Virology Exam 1

Be able to describe the basic steps of virus replication in a productive infection.

1. Attachment 2. Penetration 3. Uncoating 4. Replication 5. Gene expression 6. Assembly 7. Release 8. Maturation

What are some examples of mechanisms for regulating gene expression?

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Some viruses use a cell receptor that may be widely distributed on cells, yet the tissue tropism is restricted. What is one possible reason to explain this?

A cell with the appropriate receptor may still not contain an enzyme(eg. a splicing enzyme for a specific sequence) that is required for the replication of the virus, but not encoded by the virus.

What is meant by viral quasispecies?

A group of viruses related by a similar mutation or mutations, competing in a highly mutagenic environment.

What is meant by "one step growth curve"? What happens in the eclipse phase of this curve?

A one step growth curve represents the growth encountered when a virus is only able to undergo one cycle of replication (after burst, there are no additional host cells to be infected). During the eclipse phase, the amount of virus measured in solution decreases as the virus is taken up into the cell medium.

What is meant by "phenotypic mutant"? Give an example.

A phenotypic mutant is a virus which has mutated in a way that changes a phenotypic characteristic. Eg. Temperature sensitive mutants can only proliferate at certain temperatures.

Why might mutation of a surface protein be retained while mutation of the polymerase may not?

A surface protein mutation changes antigenicity and gives the virus a potential new attachment protein link to host cells, granting it better survival. A mutation of polymerase may render the polymerase inactive, preventing the virus from replicating.

Describe four patterns of disease. What is the difference between acute vs. persistent infections?

Acute-rapid onset and recovery Persistent-months to years before cleared

How does adenovirus attach to the cell (what viral structure is used)? How does it enter the cell? How does it get into the cytoplasm? What happens after it enters the cytoplasm?

Attaches via penton fibers. Enters by endocytosis. Gets into cytoplasm when penton toxins rupture the endosome. After it enters the cytoplasm, the capsid is removed and the core migrates to the nucleus where it is replicated.

Bovine adenovirus replicates in what tissues? In what age group are these viruses most important?

BAV replicates in the epithelial cells of the upper and lower respiratory track, conjunctiva, and intestines. Most problematic in calves.

What is "blue eye" due to CAV(Canine adenovirus) 1? What are two mechanisms for its occurrence? Why is CAV 2 used in the vaccine for CAV 1? Why is it effective?

Blue eye is due to the build up of immune complexes in the eye during convalescence. CAV2 is used in the vaccine because it is antigenically similar to CAV1 and reduces vaccine reaction.

Compare and contrast transcription of DNA vs. RNA viruses (enzymes needed). Which viruses (types, not families) may use cellular enzymes for transcription? Which virus types must carry the transcriptase into the cell with them or encode the transcriptase in their genome?

Both DNA and RNA viruses require some kind of transcriptase, except for ssRNA+. DNA viruses that penetrate the nucleus can use the cell's enzymes. DNA viruses that replicate in the cytoplasm, and RNA viruses other than ssRNA+ must have unique transcriptases coded in their genome. RNA viruses must carry this transcriptase into the cell with them, as cells do not have any enzyme to copy RNA into RNA. Retroviruses also have to encode their own reversetranscriptases as this is not a naturally occurring cellular enzyme.

What are the mechanisms of genetic recombination? What is a requirement for a virus to be able to undergo reassortment?

Breakage reunion-cutting and pasting of genes between viruses or cell mRNA/DNA. Template switching-during replication, the translational enzyme switches between templates of two different viruses. Reassortment-virus genomes are packaged with pieces from multiple viruses. *This requires that the viruses genome be replicated in pieces!*

What are two mechanisms for virus release from a cell? When is a virion considered mature?

Budding and lysis/apoptosis. The virion is considered mature when it is fully formed and released from the host cell.

Compare and contrast the tissue tropism of canine adenoviruses 1 and 2.

CAV1: endothelium and parynchyma of many tissues, esp. liver. CAV2: respiratory tract epithelium.

Where did canine parvovirus 2 come from evolutionarily (what was its origin)?

CPV 2 originated from a mutation in the feline parvo virus.

