test 1 Flashcards
(266 cards)
1
Q
Viruses are composed of _______
A
1) genome composed of ONE type of nucleic acid (DNA or RNA)
2) Protective protein or lipoprotein coat surrounding the genome
2
Q
where do viruses replicate
A
ONLY inside living cells- viruses are completely dependent on cellular synthetic machinery for replication (no cellular organelles)… Also, viruses cannot capture and store energy
3
Q
what do viruses use for replication
A
host cell synthetic machinery
4
Q
can viruses survive and replicate in cell free media
A
NO (only bacteria can)
5
Q
What is the purpose of the protective coat that surrounds the genome
A
made of proteins or lipoproteins, the coating allows the viruses to survive in the environment. Also, it GIVES THE VIRUS INFECTIVITY- without the glycoproteins attaching to cellular receptors, the virus will not be able to enter the cell
6
Q
can a naked viral genome infect the cell?
A
NO- the naked viral genome by itself is not infectious
if the naked viral genome is artificially transfected (forced in), then it will be allowed to replicate the virus
7
Q
what is a virion?
A
a mature virus particle
8
Q
what makes up a virion?
A
nucleic acid genome (either DNA or RNA) and a capsid
9
Q
what is a capsid?
A
protein coating that protects the genome, composed of capsomeres
10
Q
what are capsomeres?
A
proteins that compose the capsid (polypeptides that are held together by non-covalent bonds)
11
Q
Functions of the viral capsid
A
1) protects the genome from the environment
2) provides structural symmetry… creates stability
3) allows viruses to enter by recognizing and attaching to host cell receptors
4) Aids in virus replication- may contain enzymes necessary for virus replication (i.e. RNA transcriptase)
5) capsid proteins are very immunogenic
12
Q
what is a nucleocapsid
A
aka the virion- capsid plus the nucleic acid genome
13
Q
What distinguishes enveloped viruses and non-enveloped viruses
A
an extra outer coat to some viruses (enveloped viruses). Non-enveloped viruses are called NAKED VIRUSES
14
Q
where is the envelope derived from
A
the host cell membrane (plasma or nuclear), acquired when the virus exits infected cells by budding
15
Q
when does the virus acquire an envelope?
A
during a process called budding
16
Q
what happens before budding
A
the virus inserts viral glycoproteins (PEPLOMERS) into the cellular lipid bilayer
17
Q
where does the lipid bilayer envelope for viruses originate? Where do peplomers originate?
A
Lipid bilayer is of CELLULAR origin
the peplomers are of VIRAL origin
18
Q
Functions of the viral envelope
A
1) provides structural symmetry
2) allows virus to enter cell (peplomers attach to host cell receptors)
3) may contain enzymes essential for viral replication
4) peplomers are very immunogenic (because they are virally produced)
19
Q
4 important criteria for viral classification
A
size of the virion
symmetry of the nucleocapsid (icosahedral, helical, or complex)
presence of envelope?
type of nucleic acid
20
Q
size of the virion
A
most important tool used to classify viruses
Viruses are expressed in NANOMETERS!!!!!!
21
Q
what term describes the ability to distinguish one object from another?
A
Resolving power
normal eye ~250um
light microscope is 1000X better: ~.250um
electron microscope is 1000X better: ~0.250 nm (but the practical limit of EM is ~2.5 nm)
22
Q
what is the smallest virus that light microscopy can see?
A
Poxvirus (350 nm)
23
Q
what are the three types of nucleocapsid symmetries?
A
cubic/icosahedral
helical nucleocapsid
complex symmetry
24
Q
Symmetry of nucleocapsid: Cubic symmetry
A
AKA Icosahedral
20-sided structure consisting of 20 equilateral triangles, 12 vertices
5:3:2 rotational symmetry
25
what makes up the triangles of the icosahedral symmetry
symmetrical clusters of capsomeres (Pentons and hexons)
26
What are pentons
term describing capsomeres situated at the 12 vertices of icosahedral symmetry nucleocapsids
27
What are hexons?
term describing capsomeres in icosahedral symmetry that are not at the vertices (surrounded by 6 neighboring capsomeres)
28
What is the significance of helical nucleocapsid?
ALL ANIMAL viruses with helical nucleocapsids are ENVELOPED
29
What is the only known animal virus with complex symmetry?
Poxvirus
30
types of nucleic acids in viruses
DNA or RNA
Single stranded or double stranded
linear or circular genome
segmented or non-segmented genome
31
what are the 2 classifications of RNA virus genomes that are single-stranded
positive polarity and negative polarity
32
if an RNA virus has a positive polarity, how is it translated?
it will be translated directly into protein by cellular ribosomes
33
if an RNA virus has a negative polarity, how is it translated?
negative polarity genomes must be transcribed into a COMPLEMENTARY positive strand before it is translated into protein
34
what enzyme converts strands from negative to positive polarity?
polymerase enzyme (ex: RNA dependent RNA polymerase)
35
for negative polarity RNA viruses, is cellular polymerase sufficient for transcription to its complementary positive strand?
NO
we need RNA dependent RNA polymerase (not made by cell)
the viruses bring the polymerase into the cell
36
Virus replication steps
1) attachment of virus to cells
2) entry of virus into cell
3) uncoating of virus
4) Early transcription of viral nucleic acid into viral mRNA
5) translation of early viral proteins (enzymes) from viral mRNA
6) Transcription of parental genomes into progeny genomes (DNA -> DNA; RNA -> RNA)
7) translation of late structural proteins
8) virus assembly within the cell
9) release of virus from the cell (lysis or budding)
37
Virus attachment or Adsorption into the cell
random
nonspecific
reversible
38
what initiates attachment/adsorption
electrostatic attraction
39
when does the attachment phase become specific?
when the capsid proteins (if naked virus) or peplomers (if enveloped virus) recognize complementary cell membrane receptors
40
what is viral tropism?
the affinity of viruses to only specific cells because of the presence of the specific receptors on certain cells
41
What 2 methods can a virus enter cell?
