Download Viruses: Bacterial and Animal

Chapter 14: Animal Viruses
How do animal viruses differ
from bacterial viruses?
• Attachment or entry into the cell
• Replication of viral nucleic acid (remember
eukaryotic cells have a nucleus)
• Uncoating step is required by animal
viruses
• Exit the host cell by budding or shedding
Entry by membrane fusion
Entry by endocytosis
Replication of nucleic acid and
generation of mRNA
Release by budding
Acute viral infections
• Usually short in duration
• Host develops long lasting immunity
• Infection of the virus results in a productive
infection…host cells die as a result of
infection
General Steps of Acute Viral
infection
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Attachment
Entry into host cell
Targeting where it will reproduce
Uncoating of the capsid
Synthesis of proteins, replication of nucleic acid
Maturation
Cell lysis
Can you identify some examples of viruses
that produce an acute viral infection?
• Are they naked viruses, or viruses with
envelopes?
Persistent infections
• Virus is continually present in the body,
released by budding
• Three categories
– Latent infections
– Chronic infections
– Slow infections
Latent Infections
• Persistent infection with symptomless
period followed by reactivation of virus and
symptoms
• Example of latent viruses are found in the
family Herpesviridae
– Herpes simplex virus -1
– Herpes simplex virus -2
Latent Viral infections
• All of these viruses are in the Herpesviridae
family
Herpesviridae Family
• Double stranded DNA (dsDNA),
enveloped virus
-herpes simplex 1(cold sores)
-herpes simplex 2 (genital herpes)
-Varicella-zoster virus (chicken pox,
shingles)
-Epstein-Barr (infectious mono and
Burkitt’s lymphoma)
Herpes Simplex virus-1
HSV-1 reactivation
Herpes simplex-1
• HSV-1 causes fever
blisters, HSV-2 genital
herpes
• Symptoms: fluid filled
skin lesions
• Treatment: Acyclovir
Varicella (chickenpox) and
Herpes Zoster (Shingles)
• HSV-3 causes chicken
pox and latent
activation known as
shingles
• Acquired by
respiratory route, 2
weeks later see
vesicles on skin
• Vaccine established in
1995 for chickenpox
Epstein Barr
• Causes infectious
mononucleosis
• Acquire by saliva,
incubation period is 4-7
weeks
• Identify by
-lobed lymphocytes
-heterophile antibodies
-fluorescent antibody tests
Chronic infections
• Infectious virus present at all times
• Disease may be present or absent
• Best example is Hepatitis Type B virus
– Carriers produce virus detected in blood, saliva,
and semen
– Unique replication of dsDNA
Chronic Viral infections
Hepadnaviridae family
• dsDNA virus, enveloped
• Hepatitis B
-passes through
intermediate stage (RNA)
-three particles in blood
Dane
filamentous
sphericle
-exposure through
blood/body fluids
Hepatitis B
• Incubation period is ~12 weeks
• 10% of cases become chronic, mortality rate
is less than 1%
• About 40% of the chronic cases die of liver
cirrhosis
Flaviviridae (+) ssRNA, enveloped
• Hepatitis C virus
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–
–
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Obtain from blood/body fluids
Incubation period averages 6 weeks
Hard to screen blood for the virus
85% of all cases become chronic
Slow Infections
• Infectious agent increases in amount over a
long time during which there are no
symptoms
• Examples are HIV found in the Retroviridae
family
• Retroviruses use reverse transcriptase to
replicate ssRNA
Retrovirus
Retroviridae-multiple strands of ()RNA
• HIV
-infects Helper T cells
-requires the enzyme
reverse transcriptase
-integrates as a provirus
-is released by budding,
or lyses the cell
Viruses and tumors
• dsDNA viruses are most common to cause
viral-induced tumors
• Cancer is result of integration of viral genes
into the host chromosome
• Transforming genes are called oncogenes
• Examples: papillomavirus, herpesvirus
Viruses associated with cancers
If multiple forms infect one
cell…reassortment can
occur
Orthomyxoviridae-multiple
strands of (-)RNA
• Influenza virus
– Consists of 8 segments of RNA
– Envelope has H spikes (hemagglutinin) and N
spikes (neuraminidase)
– Incubation is 1-3 days
– Symptoms include: chills, fever, headache,
muscle aches, may lead to cold-like symptoms
Influenza virus
Ways to study viruses
• Since viruses grow in living cells….need a
live cell to culture them
– Cell culture/tissue culture
– Embryonated chicken eggs
Cell Culture
Quantitation of viruses: count
plaques
Proteinaceous infectious
particles: PRIONS
• 1982 Stanley Prusiner proposed that there
were infectious proteins
• Caused the disease “scrapie” in sheep
• Caused the “mad-cow”disease in 1987
• Human forms suggest a genetic component
Prions
• Contain no nucleic acid
• Abnormal protein promotes conformational
change to normal protein
• Results in damage to
neurons…transmissible spongiform
encephalopahthies
Brain with spongiform
encephalopathy
Infections caused by prions
Mechanism of prion replication
Picornaviridae (+) ssRNA
• Hepatitis A
-obtain through fecal-oral route, enters GI tract
and multiplies
-incubation period is ~4 weeks
-symptoms include: anorexia, malaise, nausea,
diarrhea, abdominal discomfort, fever, and chills
lasting 2-21 days
Rhabdoviridae (-)ssRNA, enveloped
• Rabies virus
-enters the skin and multiplies in skeletal
muscle and connective tissue
-virus travels along nerves to the CNS
causing encephalitis
Pathology of rabies