Describe the four types of viral capsid symmetry and be able to identify them(shape, not specific viruses) from virus pictures (not cartoons, but electronmicrographs of viruses). Why is the morphology of viruses restricted to just a few types of symmetry?

Cubic-d20 Helical-coiled or rod shaped Binal-combination of Cubic and Helical Complex-no distinct morphology Morphology of viruses is restricted because they are need to be in shapes that spontaneously assemble easily.

What are some virus factors that influence pathogenesis of a virus?

Cytocidal viruses Cytotoxins Perturbation of cellular function Enzymes Tissue tropism Dose and route of inoculation

Which viruses may use cellular enzymes for genome replication?

DNA and retrovirus RNA viruses that penetrate the nucleus of the cell.

Describe the genomic replication of adenoviruses.

DNA replication is started by a binding protein which signals the viral DNA polymerase. Replication occurs simultaneously from both ends of the genome.

How does adenovirus infection lead to shutdown of host cell functions?

Decreased cell protein synthesis, inhibition of cellular DNA synthesis, and toxic pentons eventually lead to cell death.

Describe the possible mechanisms by which a virus may disseminate from the primary site of replication. What is an important host factor that may impact the virus' ability to spread?

Dissemination can occur from cell to cell, through the blood stream, down nerves, and through lymphatics. Interferon is possibly the most important host factor that limits a virus' ability to spread.

How are multiple proteins encoded by a single transcription unit in adenoviruses? How do viruses of this family exhibit "genetic economy"?

Each strand of DNA has multiple open reading frames. Also, both strands of DNA can be transcribed, using viral RNA polymerase.

What is meant by early and late phases of transcription? How do the protein products of each differ? What separates these phases?

Early transcription phase produces proteins for use in the cell(eg. cleavage enzymes). Late phase transcription produces proteins for assembling in to new viruses. These phases are separated by mass replication of the virus' genome.

What are three mechanisms of virus entry into a cell? What are the restrictions for these methods (e.g. who can enter by fusion)?

Endocytosis-all viruses Fusion-enveloped viruses only Translocation-nonenveloped viruses only

What is one method for diagnosis of parvovirus infection? Generally, how is it controlled (e.g. in dogs and cats)?

Enteric identification of virus by EM or ELISA. The best control method for parvovirus in dogs and cats is vaccination, especially in young animals as maternal immunity is waining.

What are some examples of ways that viruses overcome their small genome to encode multiple proteins (genetic economy)?

Folding of the mRNA strand to produce multiple open reading frames. Production of proteolytic enzymes which cut large protein strands into multiple proteins. Having multiple open reading frames on one strand that produce proteins with overlapping sequences.

Why do genetic mutations occur during viral replication? What is the mutation rate of DNA vs. RNA viruses?

Genetic mutations occur during viral replication due to random errors in nucleotide insertion during the replication process. DNA viruses mutate less often than RNA viruses because they can use the cellular proofreading mechanisms. There are no proofreading mechanisms for RNA replication.

What are methods by which a virus can increase coding capacity of small genome?

Have encoding on both strands(if double stranded) Have multiple genes overlapping in a given sequence Have overlapping open reading frames Variation in termination and splicing sequences

Describe two disease syndromes of Group II avian adenoviruses (Siadenoviruses).

Hemorrhagic enteritis of turkeys Marble spleen disease of pheasants-actually causes respiratory signs. *both of these diseases have characteristic lesions on the spleen.

What are host factors that influence the pathogenesis of a virus? How can you as a veterinarian address these factors in terms of disease prevention?

Host factors: Genetic characteristics, environmental influences. Prevention: Avoid stressors, screen for breed predispositions, maintain proper health of the animal.

What parameters are considered regarding pathogenesis at the host level? At the cellular level?

Host level: How does the virus enter the host? Where does it replicate? Does it spread within the host? How? What tissues/organs does it infect? How is it transmitted? Cellular level: What is the nature of the virus receptor? What is the mechanism of viral entry? How does infection lead to alteration of host cell function? What is the mechanisms of virus release?

What other mutations might give a virus a selective advantage (what changes in phenotype, not what specific gene mutations)?

Host range mutations might allow the virus to infect a new host species, broadening the number of possible susceptible hosts.

Why have human vaccines not been developed for adenoviruses(what is the concern)?

