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Micro Notes
CHAPTER 24
1. Ten unique characteristics of viruses that add significantly to their medical impact and complications
include the following: 1) viruses are not cellular; 2) viral multiplication is completely dependent on
infection of a living host cell; 3) the viral genome can consist of either DNA or RNA; 4) the viral
genome may be either single- or double-stranded; 5) viruses lack the machinery for protein synthesis
(i.e. ribosomes); 6) viruses lack enzymes for metabolic processes including ATP synthesis; 7) in their
basic structure, virus particles consist of only nucleic acid and proteins; 8) viruses are ultra
microscopic in size and typically less than 500 nm; 9) existing viruses infect every type of cell; and
10) viruses can take control of the host genetic material to regulate the synthesis and assembly of
new virus particles.
2.
a. Specific receptors on the surface of viruses dictate their interactions with host cells.
b. The symptoms of viral diseases are a result of damage caused to host cells and the response of
the host to viral infection. These symptoms commonly include rashes, fever, muscle aches,
respiratory involvement, and swollen glands.
c.
The pathological effects of viruses vary based on the cell serving as a host, the cytopathic effects
of the virus, and the ability or inability of the virus to initiate persistent infection or transformation
of the cell.
d. During a chronic viral infection, the virus is detectable in tissue samples and is still multiplying at a
slow rate, but symptoms of infection are mild or absent. By contrast, in a latent infection, the
virus has entered a dormant phase inside the host cell and has become completely inactive. It is
not multiplying, generally is not detectable, and does not cause any symptoms.
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3. The chart below outlines the target organs and general symptoms of DNA viral infections.
Viral Infection
Target Organs
Symptoms
Variola infection (Smallpox)
Epidermal cells and
subcutaneous connective
tissue
Fever, malaise, prostration,
rash, toxemia, shock,
intravascular coagulation
Molluscipoxvirus infection
(Molluscum contagiosum)
Skin cells
Skin lesions
Herpes simplex 1 and 2
infection
Skin cells, rarely sensory
nerves
Lesions on the face (HSV1)
or genitalia (HSV2)
Varicella-Zoster virus:
Chickenpox (Shingles after
reactivation of latent infection)
Skin (sensory nerves)
Macular, popular, vesicular
rash on entire body
(vesicular-papular rash on
chest and back)
CMV (Cytomegalovirus)
disease
All organs
Varies, but severe cases may
involve enlarged liver and
spleen, jaundice, capillary
bleeding, and microcephaly in
newborns. Infection in adults
can involve fever, diarrhea,
hepatitis, and pneumonia.
Epstein-Barr virus infection
(infectious mononucleosis)
Epithelium of oropharynx,
parotid gland, B cells
Sore throat, high fever,
cervical lymphadenopathy
Hepatitis B infection
Liver
Jaundice, decreased liver
function
Adenovirus infection
Respiratory system
Fever, rhinitis, cough, lesions
of the conjunctiva
Human papillomavirus
infection
Skin and mucous membranes
Warts (common and genital)
Parvovirus infection
Skin cells, erythrocytic stem
cells
Rash on the cheeks
4.
a. DNA viruses (with the exception of Pox viruses) multiply within the host cell’s nucleus while RNA
viruses (with the exception of retroviruses) tend to be replicated and assembled in the cytoplasm.
b. In addition to active, lytic infection, DNA viruses can be carried in two ways: 1) as chronic
infections, in which the virus is multiplying at a slow rate but symptoms of infection are mild or
absent, and 2) as latent infections, when the virus is completely inactive, is not multiplying, and
causes no symptoms.
5.
a. Smallpox is no longer an active disease, but when the virus was present in the population, it was
considered one of the world’s deadliest diseases. Smallpox exposure usually occurs through
inhalation of droplets or skin crusts and initial symptoms include fever, malaise, prostration, and a
rash from which the disease gets its name.
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b. Variola major, the more severe form of the disease, is marked by toxemia, shock, and
intravascular coagulation, all part of the high mortality rate seen with smallpox.
c.
A pock is a skin pustule which leaves a small, depressed scar upon healing. Viral infection first
causes a macular rash which in turn becomes popular, vesicular, and pustular before finally
crusting over and leaving small unpigmented sites pitted with scar tissue.
d. Instead of an attenuated or inactivated strain of variola, the smallpox vaccine actually utilizes
vaccinia, a close relative of the smallpox virus.
e. The vaccination for the vaccinia virus also provides protection against smallpox and other
poxvirus infections for at least ten years.
6.
a. Molluscum contagiosum is marked by smooth, waxy nodules on the face trunk and limbs.
b. Warts, caused by infection with human papillomaviruses, could be easily mistaken for the nodules
of molluscum contagiosum.
c.
