, and eventually die. Cattle acquire BSE by
matory response composed primarily of macrophages and
eating feed supplemented with organs (e.g., brains)
CD4-positive T cells.
obtained from sheep infected with scrapie prions. (It is also
which one of the following is the most accurate?
possible that BSE arose in cattle by a mutation in the gene 3. Regarding progressive multifocal leukoencephalopathy (PML),
(A) It is caused by a defective mutant of measles virus.
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encoding the prion protein.)
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mebooksfree.com mebooksfree.com mebooksfree.com (D) Acyclovir is the drug of choice for patients in the early stages of mebooksfree.com
(B) The virus remains latent in hepatocytes for many years.
BSE is endemic in Great Britain. Supplementation of
(C) Lesions occur in several areas of the brain, resulting in diverse
feed with sheep organs was banned in Great Britain in 1988
symptoms.
and thousands of cattle were destroyed, two measures that
have led to a marked decline in the number of new cases of
PML.
BSE. BSE has been found in cattle in other European coun-
(E) It is characterized by an inflammatory reaction in the brain
tries such as France, Germany, Italy, and Spain, and there is
containing many neutrophils.
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