Human vaccinations have not been well developed because of the risk of oncogenesis. Current vaccines are only used in military personel at high risk.

How could the immune response contribute to the disease in the infected host?

Immune complexes stuck in tissues can cause damage to the host tissue. The presence of virus envelope lipids in the host cell membrane can mark it as a target for immune responses.

What are the consequences of parvovirus infection in adult dogs and cats vs neonatal dogs and cats? Why is there a difference in adults vs neonates?

In adult animals, the most common symptoms are vomiting, diarrhea, and depression; leukopenia in cats. In neonates, kittens often develop cerebellar hypoplasia, and puppies develope myocarditis. Parvovirus is able to infect a higher variety of tissues in neonates because most cells in the body are still dividing rapidly at this stage of life.

Equine adenovirus infection is a problem in what situation? What is the consequence?

In immune compromised foals (such as Arabians with SCID), EAV causes widespread distruction of epithelial cells, leading to pneumonia and death.

Where does adenovirus assembly occur?

In the host cell nucleus.

Define the following terms: incidence, prevalence, morbidity, mortality, endemic, epidemic, and pandemic.

Incidence-number of new cases in a given time. Prevalence-total number of cases either in a given period of time, or within the total of a given population. Morbidity-severity of disease caused by an agent. Mortality-likelihood that an agent will cause death. Endemic-area in which an agent is commonly found. Epidemic-higher prevalence of disease than endemically expected within an area. Pandemic-epidemic on an international/global scale.

Describe two disease syndromes of Group I avian adenoviruses (the Aviadenoviruses).

Inclusion Body Hepatitis Respiratory disease-bronchitis.

What are some ways that adenovirus evades or inhibits the immune response?

Inhibits apoptosis signals from Tc cells and downregulates MHC I expression.

Why was the development of laboratory techniques for virus propagation, such as cell cultures, such a boon to virus research?

It allowed for viral strains to be kept for study without requiring a living host organism.

What is the primary importance of parvovirus in swine (what is the major consequence of infection)?

It causes stillbirth and neonatal death, as well as weak pigs, depending on the stage of gestation on infection.

What are the four genera of this family(adenoviridae)? What species of animals can be infected in each?

Mastadenovirus-Mammals Aviadenovirus-Avians Siadenovirus-Turkeys Atadenovirus-Mammals, Avians, and Reptiles

What determines how a virus is shed from infected animals?

Method of viral shedding is primarily dependent on the tissue type involved in the infection. ie. GI viruses shed in feces, resp viruses shed by coughing/sneezing, systemic viruses may be shed by a variety of methods.

Discuss one example of viral genetic variation in nature. In the case you cite, what is the reason for persistence of the mutation?

Mutation of the host range genes of feline distemper virus to allow for the infection of dogs. This mutation persists because it allows the virus to infect a larger pool of hosts.

What determines whether or not a virus retains a mutation?

Mutation retention depends on the selective advantage given by the mutation, and the current stage of viral infection.

What is one potential mechanism for virus spread across species lines?

Mutations in the genes encoding surface attachment proteins.

In dogs and cats, how does parvovirus reach the intestinal cells it infects? How does it reach the fetus in pregnant swine?

Parvovirus enters the body through oronasal exposure, replicates in lymph tissues, then spreads in the blood to the apropriate cells. In swine, it must pass transplacentally.

What is a requirement for cells to support parvovirus replication? Why? What other factor determines the host/tissue tropism of parvoviruses? Give 3 examples of cells/tissues/hosts targeted by parvovirus.

Parvovirus targets highly mitotic cells because it does not encode it's own replication enzymes, and must have cellular replication enzymes. Furthermore, it can not induce replication in the cell, so the cell must already be dividing. Eg: intestinal crypt cells, bone marrow, fetus.

Why is the diarrhea caused by parvoviruses in dogs potentially so severe and hemorrhagic in nature (think target cells)?

Parvoviruses in dogs target the crypt cells of the intestines. These cells are responsible for producing replacement cells for the high turn over in intestinal villi. Because villous epithelium undergoes such a high turn over rate, with no replacement, the whole lining of the intestine can be quickly distroyed.

