FFP1L3 Introduction to Viruses and Viral Infections 2023 (PDF)

Leading the world to better health  RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Session I...

Leading the world to better health  RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Session ID: FFP1ML3 Introduction to viruses and viral infections Class Year 1 Course Undergraduate Medicine Lecturer Dr. Muaaz Ather Prof Manaf Al Qahtani Date 9th October 2023  LEARNING OUTCOMES 1. Discuss the basics of viral morphology 2. Explain the basics of viral replication 3. Classify viruses and give some examples 4. Name some common viral infections 5. Briefly discuss what happens when a healthy host acquires a viral infection 6. Summarise how viral infections are diagnosed in the laboratory and clinically  LEARNING OUTCOME 1. Discuss the basics of viral morphology BASICS - WHAT ARE VIRUSES? Infectious agents of small size Contain only one type of nucleic acid: DNA or RNA, but never both Are totally dependent on a host cell for replication – Rely on the cellular processes of their hosts to reproduce themselves Note: Strict intracellular parasites  BACTERIA VS. VIRUSES WHAT’S THE DIFFERENCE?  HOW CAN SOMETHING SO SMALL CAUSE SO MUCH TROUBLE? Source: ebug.eu  VIRUSES: DEAD OR ALIVE? A. Alive B. Dead C. Not sure  VIRUSES: DEAD OR ALIVE? https://microbiologysociety.org/publication/past-issues/what-is-life/article/are-vir uses-alive-what-is-life.html VIRUSES ARE A BIT LIKE PRESENTS NUCLEIC ACID IN GIFT WRAPPING  VIRION COMPONENTS Capsid: proteinaceous coat made of Nucleic Acid capsomers Nucleic acid: DNA or RNA (never both) O R Nucleic acid + Capsid = Nucleocapsid Envelope: not always present  CAPSID OVERVIEW Protein shell composed of CAPSOMERS surrounding the genome. Protective shield Viruses can have capsids with different shapes and symmetries  CAPSID: MORPHOLOGY Icosahedral 20 faces each an equilateral triangle Helical Complex  BIOLOGICAL ROLE OF THE CAPSID Introduces viral genome into host cells – Capsid proteins interact with host cell receptors Contains antigens – Stimulate the host immune system Resistant to – Drying / Heat / Detergents / Acids / Proteases  LEARNING OUTCOME 3. Classify viruses and give some examples 4. Name some common viral infections  VIRAL CLASSIFICATION Technically not living entities, therefore common rules that apply to living cellular organisms do not apply Classified according to - Phenotypic characteristics e.g. morphology ?enveloped or not - Genotypic characteristics e.g. nucleic acid type - Mechanism of replication - The hosts they infect - Lastly... the diseases they cause!  THE ENVELOPE Lipid bilayer that surrounds the capsid composed of phospholipids and glycoprotein Viruses without an envelope are called ‘non enveloped’ viruses!  Derived from host cell membranes ‘budding’. Also contains viral proteins = attach the virus to receptors on susceptible host cells  PROPERTIES OF ENVELOPED VIRUSES Confers a distinctive serological characteristic by which some viruses can be diagnosed (antigen) Surface polypeptides can bind specific receptor sites on the host cell Viral antigens stimulate host immune responses Envelope is sensitive to Drying / Heat / Detergents / Acid Consequences Must stay wet during transmission / Cannot survive in the gastrointestinal tract / Do not need to kill cells in order to spread Adapted from Murray, P.R. Rosenthal K.S., Pfaller, M.A. (2005) Medical Microbiology, 5th edition  GENETICS Circular or linear nucleic acid  GENETIC – BROAD CLASSIFICATION DNA or RNA (never both) Single-stranded or double- stranded Sense  VIRAL ACQUISITION Vectors – animal or insect Rabies, dengue Inhalation Influenza, coronaviruses, rhinovirus Faecal oral Rotavirus, norovirus, hepatitis A&E Blood borne Hepatitis B & C, HIV Sexual Hepatitis B & C, HIV Congenital Rubella, CMV, Hepatitis B & C, HIV SOME EXAMPLES OF COMMON VIRUSES Nucleic acid Family Virus Disease Herpes Herpes simplex Cold sores Varicella zoster Chickenpox DNA Shingles Epstein-Barr Infectious mononucleosis Hepadna Hepatitis B Hepatitis Orthomyxo Influenza Flu RNA Picorna Polio Hepatitis A Polio Hepatitis Retro HIV AIDS  LEARNING OUTCOME 2. Explain the basics of viral replication  VIRAL REPLICATION Viruses only reproduce within an appropriate cell Do not possess the necessary machinery themselves to replicate Must invade cell (plant, animal, human) & utilise cell’s reproduction system to make copies of itself  VIRAL REPLICATION 1. Attachment/Fusion 2. Penetration/Entry 3. Uncoating 4. Synthesis/Replication 5. Assembly 6. Release https://courses.lumenlearning.com/microbiology/chapter/the-viral-life-cycle/  ATTACHMENT AND PENETRATION Viral attachment – Capsid protein/glycoprotein to host cell receptor Virus internalised into cell – Endocytosis – Envelope fusion with the plasma membrane  UNCOATING Need to make the genome available Viral nucleic acid