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Viruses contain both DNA and RNA.
False
Viruses are considered strict intracellular parasites.
True
The protein shell of a virus is known as the capsid.
True
Viruses can reproduce independently without a host cell.
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All viruses have an envelope.
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The nucleocapsid is made of nucleic acid and capsid.
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Viruses are classified as living organisms.
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Hosts are essential for viruses to replicate.
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Viruses are classified based on their phenotypic characteristics, such as morphology.
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The capsid of a virus has no role in introducing the viral genome into host cells.
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Enveloped viruses must stay dry during transmission to survive.
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Non-enveloped viruses are also known as enveloped viruses.
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The envelope of a virus is composed of phospholipids and proteins derived from host cell membranes.
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Viral attachment involves the interaction between the capsid protein and host cell receptors.
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Uncoating allows viral nucleic acid to enter the cytoplasm or nucleus.
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Protein synthesis for viruses occurs exclusively in the nucleus.
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Non-enveloped viruses are released from cells through a process known as budding.
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Cytomegalovirus may be severe in vulnerable patients such as transplant recipients.
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Viral pathogenesis can result in local or systemic infections.
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Hepatitis C is an example of a viral infection that may be silent.
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Cytopathogenic effects can lead to a cell becoming malignant.
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Herpes simplex virus causes symptoms that are always acute and never reoccur.
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The only way viruses spread is through intercellular connections.
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Rabies is a viral infection that may be fatal.
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Neural spread of viruses occurs through peripheral nerves to the central nervous system.
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Molecular techniques like PCR are typically slower than serological tests for diagnosing viral infections.
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HIV viral load can detect resistance to antiviral drugs.
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Immunofluorescence microscopy is used for fast detection of viral DNA in specimens.
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Viruses can replicate without living cells.
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The cytopathological effect indicates that a virus is not viable.
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Serology tests can provide information about the presence of specific antibodies against viral antigens.
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Viral culture results can take up to four days to obtain.
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Hepatitis B is capable of being propagated in cell culture.
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Electron microscopy is specialized and can provide quick answers to clinical questions.
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Viruses classified as enveloped require a lipid bilayer composed of phospholipids and carbohydrates.
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The unique characteristics of enveloped viruses can aid in diagnosing certain viral infections through serology.
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Non-enveloped viruses rely on the presence of an envelope to survive through drying and heat.
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Capsid proteins play a critical role in the interaction between the virus and the host cell receptors.
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The capsid of a virus is composed of nucleic acid and a lipid bilayer.
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Viral infections are always severe and cannot be mild or self-limiting.
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Molecular techniques like PCR are known for their speed and sensitivity in diagnosing viral infections.
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Hepatitis C is an example of a viral infection that may be active and symptomatic.
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Viral infections can be both acute and chronic.
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Serological tests are typically slower than molecular techniques like PCR for diagnosing viral infections.
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Single-stranded nucleic acid can be either DNA or RNA but never both.
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Viruses can reproduce independently without invading a host cell.
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The process of viral replication includes uncoating and synthesis of new viral components.
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Fecal-oral transmission is a method of viral acquisition for viruses like HIV.
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The capsid of a virus is involved in the assembly of new virus particles.
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Cytopathological effects can confirm the viability of a virus.
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Viral culture can result in rapid results typically within one week.
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Immunofluorescence microscopy utilizes antibodies to detect viral antigens in specimens.
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All viruses can be grown in culture using artificial living cells.
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Serological testing can differentiate between viral antigens and the antibodies produced against them.
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Living cells are not necessary for the culture of viruses.
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Cytopathic effects are exclusively used to identify bacterial infections.
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Electron microscopy is mainly used for the qualitative diagnosis of viruses.
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Enveloped viruses are released from cells primarily through lysis.
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In viral pathogenicity, cytopathological activity can contribute to tissue damage and immune response.
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During the synthesis phase, viral nucleic acids are only assembled in the nucleus of the host cell.
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Intercellular spread of viruses involves the use of cellular bridges to move between adjacent cells.
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The release of viruses into the bloodstream is considered a type of extracellular transmission.
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Viral attachment involves the interaction of the virus's nucleic acid with the host cell receptor.