Viropexis: virus is engulfed by the cell membrane, enters the cytoplasm inside a phagocytic vacuole (primary way that most naked icosahedral viruses enter)
Fusion: viral envelope fuses with cell membrane (occurs with many enveloped viruses)
42
What initiates fusion of the virus envelope with the cell membrane?
the peplomers initiate fusion
43
What is viral uncoating?
the physical separation between viral nucleic acid and the viral capsid. Once a virus uncoats, the virus is no longer infectious
44
if an infected cell is broken up before the virus uncoats, will the virus still be infectious?
yes, the virus can still go infect other cells
45
if an infected cell is broken up after all virus particles have uncoated, is it still an infectious virus?
NO, you need a capsid to be infectious
46
when is viral uncoating complete?
when intracellular infectious virus particles cannot be detected in the infected cell, but a viral genome can be detected by PCR
47
what are the 2 mechanisms for uncoating
1) Digestion: digestion of viral protein coat by CELLULAR proteases
2) Conformational changes: changes in polypeptides that make up the capsomeres (due to changes in pH and ion concentration in the cytoplasm--> causes dissolution of the non-covalent bonds)
48
where do RNA viruses uncoat and replicate?
cytoplasm
49
where do DNA viruses uncoat and replicate?
uncoat in the vicinity of the nuclear pores, and uncoated genome will be immediately transported into nucleus for replication
50
what is the eclipse period?
the period where the infectious virus cannot be detected or demonstrated within the cell
begins when uncoating of tyhe virus is complete, ends with the formation of the first infectious virus progeny
51
what happens during the eclipse period
transcription of the viral genome and translation of the structural viral proteins (production of progeny viruses)
52
How do viruses usually produce the enzymes that are needed for viral replication?
the enzymes needed are produced from mRNA created during the early transcription phase of viral genomic nucleic acids into viral mRNA (mRNA is produced to go to the cytoplasm and be translated into these enzymes)
53
what is the major early protein/enzyme that is made with viruses
polymerase enzymes
54
where are the early proteins produced?
in the cytoplasm, using the host cell's ribosomes
55
for RNA viruses with positive polarity genomes, does transcription need to occur?
NO!!
POSITIVE POLARITY RNA genomes serve as the mRNA, so it directly binds the cellular ribosomes and is translated into the proteins
NO need for transcription for ss(+)RNA viruses
56
for RNA viruses with negative polarity genomes, does transcription need to occur? How does it occur?
YES
the viral RNA must be transcribed into a POSITIVE complementary strand
to do this, the virus needs RNA-dependent RNA-polymerase
PROBLEM: there is no mRNA present yet to produce the polymerase
SOLUTION: negative polarity RNA viruses carry in the RNA dependent RNA polymerase
57
for DNA viruses, does transcription need to occur?
the DNA genome is directly transcribed by CELLULAR DNA dependent RNA polymerase to yield viral mRNA
AGAIN: DNA viruses use host cell polymerase to produce the viral mRNA
the viral mRNA then leaves the nucleus and travels to ribosomes in the cytoplasm for translation of early viral proteins
58
Where does translation of viral proteins occur?
cytoplasm
59
what early protein is translated in the case for RNA viruses?
RNA dependent RNA polymerase
negative polarity RNA strands bring in some of this, but it makes more for faster replication
positive polarity RNA strands do not bring this into the cell, so all of the polymerase for pos. strands is produced this way
60
What early protein is translated in the case for DNA viruses?
DNA dependent DNA polymerase
this is needed for DNA genome replication
61
other than just polymerases, what is the function of other proteins produced in translation of early viral proteins?
disrupt cellular protein translation so that cells focus on making viral proteins
regulate how the viral genome is transcribed
upregulate or down-regulate cellular proteins
62
Late transcription of the parental genome into progeny genomes: positive polarity RNA genomes
VIRAL RNA dependent RNA polymerase transcribes the parent strand (+ ss RNA) into a complementary (-)RNA strand, and then the negative strands are each transcribed into many (+)RNA progeny strands
63
Late transcription of the parental genome into progeny genomes: negative polarity RNA genomes
VIRAL RNA dependent RNA polymerase transcribes the (-)RNA parent strand into a complementary +RNA strand. The positive strand is then transcribed into many progeny (-) RNA strands (using RNA dependent RNA polymerase again)
64
How many times is RNA dependent RNA polymerase used to produce progeny RNA genomes?
twice
1) produces complementary strand
2) takes complementary strand and transcribes progeny strand
65
for DNA viruses, how is replication of the genome done?
the parental ds DNA genome is uncoiled and replicated into many progeny by VIRAL DNA dependent DNA polymerase
66
where are DNA virus genomes replicated?
nucleus
67
How do retroviruses work?
retroviruses carry into the cell a ss RNA molecule
| the RNA is copied into a double stranded DNA molecule by reverse transcriptase (brought in by the virus)
68
what is reverse transcriptase?
RNA dependent DNA polymerase
69
What are the late viral proteins that are translated?
structural proteins: polypeptides that will produce capsomeres and peplomers, depending on whether the virus is enveloped or not
70
what is the mRNA used to translate the late viral proteins?
for +RNA viruses, proteins are translated directly from progeny RNA strands
for -RNA viruses, progeny strands are transcribed to a positive polarity strand, and this complementary strand is used to be translated to the proteins
for DNA viruses, late proteins are translated from viral mRNA derived from the progeny DNA genomes
71
What are the 2 stages of virion assembly?
Morphogenesis and encapsidation
72
What is morphogenesis?
process by which the viral capsid polypeptides come together and for assembled capsomeres and intact capsids (with no genome yet)
73
is morphogenesis spontaneous?
yes
74
what is encapsidation?
follows morphogenesis
| process where the viral genome is inserted into the already formed capsid
75
does the capsid form around the genomes?