What is the definition of pathogenesis? How does this contrast with virulence? What are possible outcomes of virus infection of a cell?

Pathogenesis is the mechanism by which viruses cause cell injury and disease signs. Virulence is the capacity of a virus to produce disease in a given cell. Virus infection leads to: cell death, latency(persistence within the cell), cellular transformation(cancers), and non-productive infections.

What are four types of persistent infection? Compare and contrast them.

Persistent Chronic Recurrent Slow

What is meant by persistent infection? Why are persistently infected animals important in the spread of viruses?

Persistent infections are infections that recur multiple times in a single host. Persistently infected animals serve as a kind of reservoir for the virus and allow it to be shed multiple times over a long period.

Why are persistent infections important to the infected host?

Persistent infections can cause acute episodes. May be associated with immune disease. Can cause neoplasia. Allows intermittent or chronic shedding to new individuals.

What is meant by positive vs. negative sense of a genome, especially in regard to ssRNA genomes?

Positive sense genomes are the same sequence as mRNA, and may be used directly for protein formation. Negative sense genomes are a mirror to the sequence of mRNA, and must be transcribed before use.

How does prior immunity in an animal influence the transmission of a virus?

Prior immunity in an animal may allow the host to clear the virus before it has a chance to infect target cells and replicate.

What is the difference between productive and abortive infections? How might an abortive infection affect a cell?

Productive infections produce functional virions that may be shed to a new host. Abortive infections do not produce mature virions at all. An abortive infection may have no change on a cell, or it may still lead to cell destruction due to the presence of foreign material on or in the cell.

What various viral features (categories) are used to classify a virus into families? Be able to provide some examples.

Properties of genome-DNA vs RNA Properties of virion-enveloped, non-enveloped Properties of proteins-size of protiens, types of replication protiens Physical properties-Shape Replication strategies-uses cellular replication, or carried replication protiens Biological properties-type of disease caused

What are three important circoviruses and how does infection manifest in the respective hosts?

Psittacine beak and feather disease(Circovirus)-necrotic feathers and beak difformity, may cause pneumonia from secondary infection Porcine circovirus-repro difficulties, wasting disease, or resp. disease. Chicken anemia virus(Gyrovirus)-RBC suppression.

Chicken anemia virus targets what cells?

RBC precursors, thymus and spleen.

What wildlife species may experience parvovirus infections with signs similar to dogs and cats?

Raccoons, mink, and wild canids.

Define recombination. Complementation. Phenotypic mixing.

Recombination-the exchange of genetic material between 2 viruses infecting the same cell. Changes the genome of the viruses. Complementation-the exchange of gene products(proteins) between two viruses being replicated in the same cell. Does not change the genome of the viruses. Phenotypic mixing-the fully packaged virus exiting the cell has phenotypic characteristics of both viruses inhabiting the cell.

What is egg drop syndrome caused by adenovirus?

Reduction in shell hardness caused by atadenovirus. Eggs are laid with soft, incomplete, or no shell.

How are infected cells killed/lysed by adenoviruses?

Reduction of cellular function from being hijacked, apoptosis from the build up of new viruses in the cell.

What are two theories about the origin of viruses?

Regressive theory-viruses are degenerate portions of other parasites. Independent theory-viruses developed from self replicating molecules.

Where does the virus replicate in the cell? Describe the transcription of adenoviruses, including the source of the RNA polymerase. What separates early from late transcription?

Replication occurs in the nucleus. Transcription uses RNA polymerases, early phase proteins are used for cell regulation, immune evasion, and pushes the cell into the cell cycle to boost reproductive machinery. Early and late transcription is separated by the replication of the genome. Late transcription uses the replicated genome to produce structural protiens.

To what virus is circovirus replication similar?

Replication of circovirus is similar to parvovirus in that it requires active cellular replication machinery.

What are some components of innate and specific immunity protecting the respiratory tract? The gastrointestinal tract?

Resp: Innate-turbulence, mucocilliary mechansim. Specific-macrophages GI: Innate-pH(acidity). Specific-macrophages, antigen presentation by peyer's patches.

What are some mechanisms of virus transmission? Give an example (mode, not specific virus).