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Latency in viruses means they remain inactive but can reactivate later, as seen with some herpes viruses.
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All viruses are capable of infecting host cells through the same methods of endocytosis and membrane fusion.
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Viruses with an envelope are referred to as 'non-enveloped' viruses.
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The capsid of a virus plays no role in stimulating the host immune system.
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Complex viruses have a capsid structure that is fundamentally the same as helical viruses.
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Enveloped viruses can survive in the gastrointestinal tract because they are resistant to acids.
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Viral classification can be based on the diseases they cause and the hosts they infect.
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Viruses can only reproduce if they invade a host cell to utilize its reproductive machinery.
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Viral transmission via inhalation is associated with the spread of polio.
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The synthesis of new viral components occurs prior to uncoating during viral replication.
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Congenital transmission can lead to infections such as rubella and HIV.
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Hepatitis B is an example of a virus that can only be transmitted through sexual contact.
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Viruses must always possess a lipid bilayer envelope to be viable.
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The nucleocapsid of a virus consists of both nucleic acid and capsomers.
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Viruses are classified as autotropic organisms due to their reliance on host cells.
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Cytopathogenic effects of viruses can lead to the transformation of host cells to malignant forms.
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Viruses can replicate in both living and non-living environments.
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All known viruses are either made up of DNA or RNA but never both.
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A virus's capsid serves no function in the delivery of its genetic material into host cells.
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All viral infections lead to severe symptoms that are always long-lasting.
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Viral infections are always severe and never self-limiting.
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Molecular techniques like PCR are known for their speed and lower sensitivity in diagnosing viral infections.
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Herpes simplex virus symptoms are always mild and do not reoccur.
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Hepatitis C is an example of a viral infection that may present silently without symptoms.
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Electron microscopy is a method used for visualizing viral structures and can provide quick answers in clinical settings.
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Viruses can spread intra-cellularly through inter-cellular bridges.
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Endocytosis is a process that viruses utilize to release new virions from an infected cell.
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The synthesis of viral nucleic acids can occur in both the cytoplasm and the nucleoplasm.
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Budding is a method only used by non-enveloped viruses for releasing new viral particles.
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Viral pathogenesis can lead to either local or systemic infections, including a condition known as viraemia.
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The primary role of viral nucleic acid during the viral life cycle is to facilitate the attachment to host cells.
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Cytopathic effects caused by viruses may lead to cell death and sometimes result in malignant transformations.
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Viral genomes can become integrated into host genomes, leading to potential passage through generations of cells.
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Study Notes
Introduction to Viruses and Viral Infections
- Viruses are infectious agents of small size.
- Contain only one type of nucleic acid: DNA or RNA.
- Totally dependent on a host cell for replication.
- Rely on cellular processes of their hosts to reproduce themselves.
- Strict intracellular parasites.
Viral Morphology
- Viruses are not alive - they contain nucleic acid encased in a protein coat called a capsid.
- The capsid can either be enveloped (with a lipid bilayer membrane) or non-enveloped.
- Enveloped viruses are derived from host cell membranes and contain viral proteins for attachment to host cells.
- The envelope is sensitive to drying, heat, detergents, and acids.
- The capsid protects the virus and helps it attach to host cells, and is resistant to: drying, heat, detergents, acids, and proteases.
- Capsids come in different shapes and symmetries: Icosahedral, Helical, and Complex.
Viral Replication
- Attachment and Penetration: The virus binds to the host cell receptor via its protein/glycoprotein capsid. The virus then enters the cell through endocytosis or by fusing its envelope with the plasma membrane.
- Uncoating: The viral nucleic acid is released into the cytoplasm or nucleus, making it available for replication.
- Synthesis and Assembly: The virus synthesizes its nucleic acid and proteins. Nucleic acid synthesis can occur in the nucleus or cytoplasm, while protein synthesis always occurs in the cytoplasm.
- Assembly and Release: Viral components assemble into complete viruses within the host cell. Non-enveloped viruses are released by lysis or exocytosis. Enveloped viruses are released by budding.
Viral Classification
- Viruses are classified by:
- Phenotypic characteristics (e.g., morphology, enveloped or not).