NO!!! the genome gets inserted into pre-formed capsids
76
What is released with many intact viruses?
many non-infectious, empty capsids
| not all pre-formed capsids get encapsidated by genomes
77
where does assembly of the virion take place?
where the virus replicates
for RNA viruses, assembly takes place in the cytoplasm
for DNA viruses, assembly takes place in the nucleus
78
What are the 2 ways that a virus can be released from the host cell?
Lysis: physical bursting of the infected cell
Budding: opposite of phagocytosis
79
where does budding take place?
areas of the plasma or nuclear cell membrane that has been modified by the insertion of viral proteins
80
is budding destructive?
no
81
what is yield?
the number of infectious virus particles produced from each cell
82
replication of Picornavirus (foot and mouth disease)
| non-enveloped (+) ssRNA virus with icosahedral capsid
1) attaches to specific cell receptors on the cell surface, enter via viropexis and uncoats in cytoplasm
2) +RNA virus, so no transcription needed
3) RNA dependent RNA polymerase translated form parental genome (parental genome serves as mRNA)
4) viral RNA dependent RNA polymerase binds to parent RNA strand and makes complementary negative strands, then does the same to the complementary strands to make progeny viral genomes
5) progeny strands are then translated by cell ribosomes into one long polypeptide strand tha tis then cleaved by viral proteases
6) morphogenesis of progeny occurs in cytoplasm, then encapsidation of progeny genome
7) virus is released via cell lysis
83
Replication of Herpesvirus
| enveloped ds DNA virus with icosahedral capsid
1) peplomers bind and attach to cellular receptors, herpesvirus enters by fusion and uncoats in the vicinity of nuclear pores
2) uncoated DNA enters the nucleus before it is digested by cytoplasmic enzymes
3) DNA genome is transcribed into viral mRNA in the nucleus by host DNA dependent RNA polymerase, then the mRNA moves to cytoplasm to be translated into viral DNA dependent DNA polymerase and thymidine kinase... these then go back to the nucleus
4) polymerase and thymidine kinase initiate massive round of viral DNA replication in the nucleus to produce progeny viral DNA genomes
5) mRNA is transcribed from progeny DNA and goes to cytoplasm for translation into viral structural proteins (that move back to nucleus)--> capsomeres and peplomers
6) morphogenesis occurs in nucleus, followed by DNA genome insertion via encapsidation
7) virus leaves cell after viral peplomers are inserted into the nuclear membrane--> then virus leaves cell by budding, acquires envelope from nuclear cell membrane
84
Replication of retroviruses- what makes them special?
enveloped RNA viruses with helical nucleocapsid surrounded by icosahedral capsid
genome is single stranded and diploid
85
Replication of retroviruses
1) peplomers on envelope recognize and attach to CD4 receptors on Th lymphocytes, enter cell by fusion, uncoats in cytoplasm
2) Reverse transcriptase (RNA dependent DNA polymerase) is brought into cell by virus... it binds to uncoated viral RNA in cytoplasm and makes complementary double stranded DNA copy of the RNA genome (viral ds DNA moves to nucleus)
3) the viral ds DNA is then ligated into the cell DNA chromosomes
4) the viral DNA is transcribed into small viral mRNAs, move to cytoplasm, and are translated into viral proteins (peplomers, capsids, RT enzyme)
5) One long mRNA strand is transcribed form the entire DNA genome--> serves as the viral genome
6) morphogenesis occurs in cytoplasm
7) the viral nucleocapsid is encapsidated into the icosahedral capsid ALONG WITH reverse transcriptase
8) viral peplomers are inserted into the PLASMA membrane and virus is released by budding
86
What is a gene?
a nucleotide sequence that encodes for a specific polypeptide or protein
87
What is a mutation/
results from changes in nucleotide sequence within viral genes
88
what are the types of mutations?
substitution mutation: one (point) or several nucleotides are substituted
deletion mutation: one or several nucleotide bases are deleted from the genome
89
What are spontaneous mutations?
occur during normal virus replication
low rate of spontaneous mutations for DNA viruses (10^-8), but high rate of spontaneous mutations for RNA viruses (10^-3)
90
What are consequences of mutations?
- host range changes: virus adapts to unnatural host
- increase or decrease in virulence
- susceptibility to drugs can change (resistance)
- antigenic makeup of virus changes
91
What is phenotypic mixing/
occurs when 2 closely related viruses infect one cell and produce different progeny viruses
92
What is a pseudotype?
virus progeny that have acquired the genotype of one parent and phenotype of the other parent (A genotype, B phenotype)
93
What is a mixed phenotype?
virus progeny that have acquired the genotype of one parent and phenotype of both parents (A genotype, A and B peplomers/phenotype)
94
Are pseudotypes and mixed phenotype viruses genetically stable?
NO
if one of these infects a cell, they will only be able to produce the phenotype of the genome they have (A genotype can only produce A phenotype)
95
How does Genetic recombination occur?