into cytoplasm/ nucleus  SYNTHESIS & ASSEMBLY Many strategies for synthesis of viral nucleic acid & protein – Nucleic acid can be made in nucleus or cytoplasm – Protein synthesis always in cytoplasm ASSEMBLY & RELEASE OF NEW VIRUS FROM CELL Viral components must assemble into complete viruses to go from one host cell to another Non-enveloped: lysis or exocytosis Enveloped: budding  LEARNING OUTCOME 5. Briefly discuss what happens when a healthy host acquires a viral infection  VIRAL PATHOGENESIS Interaction with target tissue – Can organism establish a local infection/a systemic infection i.e. a viraemia Cytopathological activity – What effect does it have on host tissue? Immune response – Can virus escape immune responses? Immunopathology – What type of immune response does the virus initiate?  Effects of Viruses on Cells LATENT DEAT TRANSFORMA INFECTI H TION ON Cell becomes malignant or May Cytopathog subsequent enic effect pre- malignant ly (CPE) seen reactivate on cell e.g. papillomavir e.g. herpes us viruses  VIRAL SPREAD 1. Intracellular (cell to cell) Intercellular bridges e.g. HSV 2. Extracellular Virus is released from infected cells and spreads to other cells locally and distally (haematogenous / lymphatic cells) e.g. Influenza 3. Neural Peripheral nerves to CNS e.g. Rabies, HSV 4. Nuclear Viral genome incorporated into host genome – passed onto succeeding cell generations e.g. HIV  OUTCOME OF VIRAL INFECTIONS Many viral infections are mild & self limiting – e.g. Coronaviruses causing common cold May be severe in vulnerable patients – e.g. Cytomegalovirus in transplant patients May be silent – e.g. Hepatitis C May reoccur intermittently – e.g. Herpes simplex virus (cold sores) May be fatal – e.g. Rabies N.B. Infections may be acute or chronic  LEARNING OUTCOME 6. Summarise how viral infections are diagnosed in the laboratory and clinically  CLINICAL DIAGNOSIS ‘It’s probably “just” viral…’ – What I imagine the doctor said to the first person who presented with COVID-19 symptoms Often very difficult to distinguish from bacterial infection But sometimes pretty obvious … Images from: www.dermnetnz.org  LABORATORY DIAGNOSIS OF VIRAL INFECTION Qualitative: - Detect and identify. - Is it there - What is it? - Is it viable? Quantitative: - How much of it is there?  Detect and/or Identify the Virus Molecular Serological Microscopy Visualize Detect Virus Detect virus (EM) Detect Viral Specific Antibody to or effects of Antigen DNA/RNA the Virus (LM) EM – Electron Microscopy LM – Light Microscopy  MOLECULAR E.G. PCR & RT-PCR Fast - (Relatively – many serological tests are faster) Usually very sensitive Quantification Used for – rapid diagnosis of viral infection – guide treatment e.g. HIV viral load – detection of resistance to antiviral drugs  MICROSCOPY ImmunoFluorescence (Microscope) – Rapid detection of viral antigens in specimen using commercially produced antibodies 🔬 Light Microscope - Inclusion Bodies – Virus-induced masses within cytoplasm of cells – Can be pathognomonic without need for culture (negri bodies in hippocampal cells in rabies) Electron Microscope (EM) – Specialized – Can give quick answers to clinical questions  VIRAL CULTURE Viruses only replicate within living cells so LIVING cells needed for culture Living human or animal cells are grown in artificial culture Examine the cells for the effect of viral growth – ‘Cytopathological effect’ – Haemadsorption (cells acquire the ability to stick to mammalian red blood cells) Confirm the ID of the virus by immunofluorescence / neutralization / haemadsorption inhibition  DISADVANTAGES OF VIRAL CULTURE  Slow - up to 4 weeks required for result 🗓😴  Often very poor sensitivity, sensitivity depends to a large extent on the condition of the specimen  Susceptible to bacterial contamination  Susceptible to toxic substances which may be present in the specimen.  Many viruses will not grow in cell culture e.g. Hepatitis B, diarrhoeal viruses, parvovirus, papillomavirus.  MAJOR ADVANTAGE OF CELL CULTURE Can determine viability! If cytopathological effect (CPE) is observed - virus is viable  SEROLOGY Testing for viral antigen e.g. HBsAg (Hepatitis B surface antigen) AND/OR Testing for the presence of specific antibodies produced against viral antigens – Ideally wo samples required (paired sera): one at the onset of illness & a second after 7-14 days – Rise in antibodies confirms infection  SUMMARY Viruses are composed of nucleic acid, a protein coat (capsid) and occasionally an envelope Totally dependent on host cell for replication Are classified according to shape, nucleic acid, disease, host and mechanism of replication Cause a diverse range of infections Diagnosis can be made clinically Laboratory diagnosis may be qualitative or quantitative and incorporates a number of techniques including molecular, serological, microscopy and culture Thank you