- Genotypic characteristics (e.g., nucleic acid type).
- Mechanism of replication.
- The hosts they infect.
- The diseases they cause.
Viral Pathogenesis
- Viral infections can be local, systemic, or both depending on the tissue targeted.
- Different viruses have varying cytopathological effects (CPE) on host tissue.
- Viral infections trigger an immune response, which can be either protective or contribute to the pathology of the infection.
- Latent infection: the virus remains in the host without causing any symptoms.
- Transformation: the virus induces malignant or pre-malignant changes in the host cell.
- Active infection: the virus causes symptoms.
Viral Spread
- Viral infections spread through:
- Intracellular (cell to cell): through intercellular bridges (e.g., herpes simplex virus).
- Extracellular: the virus is released from infected cells and spreads to other cells locally and distally (e.g., influenza).
- Neural: the virus travels along peripheral nerves to the central nervous system (e.g., rabies, herpes simplex virus).
- Nuclear: the viral genome is incorporated into the host genome, leading to spread to subsequent cell generations (e.g., HIV).
Outcomes of Viral Infections
- Many viral infections are mild and self-limiting.
- Viral infections can be severe in immunocompromised individuals.
- Some viral infections are silent (e.g., hepatitis C).
- Some viral infections can reoccur intermittently (e.g., herpes simplex virus).
- Some viral infections can be fatal.
Diagnosis of Viral Infections
- Clinical diagnosis: Diagnosis based on symptoms. Can be difficult to distinguish from bacterial infections.
- Laboratory diagnosis: Used to detect, identify, and quantify the virus.
- Molecular methods (e.g., PCR, RT-PCR): detect viral DNA/RNA, quantify viral load, and detect resistance to antiviral drugs.
- Serological methods: Detect viral antigens or antibodies to the virus.
- Paired sera samples (acute and convalescent) are used to confirm recent infection.
- Microscopy: Includes electron microscopy (EM), light microscopy (LM), and immunofluorescence microscopy.
- EM: visualize viruses directly.
- LM: visualize viral inclusion bodies within infected cells.
- Immunofluorescence: rapid detection of viral antigens in a specimen.
- Viral Culture: Viruses grow in living cells grown in culture. Examine cells for cytopathological effects (CPE) or haemadsorption to identify the virus.
Advantages and Disadvantages of Diagnostic Methods
-
Viral Culture:
- Advantages: Determines viability.
- Disadvantages: Slow, poor sensitivity, susceptible to contamination.
-
Serology:
- Advantage: Can confirm recent infection.
- Disadvantage: Requires paired sera samples, which can be time-consuming.
-
Molecular Methods:
- Advantages: Fast, highly sensitive, quantifiable.
- Disadvantage: Not always readily available, can be expensive.
Key Concepts and Terms
- Virion: The complete infectious viral particle.
- Capsid: The protein coat surrounding the viral genome.
- Nucleocapsid: The complex of the viral genome and capsid.
- Envelope: A lipid bilayer membrane surrounding the capsid of some viruses.
- Cytopathological Effect (CPE): Visible changes in cells caused by virus infection.
- Inclusion Bodies: Masses within the cytoplasm of infected cells caused by viral infection.
- Haemadsorption: The ability of infected cells to stick to red blood cells.
- Antigens: Substances that induce an immune response.
- Antibodies: Proteins produced by the immune system that bind to specific antigens.
- Viraemia: The presence of virus in the blood.
Capsid Morphology
- Capsids are the protein coat that surrounds the genetic material of a virus.
- They can have different shapes and symmetries.
- Icosahedral capsids have 20 faces, each an equilateral triangle.
- Helical capsids have a cylindrical or rod shape.
- Complex capsids do not fit into either of the other categories.
Biological Role of the Capsid
- The capsid introduces the viral genome into the host cell.
- Capsid proteins interact with host cell receptors to facilitate entry.
- The capsid contains antigens that stimulate the host immune system.
- It is resistant to drying, heat, detergents, acids, and proteases.
The Envelope
- The envelope is a lipid bilayer that surrounds the capsid.
- It is derived from host cell membranes during a process called "budding."