2 genetically different but related viruses infect the same cell and replicate... And they exchange nucleic acid between parental viruses
progeny viruses are then formed whose nucleic acid is a combination of both parents
96
In essence, what is a recombinant?
part of one virus' genome in the genome of another virus
97
When do recombinant viruses occur naturally?
when cellular genes are inserted into the viral genome during viral replication in animals
98
What is reassortment?
a type of genetic recombination specific to influenza virus
if a cell is infected with 2 different influenza viruses, progeny viruses have a genome that consists a mixture of segments derived from both viruses
99
How does influenza form now pathogenic strains?
genetic reassortment
100
Recombinant virus vaccines
Insertion of the GENE that encodes for an antigen (ex: G glycoprotein of rabies) into the genome of another virus (Ex: Pox virus). When the vaccine is inoculated into an animal, the rabies gG is translated along with the other pox proteins. The immune system sees the gG, resulting in humoral and cell mediated response to gG
101
Problems with Antiviral chemotherapy
>drugs can be toxic to cells and thus toxic to animals
>by the time clinical disease is evident, virus replication has already peaked and the immune response is underway
>Resistance to antiviral drugs occur quickly (RNA viruses mutate at high rates)
102
Strategy of antiviral drugs
kill the virus only, not normal cells
103
Idoxuridine and Trifluridine
analogue of thymidine which is incorporated into DNA in place of thymidine
good anti-herpesvirus drug
**Used only as ophthalmic drug to treat for herpes eye infections
converted into IdoxTP (analog of TTP and is inserted into the herpesvirus DNA genome by DNA dependent DNA polymerase--> blocks DNA replication)
104
Adenine arabinoside
nucleotide analog of dATP
inhibits DNA synthesis (inserted into genome by DNA dependent DNA polymerase--> blocks DNA replication)
105
Acyclovir
nucleoside analogue
transported into cells by guanine transporter and then phosphorylated by thymidine kinase into an active metabolite (thymidine kinase is only present in herpesvirus infected cells, so only active in infected cells)
once phosphorylated, it inhibits herpesviral DNA polymerase
ACV is also inserted into DNA terminating DNA synthesis
106
Amantadine
prevents influenza A infection
blocks the uncoating process of influenza by binding M2 protein (which is needed for uncoating)
107
Azidothymidine (AZT)
pyrimidine nucleoside analogue
inhibits reverse transcriptase
100X more affinity for HIV-RT than for DNA polymerase
108
Ribavarin
prodrug that is metabolized into nucleotides resembling RNA nucleotides
nucleotides are inserted into viral RNA, causing translation into non-functional viral proteins and non-infectious viral genomes
109
Oseltamivir (Tamiflu)
neuramidase inhibitor for Influenza A and B
prevents new viruses from emerging from infected cells
110
Protease inhibitors
inhibitor of post-translational proteases (essential for post-translational cleavage of polypeptide sequences)
inhibits the formation of infectious virus
block the late stages of viral replication
111
What are cytopathic effects
microscopically visible morphological changes in cell cultures induced by viruses
presence of CPE usually indicate virus replication
112
What causes the visible morphological changes of cytopathic effects?
due to cell dying as a result of viral infection
1) cell swelling/rounding/shrinking
2) presence of inclusion bodies within infected cells
3) cell fusion
113
What causes cytopathic effects
the shutting down of cellular protein synthesis, which occurs when viruses replicate in permissive cells
this can also occur due to virus infection into non-permissive cells (cells that the virus can't replicate in)... these effects are caused by direct toxicity of the viral capsid to the host cell
114
What is a viral plaque?
In the lab, viruses are grown on a tissue culture monolayer of cells attached to plastic container
when virus replicates in cells, it spreads to surrounding cells-> when the infected cells are killed, they shrink away from the surrounding cells and detach from the plastic. This leaves behind a small denuded area in the cell monlayer called a plaque
115
What are inclusion bodies?
intracytoplasmic or intranuclear masses observed in STAINED viral infected cells
seen both in vivo and in vitro
the inclusion bodies are very characteristic of certain viruses
116
do all viruses cause inclusion bodies?
no
117
What are the types of inclusion bodies?
Intranuclear, intracytoplasmic, or both
DNA viruses cause intranuclear inclusions (Poxvirus is an exception)
RNA viruses cause intracytoplasmic inclusion
118
What are inclusion bodies? (3 part answer)
site of virus replication where viral proteins accumulate
aggregate of complete virions (called crystalline masses)
Scars resulting from degenrative changes in the cell
119
Are inclusion bodies specific to certain viruses
yes, some viruses will exhibit very distinctive inclusion bodies--> histopathology serves as a very important criterion for diagnosing certain viral infections
120
Rabies inclusion bodies?
RV causes intracytoplasmic inclusions called NEGRI BODIES
121
What are Rabies inclusion bodies called?
Negri bodies
122
Adenovirus inclusion bodies?
Adenovirus causes intranuclear inclusions that are BASOPHILIC (in dogs, mostly in liver)
123
Distemper inclusion bodies?
Distemper causes BOTH intracytoplasmic and intranuclear inclusions in the BRAIN
causes mostly intracytoplasmic inclusions in other organs
124
What is a viral syncytium?
certain viruses induce a fusion of the plasma membranes of surrounding infected cells
results in formation of a multinucleated giant cell called a synctytia
125
what causes viral syncytium?
specific peplomers inserted into the cell membrane during viral replication
126
what viruses induce viral syncytium?
parainfluenza virus and herpesvirus
127
What are non-cytopathic viruses?
viruses that replicate in cells without dmaging or killing the cell
cause no cytopathology
128
what are the mechanisms of non-cytopathic viruses?
persistent infection
latent infection
cell transformation
129
what is a persistently infected cell?
cells that are infected and continuously release infectious virus particles while showing no cytopathology
FeLV and feline calicivirus
the non-cytopathic type of bovine viral diarrhea virus causes persistent infections in cattle
130
What is a latently infected cell?
cells that have virus in them but the VIRUS IS NOT REPLICATING
131
do latently infected cells produce infectious particles?
no, the virus is not replicating
132
when do latently-infected cells stop being latently infected?
when the virus begins to replicate and produce Cytopathic effects
133
what is the term for a virus that was in a latently infected cell and is now replicating in that cell?
viral recrudescence
134
What causes viral recrudescence?
stress, cytokines, irritation, etc.
135
what viruses cause latent infections?
retroviruses (HIV, FIV) cause latent infections in lymphocytes
herpesvirus typically causes latent infections in neurons
136
What is a virus transformed cell?
a cell that has acquired a different DYSREGULATED growth characteristic as a result of virus infection
137
what do virus transformed cells lead to?
virus transformed cells are neoplastic, cause tumors
138
Do cells become virus transformed cells due to infection by DNA or RNA virus infections?