Respiratory droplets, fecal/oral contamination, vector, direct contact, congenital(in utero). Eg. Respiratory virus shed in the mucous expelled by a sick animal lands on the mucous membranes of a susceptible animal.

Why are the rodent parvoviruses important?

Rodent pravoviruses affect laboratory mouse colonies, causing still birth. The presence of immune response to the virus can also confound research findings.

What are other factors that influence the mechanism of virus transmission? In what way do these factors impact virus transmission?

Route of entry and shedding-the closer the virus enters to it's target tissue, the less likely it is to get wiped out by host defenses. Duration of shedding-The longer a virus is shed, the more likely it is to contact a new host. Hardiness in the environment-The longer a virus lasts in the environment the more likely it is to contact a new host before breaking down. Existence of reservoirs-Maintain the virus in the environment and allow it to be transmitted to new hosts many times.

What determines how the various adenoviruses are shed from infected animals? Give some examples.

Shedding method depends on infective method. Respiratory forms are spread through respiratory droplets. Gastroenteric forms are spread through fecal-oral contamination. CAV 1 is spread from the urine.

What components of innate immunity protect the epithelial surfaces of an animal? What components of specific immunity protect at epithelial surfaces? What are ways that a virus may overcome the epithelial barriers to infection?

Skin and mucosal membranes, acidity of the GI tract, mucocilliary apparatus in the respiratory tract. Macrophages, peyers patches, inflammatory response. Breaks in the epithelial layer, mechanisms for surviving environmental disturbances(envelopes), inoculation of host by a vector.

What is meant by structural vs. non-structural proteins?

Structural proteins are any proteins carried by the mature virion(capsomeres, attachment proteins, etc). Non-structural proteins are proteins only expressed in the infected cell(replication proteins, enzymes, etc).

Generally, what are the products of late transcription in adenoviruses?

Structural proteins.

Describe the structure of adenoviruses, including the genome. What is a hexon? What is a penton? What are other structural proteins in the mature virion?

Structure: icosohedral, non-enveloped, protruding fibers on vertices, linear dsDNA. Hexons are capsomers covering the faces and edges of the icosohedron. Pentons are the attachement protiens for the fibers protruding from the vertices. The mature virion also includes histone like proteins.

Describe the structure of parvoviruses, including the genome. How is DNA synthesis primed in genomic replication? Most DNA viruses have a lower mutation rate as compared to RNA viruses, yet parvoviruses have a rate approaching RNA viruses - what is one reason that may explain this?

Structure: very small, icosohedral, no envelope, ssDNA(-), linear with hairpin turns at end. DNA synthesis is primed by the hair pin turns at each end of the genome. High mutation rate because DNA replication requires several intermediate steps, and lots of splicing.

What subfamily and genera include the parvoviruses of veterinary importance? Humans?

Subfamily: Parvovirinae Veterinary genera: Parvovirus, Bocavirus, Amdovirus Human genera: Erythrovirus

What are some reasons that a virus may have seasonal variation?

Susceptibility of host may be linked to stress caused by weather change. Some viruses may also be spread more easily in dry or wet conditions, which change with the season.

What are the target cells of PBFD virus? How is it transmitted? What is an important consequence of infection (may contribute to the cause of death)?

Targets epithelial basal layer. Shed in feather dander and droppings, can be transmitted vertically. Also targets the thymus and bursa of Fabricius-causing immune suppression.

In the cellular effects of viruses, what is the "virus goal" that as a result induces these effects? How does this contrast with, for example, a bacterium that secretes a toxin?

The "virus goal' that results in cellular disruption is replication of the virus. The virus hijacks cellular components and machinery to replicate, inhibiting the cell from using it's own supplies. Bacterial toxins on the other hand, produce cellular disruption without using the cell's components for any use of its own.

5. How is attachment of virus to a cell mediated? What is a major factor in determining the susceptibility of a cell to infection by a virus?

The attachment of virus to a cell is mediated by cell receptors on the cell surface. Susceptibility of a cell to viral infection depends on the presence of the cell receptor used by that virus.

Describe one event/discovery in history of virology, and it's impact (your choice; does not have to be one I mentioned).

The development of the electron microscope let us visualize viruses for the first time.

How does the stability of a virus in the environment impact its ability to be transmitted?