- The envelope contains viral proteins that help the virus attach to receptors on susceptible host cells.
- It confers a distinctive serological characteristic that can be used to diagnose some viruses.
- The envelope is sensitive to drying, heat, detergents, and acids.
Viral Genetics
- Viruses have either DNA or RNA as their genetic material, but never both.
- The nucleic acid can be single-stranded, double-stranded, or circular.
Viral Acquisition
- Viruses can be acquired through various routes, including:
- Vectors: Animals or insects transmit viruses like rabies and dengue.
- Inhalation: Viruses like influenza, coronaviruses, and rhinovirus enter through the respiratory system.
- Faecal-oral: Viruses like rotavirus, norovirus, and hepatitis A & E are spread through fecal-oral contact.
- Bloodborne: Hepatitis B, hepatitis C, and HIV are transmitted through blood.
- Sexual: Hepatitis B, hepatitis C, and HIV can be acquired through sexual contact.
- Congenital: Rubella, CMV, hepatitis B, hepatitis C, and HIV can be transmitted from mother to child during pregnancy.
Viral Replication
- Viruses depend on host cells to replicate, as they lack the necessary machinery.
- The replication process consists of six stages:
- Attachment/Fusion: The virus attaches to the host cell.
- Penetration/Entry: The virus enters the cell.
- Uncoating: The viral genome is released from the capsid.
- Synthesis/Replication: The viral genome is replicated.
- Assembly: Viral components assemble into new viruses.
- Release: New viruses are released from the host cell.
Attachment and Penetration
- Viral attachment occurs when the capsid proteins or glycoproteins bind to specific receptors on the host cell.
- Viruses enter the cell through endocytosis or by fusing their envelope with the plasma membrane.
Uncoating
- Uncoating frees the viral genome from the capsid, making it available for replication.
Synthesis & Assembly
- Viral nucleic acid and proteins are synthesized through multiple strategies.
- Nucleic acid synthesis can occur in the nucleus or cytoplasm.
- Protein synthesis always occurs in the cytoplasm.
Assembly and Release
- New viruses are assembled from newly synthesized viral components.
- Non-enveloped viruses are released through lysis or exocytosis.
- Enveloped viruses are released by budding.
Viral Pathogenesis
- Viral infection can lead to:
- Local infection: The virus remains confined to the site of infection.
- Systemic infection (Viraemia): The virus spreads throughout the body.
- Cytopathological activity: The virus causes damage to host tissues.
- Immune response: The host mounts an immune response against the virus.
- Immunopathology: The immune response itself can contribute to tissue damage.
Viral Spread
- Viruses can spread within and between host cells:
- Intracellular (cell to cell): Viruses can spread through intercellular bridges.
- Extracellular: Viruses are released from infected cells and spread to other cells.
- Neural: Viruses travel along peripheral nerves to the central nervous system.
- Nuclear: Viral genome integrates into the host genome, leading to viral transmission to subsequent generations of cells.
Outcome of Viral Infections
- Viral infections can have a variety of outcomes:
- Mild and self-limiting: Many viral infections are mild.
- Severe in vulnerable patients: Viral infections can be severe in individuals with compromised immune systems.
- Silent: Some infections may not cause any noticeable symptoms.
- Intermittent recurrence: Some infections reappear periodically.
- Fatal: Some viral infections can be fatal.
Clinical Diagnosis of Viral Infection
- Viral infections can be difficult to distinguish from bacterial infections based on clinical symptoms.
- Infections can be acute (short-term) or chronic (long-term).
Laboratory Diagnosis of Viral Infection
- Laboratory methods used for diagnosing viral infection:
- Molecular techniques: DNA/RNA-based methods like PCR and RT-PCR.
- Serological tests: Detect viral antigens or antibodies to the virus.
- Microscopy: Visualize viruses or effects of infection using electron microscopy (EM) or light microscopy (LM).
- Viral culture: Grow viruses in living cells to observe the effect of viral growth.
Molecular Techniques
- Molecular techniques like PCR and RT-PCR are fast, sensitive, and allow for quantification.
- They are used for rapid diagnosis, guiding treatment, and detecting antiviral drug resistance.