```
BOTH
DNA viruses (herpesvirus, papillomavirus)
RNA viruses (retroviruses)
```
139
what is expressed due to the virus infection that causes the virus transformed cell?
Viruses cause the expression of one of two things:
1) expression of viral oncogene (v-onc) that is carried in by the virus as part of its viral genome. The viral products translated from these oncogenes are the transforming agents
2) expression of cellular oncogenes (c-onc)--> get turned on by the virus when it infects the cell.
140
What do cellular oncogenes cause in the virus transformed cell?
one of 4 things is turned on
1) a proto-oncogene, which regulatees cell growth and differentiation
2) a tumor suppressor gene
3) a gene that regulates apoptosis
4) a gene that is necessary for DNA repair
A tumor is thought to arise form the outgrowth of a single cell (monoclonal in origin) that has accumulated multiple genetic mutations in more than one of the above-mentioned oncogenes
141
Are infectious virus particles released from virus transformed cells?
if infected with a retrovirus, the virus is replicated
in cells transformed by DNA viruses, the genome of the virus is present in the cell but infectious particles are not detected
in papilloma viruses, infectious particles are only seen in dead cells
142
characteristics of virus-transformed tumor cells
- greater growth potential (immortal and divide indefinitely, lose contact inhibition, increased mitotic rates)
- cells are altered morphologically and metabolically (serum requirements are less)
- contain viruses and display viral encoded proteins (cell membranes display new surface proteins)
- cells have the capacity to produce malignant tumors when injected into immunosuppressed lab animals
- show chromosomal abnormalities (heteroploidy)
143
What is required for an infection to occur?
source of virus/infection
means of virus transmission
source of susceptible animals
144
How do viruses survive outside the host?
virus resistance- survive in environment for long time
infection of arthropod vectors (virus replicates in arthropod)
145
What is the extrinsic incubation period?
the period of time the vector is infected to the time the virus can be transmitted by the vector
146
How do viruses survive within the host?
Establish persistent or latent infections
change antigenic makeup to escape immune response
cause immunosuppression
147
What are enteric viruses (rotavirus and enteroviruses) resistant to?
environment, low pH of the stomach, detergent action of bile, and proteolytic digestive enzymes
the enzymatic degradation of the outer capsid wall enhances the infectivity of the irus
148
How are enteric viruses transmitted?
typically through contaminated feces or fomites
149
How do respiratory viruses contact the respiratory system?
usually via aerosol droplets (droplets 10 um are deposited on mucosa of nasal turbinates)
150
what enhances the transmission of a respiratory virus
- atmospheric conditions (humidity)
| - close contact between animals
151
are most respiratory viruses enveloped or non-enveloped?
enveloped, so these viruses are usually easily inactivated
152
what do respiratory viral infections predispose the host to?
secondary bacterial infections of the respiratory system
153
How do viruses get access to the body through the skin?
through a break in the skin (trauma, arthropod bite, needles/iatrogenic)
154
How do viruses gain access to the urogenital system?
via sexual relations
the fetus can be infected transplacentally or via the sperm
155
How do viruses enter the body?
```
alimentary system
respiratory system
urogenital tract
conjunctiva (infectious bovine rhinotracheitis virus)
skin
```
156
What happens after the virus enters the body?
viruses may remain localized or they may spread and cause lesions in different organs
157
What are the first cells that the virus encounters? what are the possible outcomes?
Macrophages
1) virus can be destroyed by the macrophage
2) replicate in the macrophages and disseminate throughout the body
3) be taken up by the macrophages in organs and spread to adjacent cells and set up an infection in that organ
158
What is viremia?>
presence of virus in the blood (virus can be free in the plasma or be cell associated within leukocytes or RBC)
159
Where is the first site of virus replication?
at the site of virus entry
160
Mousepox replication
site of entry is skin
- after local replication in skin, virus goes to LN and undergoes another round of replication
- then invades the blood stream and reaches the liver (primary viremia)
- after replicating in liver/spleen, it is released in the blood stream in HIGH CONCENTRATIONS (secondary viremia)
- then goes to skin (target organ), where the virus will replicate again and cause typical mousepox
161
What are the 2 ways viruses can reach the CNS?
via peripheral nerves via axon cytoplasm flow
via the bloodstream
- passive transport between/thru blood vessel endothelium
- infect vascular endothelial cells and reach the brain parenchyma
- transport across blood vessels via infected leukocytes
- infect brain parenchyma via CSF
162
What is perivascular cuffing?
small lymphocytes surrounding blood vessels- noted histopathologically in most viral infections of the CNS
163
what causes the CNS lesions during viral infections?
immune response to the virus, not direct damage by the virus itself
164
How is a fetus infected via a virus in the dam?
viremia leads to direct fetal infection or infection of uterus/placental
165
do fetal infections result in death of the fetus?
not necessarily- if fetal death occurs, the consequence could be SMEDI (stillbirth, mummification, embryonic death, infertility)
166
if the fetus survives infection, what is the result?
either born malformed or born normal and immune to the virus
typically, fetal infection during the 1st trimester is more apt to result in congenital defects
infection later in pregnancy results in fetal death or survivability
167
What are the physical barriers to infection?
skin
respiratory tract
digestive tract
reproductive tract
168
physical barriers to infection: skin
most difficult barrier to penetrate
| cells on surface are dead, so no virus replication
169
physical barriers to infection: respiratory tract
epithelial cells have cilia and are covered by mucus
| viruses get trapped in mucus and cilia move the mucus towards respiratory opening (swallowed or spat out)
170
Physical barriers to infection: digestive tract
saliva inactivates many viruses
pH and trypsin in stomach and digestive enzymes in duodenum destroy most viruses by breaking down the lipid envelope and capsid
171
Physical barriers to infection: Reproductive tract
stratified epithelium covered by mucus
172
Innate immune response
non-specific response (recognizes all viruses)
does not develop memory cells
activated within minutes of infection
173
what are the 3 most important components of the innate immune system
interferons (alpha and beta)
macrophages and dendritic cells
natural killer cells
174
what role neutrophils have in innate immunity to viruses?
none
| neutrophils only work on bacteria
175
What is CPRR?
cellular pattern recognition receptors
present on macrophages, dendritic cells, nk cells, fibroblasts, etc
recognize pathogen-associated molecular patterns (PAMP)
176
What are examples of CPRRs?
toll like receptors
177
what is the most important CPRR and where is it located?