The longer a virus can remain stable in the environment, the longer the virus has to come in contact with a new host.

The mRNA of some viruses is functionally monocistronic while technically polycistronic - what is meant by this statement?

The mRNA produces one long strand of protein which is then spliced into multiple proteins by proteolytic enzymes.

What is the origin of viral lipid (envelope)? What virus proteins are most likely to be glycoproteins?

The viral lipid envelope comes from the original host cell in which the virus was formed. Peplomers are more likely to be glycoproteins than other virus proteins.

The pathogenesis of Aleutian disease of mink is due not only to virus replication but also to the immune response - describe briefly the pathogenesis of this disease.

The virus is not completely cleared and immune complexes can become lodged in the tissues, leading to glomerulonephritis, vasculitis, hepatitis, anemia, and death. Mink with the Aleutian color seem to be more susceptible.

What are some of the functions of the early proteins in adenoviruses? How do they lead to transformation of a cell (oncogenesis)?

Transcription activation, induces cell cycle, protects cell from immune response, and increases protein synthesis for DNA replication. Oncogenesis occurs when the virus induces the cell cycle in the host cell but is unable to complete replication, and leaves the cell alive and actively, permanently replicating.

What is meant by "tropism"? What factors determine the tropism of a virus? How does the virus' cellular/tissue tropism manifest at the level of the animal/host?

Tropism is the limitation of viral infections to a certain type of tissue. Factors influencing tropism include appropriate cell receptors, and cellular hospitality. Virus tropism manifests as disease signs associated with the cell type/tissue selected for.

What is meant by virus "tropism"? What are factors that impact virus tropism? How does variation in tropism evidence at the level of the host?

Tropism represents the ability of a virus to infect a certain type of host cell. Variation in tropism might be evidenced in the host as the presence of virus only in one single type of cell, or a greater degree of virulence in one cell type over another.

Why was the discovery of reverse transcriptase controversial?

Until the discovery of reverse transcriptase, it was believed that DNA to RNA transcription was a unidirectional process and could not be reversed.

How are adenoviruses used as vectors for gene therapy?

Up to 30kb of the adenovirus genome can be replaced with other genes. These genes can then "invade" the host cell and be used for protein synthesis by the cell.

In parvovirus, how are 3 structural proteins (VP1-3) encoded by one ORF? What are the functions of the nonstructural proteins NS1 and 2?

VP1 and VP 2 are produced by alternative splicing of the same mRNA. VP 3 is produced by further splicing of VP 2. The nonstructural proteins function in gene expression regulation and preventing cellular DNA from being replicated.

What are the properties of circovirus? What is meant by 'ambisense' when referring to the genome?

Very small, non-enveloped, circular ssDNA(ambisense). Ambisense refers to the fact that the genome can be read from the 5' end or the 3' end.

Know the definitions of terms: Virion, capsid, capsomere, nucleocapsid, peplos, peplomer.

Virion - mature virus particle. Capsid - protein coat. Capsomere - capsid subunit. Nucleocapsid - n.a + protein shell. Peplos - lipid envelope. Peplomer - projections on some envelopes, usually glycoproteins.

What properties distinguish a virus from other microorganisms? What properties define a virus?

Viruses are distinguished from other microorganisms in that they lack any kind of energy producing mechanism. Binary fission does not occur, and viruses can only replicate inside of another living cell. The genetic material of the virus is either DNA or RNA, not both.

What are potential consequences of circovirus infection in swine?

Young pigs with wasting disease syndromes - postweaning multisystemic wasting syndrome (PMWS). Respiratory disease, pneumonia. Dermatitis and nephropathy - immune-mediated? Reproductive disease, enteritis, CNS disease. Can be a cause of fetal death(PCV1)

Which viruses are able to undergo splicing of the mRNA?

mRNA is not spliced. However, some viral mRNA strands(typically in ssRNA+ viruses) the mRNA can be folded to give the appearance of multiple open reading frames.

What is the first step following uncoating for positive sense ssRNA viruses (except Retroviruses)? How does this differ from the other virus types?

ssRNA+ viruses are able to bind directly to ribosomes and begin translation as soon as they are uncoated. All other viruses must undergo some kind of transcription first.