Microscopy
- Immunofluorescence microscopy: Rapid detection of viral antigens in specimens using labeled antibodies.
- Light microscopy: Detects virus-induced masses within the cytoplasm of cells ("inclusion bodies").
- Electron microscopy (EM): Requires specialized equipment, providing quick answers to clinical questions.
Viral Culture
- Viral culture involves growing viruses in living cells like human or animal cells.
- The effects of viral growth, such as cytopathological effects (CPE) or haemadsorption, are observed.
Disadvantages of Viral Culture
- Viral culture is slow, often insensitive, susceptible to bacterial contamination, and susceptible to toxic substances.
Serology
- Serological tests detect either viral antigen (e.g., HBsAg for hepatitis B surface antigen) or antibodies produced against the virus.
- Paired sera samples (one collected at the onset of illness and another 7-14 days later) are used to confirm infection if there is a rise in antibodies.
Summary
- Viruses are composed of nucleic acid, capsid, and sometimes an envelope.
- They rely on host cells for replication.
- Viral classification takes into account factors like shape, nucleic acid, disease, host, and replication mechanism.
- Viral infections cause a wide range of diseases.
- Clinical diagnosis may be difficult.
- Laboratory diagnosis utilizes molecular, serological, microscopic, and culture techniques.
- Both qualitative and quantitative methods can be used to identify and characterize viral infections.
Viruses
- Small infectious agents that contain either DNA or RNA, but never both.
- Depend entirely on a host cell for replication.
- Classified based on morphology, genetics, replication mechanism, host they infect, and disease caused.
- Cannot be classified using the same rules as living organisms.
Virion Structures
- Contain a capsid: a protein shell surrounding the genome composed of capsomers that provide a protective shield.
- Possess different shapes and symmetries, including icosahedral, helical, and complex.
- Can have an envelope: lipid bilayer that surrounds the capsid derived from host cell membranes during ‘budding’.
- Envelope contains viral proteins that help attach the virus to host cell receptors.
Viral Classification:
- Phenotypic: morphology, such as the presence or absence of an envelope.
- Genotypic: nucleic acid type, such as DNA or RNA, single-stranded or double-stranded.
- Mechanism of Replication: how the virus replicates.
- Hosts: what types of organisms they infect.
- Disease Caused: the illness resulting from infection.
Enveloped Viruses
- Possess a distinctive serological characteristic used for diagnosis.
- Can bind to host cells through surface polypeptides.
- Have antigens that stimulate host immune responses.
- Are sensitive to drying, heat, detergents, and acids.
Virus Acquisition
- Can be acquired through various modes: vectors, inhalation, faecal-oral, bloodborne, sexual, and congenital.
Viral Replication Steps
- Attachment/Fusion: viral proteins bind to host cell receptors.
- Penetration/Entry: virus internalized into the cell through endocytosis or envelope fusion.
- Uncoating: viral nucleic acid is released into the cytoplasm or nucleus.
- Synthesis/Replication: viral nucleic acid and proteins are synthesized using host cell machinery.
- Assembly: viral components are assembled into complete viruses.
- Release: new viruses are released from the cell through lysis, exocytosis, or budding.
Viral Pathogenesis
- Interaction with target tissue: local or systemic infections can occur.
- Cytopathological activity: effects of the virus on host tissue.
- Immune response: can the virus escape the immune system?
- Immunopathology: type of immune response initiated by the virus.
Outcomes of Viral Infections
- Can range from mild and self-limiting to severe and fatal.
- Can be acute or chronic.
Clinical Diagnosis of Viral Infections
- Often difficult to distinguish from bacterial infections.
Laboratory Diagnosis of Viral Infections
-
Qualitative:
- Detect and identify the virus.
- Determine the type of virus.
- Assess the viability of the virus.
- Quantitative: determines the viral load.
- Methods: molecular (PCR, RT-PCR), serological (detect antibodies), microscopy (visualizing viruses or effects on cells).
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Description
This quiz explores the fundamental characteristics of viruses, including their structure, replication processes, and the nature of viral infections. Test your knowledge on viral morphology, including the differences between enveloped and non-enveloped viruses. Understand how viruses depend on host cells for replication and their role as intracellular parasites.