TLR-3, located inside endosomes (NOT on cell surface)
178
what does TLR-3 recognize, and what cascade does it initiate?
TLR-3 recognizes viral dsRNA and ssRNA
TLR-3 activates cytoplasmic kinases that phosphorylate NFkB, which then translocates to the nucleus where it initiates the transcription of cytokine mRNA.
the cytokine mRNA are translated into cytokines and are exported by the cell to initiate inflammation and antiviral response
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what cytokines plaly a role in controlling viral infections?
```
TNF-alpha
IL-12
IFN-alpha
IFN-beta
IFN-gamma
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what is the most important arm of the innate resistance to viruses?
the interferon system
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what type of cells produce IFN-alpha, IFN-beta, and IFN-gamma
IFN alpha and beta are made by any viral infected cell
IFN-gamma is produced onlly by activated NK cells and Th cells
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How are Interferons produced?
When a virus infects a cell, the viral RNA binds to TLR3 within an endosome. This activates cellular kinases that phosphorylate NFkB that activates the cellular IFN genes that encode for IFN-alpha and beta mRNAs. The mRNA is then translated into IFN-alpha and IFN-beta that are exported out of the viral infected cell
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are IFN-alpha and IFN-beta active in the induced cell (i.e. the cell that produces them)?
NO, they are exported out of the cell and interact with specific receptors on adjacent, non-infected cells
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what happens to the cell that produces the interferons?
the infected cell will usually die from viral infection
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What happens when IFN-alpha and IFN-beta bind to surface receptors of non-infected host cells?
the non-infected cells induce the production of antiviral proteins--> these antiviral proteins cause the cells to resist a viral infection
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are IFN-alha and beta antiviral?
NO
| the interferons are not directly antiviral, but they induce antiviral proteins which causes the cells to destroy viruses
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what are the jobs of IFN alpha and beta?
induce antiviral proteins
| activate NK cells in the vicinity of the infection
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what do NK cells do?
secrete IFN-gamma that then activates macrophages and dendritic cells to secrete TNF-alpha and IL-12
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What do activated macrophages and dendritic cells do?
they produce TNF-alpha and IL-12
dendritic cells move to the LN to present viral Ag to T-cells
the macs initiate an inflammatory response in the vicinity of the viral infected cells
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What are the three antiviral proteins?
2-5(A) Synthetase
protein kinase (PK-1)
RNase L enzyme
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where are the antiviral proteins present?
all are present in the cytoplasm in an inactive form
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when are antiviral proteins activated?
when the cells are infected by a virus
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How is 2-5(A) Synthetase activated, and what does it do?
it is activated by dsRNA
| it synthesizes 2-5(A), which activates RNase L which chews up viral mRNAs
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What activates PK-1, and what does it do?
PK-1 is actvated by dsRNA
it phosphorylates the translation of initiationg factor (eIF-2) which binds to viral mRNA to prevent viral protein translation and formation
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are IFNs broad or narrow spectrum? potent or not potent?
IFNs are very broad spectrum (not virus specific) and extremely potent
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What are natural killer cells?
large granular lymphocytes that exhibit spontaneous cytotoxicity against viral infected cells
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what percent of the circulating leukocytes does natural killer cells make up?>
10%
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do NK cells need to be sensitized to a virus before they attack?
NO
NK cells are broad spectrum- they can kill all virally infected cells without needing to be sensitized to the specific virus
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how are NK cells activated?
they are activated by IFN-alpha and beta or by the inflammatory cytokines TNF-alpha and IL-12 produced by macrophages
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What are the 3 receptors on NK cells?
Natural cytotoxic receptors- recognize and bind structural viral protiens (peplomers) on viral infected cells
Inhibitory MHC-1 receptors-recognize MHC-1 molecules naturally present on all host cells
Fc receptors that bind to Fc portion of Ab that have attached to peplomers on viral infected cells
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what is the dominant receptor on NK cells?
the inhibitory MHC-1 receptor
if it is activated by the present on normal MHC-1, it will prevent the natural cytotoxic receptor from activating the NK cell
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what happens when the NCR is activated by viral infected cells that do not display normal MHC-1
NK cell kills the cell by secreting PERFORIN and then injecting GRANZYME in the cell (Granzyme activates the death proteins which induces apoptosis)
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what does the NK cell use to kill virus infected cells
first, inserts perforin, then injects granzyme to induce apoptosis
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What is ADCC?
Antibody dependent cell cytotoxicity
when the NK cell binds to an Ab that is attached to a virus infected cell
NK kills these cells using perforin/granzyme pathway
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When does NK cell activity peak?
3 days post infection, then it declines
| CTLs appear later, around 7 days
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What is the function of NK cells
to build up rapidly following virus infection and bind non-specifically to limit viral infection while CTLs are being built (takes about a week). CTLs are more efficient at recognizing and killing viral infected cells
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what is more efficient at killing viral infected cells: NK cells or CTL?
CTLs
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when does the level of IFN produced peak? When do NK cells peak? When do CTLs appear? When do Ab appear?