The discontinuation of the smallpox vaccination has led to a population with no acquired immunity
to any of the pox viruses. This is unimportant in the case of smallpox, which no longer exists in
the wild, but has led to an increase in monkeypox infection.
7. Although members of the herpesvirus family differ in many of their characteristics, they all share the
ability to cause recurrent, persistent, and latent infections.
8.
a.
HSV-1
HSV-2
Skin lesions
Type of disease
Skin lesions
Usually oropharynx, rarely
genitalia
Body areas affected
Usually genitalia, rarely oral
mucosa
Herpatic keratitis; herpetic
whitlows; approximately 30%
of cases of neonatal
encephalitis
Possible complications
Approximately 70% of cases
of neonatal encephalitis;
herpetic whitlows
b. Newborns infected with herpes have no acquired immunity and are therefore protected only by
passive immunity from their mother. This lack of a robust immune response is responsible for the
severity of symptoms in the disease.
c.
Diagnosis usually begins with observation of symptoms. Both oral and genital herpes manifest
one or more small fluid-filled vesicles which burst and crust over. Scrapings from these lesions
may be stained and observed with light microscopy to reveal multinucleate giant cells typical of
herpes infections. To confirm the diagnosis and determine the type of herpes, subtyping may be
performed with fluorescent antibodies and specific DNA testing.
The most effective treatments and preventives are the “cyclovirs” such as acyclovir, famciclovir,
and valacyclovir. These may be prescribed in topical or oral drug form. Some over the counter
ointments may be of some use in relieving pain and shortening the time of infection.
d. Recurrent attacks of HSV originate in the tendency of herpes simplex viruses to multiply in
sensory neurons during an attack and subsequently enter a latent period in the nerve trunks and
ganglia cells. They remain in an inactive state until some stimulus causes them to become
reactivated and migrate back to the original site of infection and cause symptoms. Some of the
factors that appear to stimulate recurrency are trauma, sunburn, stress, and fever.
9.
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a. Chickenpox is the result of initial infection with the varicella-zoster virus. Shingles is reactivation
of the same virus.
b. Shingles lesions appear on the skin of the face and trunk as the virus migrates down the
trigeminal and thoracic nerves. Lesions appear on the skin of areas innervated by these nerves,
producing an asymmetrical rash characteristic of the disease.
c.
Both ZIG and VZIG are specific immune globulins that work by vastly increasing the titer of
varicella-zoster specific antibodies in the blood. They generally lessen the symptoms of infection
but do not entirely prevent disease.
10.
a. Cytomegalovirus targets the liver, spleen, lungs, blood cells, and retina.
b. Transmission of CMV usually involves sexual contact, vaginal birth, transplacental infection,
blood transfusion, and organ transplantation
c.
The three groups at highest risk for disease are fetuses, newborns, and immunodeficient adults.
11.
a. The Epstein-Barr virus is associated with infectious mononucleosis, Burkitts lymphoma, and
nasopharyngeal carcinoma.
b. Burkitts lymphoma is thought to arise as a persistent EBV infection in young children and give
rise to transformed B lymphocytes while other chronic infections overwhelm the T-cell effecter
response needed to stop the overgrowth of these unhealthy B-cells.
c.
Due to improvements in sanitation and less frequent and intimate contact with other children,
70% of Americans enter young adulthood never having had HBV infection. The more intimate
associations common to adulthood lead to a much greater rate of infection such that 90% to 95%
of all people show serological evidence of infection by midlife.
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12.
Virus Name
HSV-1 / HSV2
Varicellazoster
Cytomegalovirus
Epstein-Barr
virus
HHV-6 /
HHV-7
Tissue
infected
Oral (HSV-1)
and genital
(HSV-2)
mucosa
Skin,
sensory
nerves
Liver, spleen,
lungs, blood cells,
and retina
Lymphoid
tissue and
salivary glands
Lymphoid
tissue and
salivary
glands
Names of
diseases
Oral herpes,
genital herpes
Chicken
pox,
shingles
CMV disease
Mononucleosis
Roseola
Special
characteristics
Complications
include
whitlows and
neonatal
encephalitis
Shingles is
reactivation
of latent
viral
infection
Far more virulent
in fetuses,
newborns, and
immunosuppressed
Complications
include Burkitts
lymphoma and
nasopharyngeal
carcinoma
May be
linked to
development
of multiple
sclerosis
and
Hodgkins
lymphoma
13.
a. Viral hepatitis is an inflammatory disease of the liver in which the death of hepatocytes and the
mononuclear response swells and distorts the liver, interfering with the excretion of bilirubin into
the intestine. When bilirubin accumulates in the blood and tissue, it lends a yellow tinged
appearance (jaundice) to the skin and eyes. Hepatitis (HAV) is an RNA virus responsible for
infectious hepatitis, which is spread via contaminated food and water and is the least severe of
the three main hepatitis viruses. HBV is a DNA virus responsible for serum hepatitis. It is
contracted through close (often sexual) contact and can lead to chronic hepatitis and hepatic
cancer. HCV is an RNA virus responsible for post-transfusional hepatitis. It is contracted in the
same manner as HBV and leads to chronic inflammation of the liver and cirrhosis.
b. The delta agent (hepatitis D virus) is a defective RNA virus that can only cause infection when a
cell is also infected with HBV.