IFN peaks 3-5 days post infection
NK cells peak 3 days post infection
CTLs appear 7 days post infection
Ab appear 10 days post infection
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Where are macrophages seen?
they are seen in circulation as monocytes and found fixed in tissues/mucosal surfaces (kupffer cells, dendritic cells, lung magrophages)
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What is the function of macrophages and DC?
take up viruses by phagocytosis (macs) or micropinocytosis (DC) and bring them to LN
Both also recruit NK cells and Th cells by producing IL-12 and TNF-alpha
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How can macrophages destroy viruses that are inside them?
only when macrophages are activated by IFN-gamma (produced by NK cells and T-lymphocytes)
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What is humoral immunity?
adaptive immunity that includes antibodies
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What is the significance of IgM?
it is important in primary response to viral infection, but level of IgM quickly drops off
IgM activates complement and is important in opsonization, neutralization, and agglutination
IgM is only present in high levels during active infection, so can determine active infection vs prior exposures
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significance of IgG
enters tissue spaces during inflammation and can escape onto damaged mucosal surfaces
able to opsonize, agglutinate, and neutralize
IgG persista for a very long time, so can't be used to determine active infection
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significance of IgA
secretory Antibody
protects mucosal surface
dimer with J-chain and secretory piece
the major Ig in body secretions (tears, respiratory mucosa, gut, milk)
agglutinates and neutralizes the virus... PREVENTS VIRAL ADHERENCE
important in lactogenic immunity
trypsin-resistant (since it can be found in gut)
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Define Seronegative, Seropositive, and seroconversion
Seronegative: animal has NO circulating antibodies to a particular virus
seropositive: animal has circulating Ab to a particular virus
seroconversion: when an animal changes from seronegative to seropositive (4 fold)
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What are the ways antibodies prevent viral infection?
neutralization- Ab binds epitopes on the virion surface and prevents viral attachment to cells (can also cause conformation changes so it can't bind)
lysis- Ab bound to virus particles activates complement cascade, resulting in viral lysis
aid in phagocytosis- Ab arms macrophages or opsonize viral particles. Fc receptors on macs bind Abs and initiate phagocytosis... or complement system may be activated
cause lysis of host cell- Ab bind to virus infected cells, activate complement system or ADCC (NK cells/CTLs). Phospholipases are released and cause cell lysis, and cells are lysed before virus particles released, preventing virus spread
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What regulates cell mediated immunity
T cells regulate cell mediated immunity via secretion of cytokines
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What kind of antigens do CTLs respond to?
endogenous antigens
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how do CTLs work?
during virus replication, small viral peptides are formed which bind to the groove on MHC-1 molecules and are transported to the cell surface and exposed to CTLs.
CTLs that recognize their peptide:MHC molecule are activated and become cytotoxic- they recognize and lyse any viral infected cells that present these specific viral peptides to their MHC-molecules
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what do Th cells do?
produce various cytokines that activate B cells and T cells
Th1 is primarily responsible for stimulating cell mediated immune responses
Th2 is primarily responsible for stimulating humoral response
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what are the 2 types of vaccines?
inactivated/killed vaccine- whole virus or part of the virus is inactivated
attenuated/live vaccines- live virus that has been rendered less pathogenic/virulent
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how are inactivated vaccines presented?
presented as exogenous antivens
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what response does inactivated vaccines give?
humoral Ab response dominated by Th2 cells
very little or no cell mediated immune response
225
how are attenuated vaccines presented?
presented as both exogenous and endogenous antigens
226
what response does attenuated vaccines give?
both humoral Ab and cell mediated immune response via Th1 and Th2 cells
227
do viruses in attenuated vaccine replicate within body?
yes, they replicate within the animal, so they present as both endogenous and exogenous antigens
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What are the 4 types of live vaccines?
Live virulent virus (rarely used)
Attenuated- modified live vaccine
Genetically altered viruses
Recombinant vector vaccines
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How are live virulent virus vaccines given?
given by an abnormal route or given to another host to induce immunity (i.e. given at the wrong site, chicken getting turkey virus)
230
what is the main disadvantage of live virulent virus vaccines?
the virus can spread to other animals
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What is the basis behind modified live vaccines?>
the virulence of the virus or bacteria reduced by growing and passaging the virus through tissue culture cells, eggs, or animals.
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what are ways to produce modified live vaccines?
1) passing it through a wrong animal (ex: passing virus thru rabbit--> inoculating rabit>allowing rabbit to develop disease>re-isolating virus from rabbit>re-inoculation of virus into another rabbit... basically virus adapts to causing disease only in rabbit and not original host--> when virus is re-inovulated into the original host, a mild infection follows, which causes strong humoral and CMI responses
2) UV light mutagenization
3) Temperature sensitive mutants (viruses are selected that cannot grow at normal body temperature
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What are most virus vaccines used in vet med?
Modified live vaccines that have been stabilized by lyophilization
confers good immunity after only one inoculation (since the virus replicates in the animal)
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What is the basis behind genetically altered viruses?
gene deletion or site-directed mutagenesis causes attenuation
the GENE DELETED VIRUS can infect animals but cannot cause disease
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What are some examples of genetically altered viruses?
Thymidine kinase deficient vaccines or glycoprotein E deleted vaccines
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How does genetically altered virus vaccines work with test and slaughter programs?
Differentiation/discrimination of infected from vaccinated animals (DIVA) strategy
pigs are inoculated with TK-deficient pseudorabies vaccine. the pigs do not make Ab to TK protein since it is not in the vaccine. Therefore, if a vaccinated pig becomes infected with wild-type pseudorabies, it will make antibodies to TK protein expressed by the wild virus.
By testing annually for TK protein, we can do serology do determine which have been infected with PRV and slaughter
serology we use is indirect ELISA
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What is the basis of recombinant vector vaccines?>
a gene that encodes for an important protective immunogenic protein of one virus is inserted into (recombined) with the genome of another virus, usually pox virus. The recombinant vaccinia virus vaccine now carries an extra gene. When foxes/raccoonns eat bait laced with this vaccinia virus, the virus replicates and the rabies virus protein is translated with the rest of the virus. The body now mounts an immune response and a CMI and humoral immune response now recognizes rabies.
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What are the types of non-replicating, killeld virus vaccines?
Inactivated whole virus
Purified native viral proteins
subunit vaccines (expression of viral proteins in cells)
vaccines produced by expression of viral proteins that self-assemble into virus-like particles
Naked DNA vaccines
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inactivated whole virus
virus inactivated by agents render them non-infectious or non-toxic while retaining antigenic sites
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what are commonly used agents to inactivate whole viruses?
formaldehyde
alkylating agents (B-propiolactone)- cross links the nucleic acid but leaves the proteins intact
241
Purified native viral protiens
lipid solvents solubilize enveloped viruses and release the glycoproteins which are then separated and harvested for injection in animals
basically inject glycoproteins to allow immune response
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Subunit vaccines
produced by expression of viral proteins in cells
yeast, mammalian cells, insects, and bacteria can produce viral proteins in large quantities (the viral gene of interest is translated into viral proteins in those eukaryotic cells)
Ex: hepatitis B, Hemagglutinin of influenza A, subunits of e.coli and FeLV
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vaccines produced by expression of viral proteins that self-assemble into virus like particles
by mixing capsomeres and different proteins together, they self-assemble into virus-like particle
do not produce a genome, just a capsid
244
what do inactivated vaccines need to be injected with?
inactivated vaccines must be injected with adjuvants to maximize immunogenicity
245
what are adjuvants?
materials that are added to vaccines to potentiate the humoral and CMI responses (increase the duration of immunostimulation)
adjuvants act as vehicles that slow down the degradation of the Ag so that it is released over a longer period of time
adjuvants also act as immunomodulators that localize tha antigen creating non-specific prolonged cell-mediated inflammatory response
246
what are examples of adjuvants
aluminum salts
bacterial cell wall fractions such ass LPS
Surface active agents
Freund's complete adjuvant (inactivated mycobacterium oil)
247
what is the problem with Freund's complete adjuvant?
causes sterile granulomas/abscesses
248
What is the basis of naked DNA vaccines?
inoculation of a naked gene instead of protein
when the gene is inoculated IM, the myocytes take up the naked DNA--> DNA is transcribed into mRNA and mRNA is translated into immunogenic protein that stimulates immune response
the protein is treated as endogenous and exogenous protein, stimulating both CMI and humoral immunity
249
what is the disadvantage of naked DNA vaccines?
they can integrate in the DNA and activate ancogenes
250
What type of protection do vaccines provide?
Partial or incomplete protection
| a vaccinated animal may get a subclinical infection or superficial mucosal infection
251
what do vaccines protect against
vaccines protect against disease, but do not prevent sub-clinical infection (they can still be infected and shed virus)
252
are vaccination and immunization synonymous?
no, you can vaccinate without immunizing
253
what are precautions you should take when handling vaccines?
read label
protect vaccs against excessive heat and light
don't mix vaccines into one vial and give as cocktail
be careful with disinfectants (MLV can be destroyed)
one vaccine may block immune response of another... ex: distemper is immunosuppressive
254
adverse consequences of vaccination
```
residual virulence
toxicity
allergic reactions
harmful effects on fetus
post-vaccinal sarcoma
```
255
what are the toxicity concerns with vaccination?
local reaction at sites of vaccination for a few days
vaccines containing killed gram-negative organisms may be extrinsically toxic due to presence of endotoxins
256
What harmful effects on the fetus can vaccines have?
modified live vaccines can cross placenta and affect fetal development (blue tongue virus)
257
where have we seen post-vaccinal sarcomas?
mostly cats at the site of vaccination
cats receiving FeLV are 5.5 times more likely to develop sarcoma
number of vaccs cats receive is important in increasing incidence of sarcomas: 50% with 1 vaccine
127% with 2 doses
175 with 3-4 doses of vaccine
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What are the major causes of vaccine failure?
1) vaccination with a different virus or serotype than the one causing disease
2) presence of maternal Abs
3) Vaccine may be inactive (improper storage, expired)
4) Incorrect manufacturing (too little Ag, contamination)
5) not enough protection was elicited from vaccine
6) short time between vaccination and infection with virulent virus
7) IgA was not produced
8) vaccine given incorrectly (SQ instead of IM)
259
How do maternal antibodies cause vaccine failure?
maternal antibodies prevent MLV from replicating if levels of Ab is high. Attenuated viruses get neutralized by maternal Ab
260
Why is IgA important for enteric and respiratory viruses, and when is IgA not produced when vaccinated?
IgA is important because most enteric and respiratory viruses cause local disease and not systemic--> IgA protects the mucosa better than IgG (better for systemic viral infections)
IgA is not produced well during parenteral vaccines
261
What is passive immunization?>
antibodies (in the form of antiserum) are injected into animals to protect them against infection
262
When is passive immunization important?
important in protection against toxins and against life threatening infections
263
what is the problem with passive immunization?
antiserum administration may interfere with active immunization
264
Advantages of passive immunity
provides immediate protection
| works well against pathogens that are poor immunogens
265
Disadvantages of passive immunity
protection is short lived
interferes/delays the ability to vaccinate animals
because it is Ab produced by other animals, can cause hypersensitivity reaction (type I or III)
antisera can transfer blood borne diseases (FeLV, FIV, etc)
266
What is heterologous antisera?
basically like the mouse monoclonal antibody (MAb) given to dogs
type of passive immunization- quickly catabolized by recipient animal via trypsin into Fc and Fab fragments
Fab fragments are catabolized more slowly then the whole Ab, and they are as efficacious as the entire Ab molecule to neutralize pathogens
for enteric pathogens (E. coli), heterologous antisera can be given orally