14.
a. Most of the genome of the hepatitis B virus consists of a double stranded DNA molecule, but a
portion of the genome exists as single stranded DNA.
b. Because HBV cannot be grown in culture, viral fragments, antibodies, and in some cases entire
virions, all of which have been isolated from the blood of infected patients, are used as the
primary source of viruses for study.
c.
People infected with HBV usually have a very high viral load, and, because the infectious dose for
the virus is very low, infection is very easily accomplished. Care must be taken not to share items
such as toothbrushes or razors. Mosquitoes, too, have been known to serve as vectors.
d. People most at risk for contracting hepatitis B include those living under crowded conditions, drug
addicts, the sexually promiscuous, and people whose occupation requires them to routinely
handle blood or blood products.
15.
a. The hepatitis B virus enters the body through a break in the skin or by injection into the
bloodstream. Eventually it reaches the liver cells where it multiplies and releases viruses into the
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blood during an incubation period of 4 to 24 weeks. Most people exhibit few overt symptoms and
eventually develop immunity to HBV. Others experience malaise, fever, chills, anorexia,
abdominal discomfort, and diarrhea. Some of these persons will go on to develop fever, jaundice,
rashes, and arthritic symptoms, while an even smaller subset will develop glomerulonephritis and
arterial inflammation. The hepatitis B virus can leave the body through bloodletting procedures
and can also be passed to neonates during the birth process.
b. The most serious complications of HBV infection include chronic liver disease in the form of
necrosis or cirrhosis, arthritic symptoms, and a vastly increased risk of liver cancer.
16.
a. Two types of vaccines exist for HBV. The first (Recombivax, Energix) contains HBV surface
antigens which have been grown in yeast. The second (Heptavax) is made from the purified
sterile antigen extracted from the blood of carriers. It is used primarily for people who are allergic
to yeast.
b. Vaccination is required for medical and dental workers, students, patients receiving multiple
transfusions, immunodeficient persons, and cancer patients. It is now strongly recommended as
part of the routine childhood immunization schedule.
17.
a. Adenoviruses cause conjunctival infections, mild respiratory diseases (colds), and acute
hemorrhagic cystitis.
b. Adenoviruses are spread via respiratory and ocular secretions.
18.
a. Painless, elevated, rough growths on the fingers or other parts of the body are known as
common, or seed, warts. Plantar warts are deep, painful papillomas on the soles of the feet.
Genital warts have a varying morphology but appear on the genitals.
b. Genital warts are most serious because they carry with them a greatly increased risk of cervical
and penile cancer.
c.
Warts are caused by the human papilloma virus (HPV). The lesions of warts are typically fleshy,
skin-colored growths that range from flat and smooth to raised irregular masses. To confirm the
diagnosis, a biopsy may be done to look for signs of the virus. This may require examination with
an electon microsope to identify the virus directly. A pap smear of the cervix can reveal
abnormal cells that are often a feature of genital warts. Another useful test involves a DNA probe
to detect HPV DNA. This is currently a screening test given as part of the gynecological exam to
rule out the presence of HPV in women.
d. These are recombinant vaccines carrying a surface antigen from the virus (the whole virus is not
used). Both Gardasil and Cervarix contain the antigens of the most common strains of viruses
involved in cervical cancer. The vaccines are meant to be a preventive for HPV infections in girls
and young women that will protect them against getting cervical cancer. It is effective only if given
before they have become infected.
19. The most important human parvovirus is B19, the cause of erythema infectiosum.
20.
a. Oncogenic DNA viruses are Epstein-Barr virus, human herpesvirus 8, hepatitis B virus, and
humanpapilloma virus (HPV).
b. Sexually-transmitted DNA viruses are molluscum contagiosum, herpes simplex, type 2, hepatitis
B virus, and some types of humanpapilloma virus.
c.
Those spread by blood or blood products include EBV, HBV, and HCV.
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d. DNA viruses spread by respiratory droplets, kissing, and other nonsexual intimate contact:
Smallpox, molluscum contagiosum, monkeypox, HSV-1, HSV-2, varicella-zoster,
cytomegalovirus, EBV, HBV, HCV, adenovirus, HPV.
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