Medical Virology PDF

3/5/2024 MEDICAL VIROLOGY - We live and prosper in a literal cloud of viruses - The number of viruses on Earth is staggering A virus is an obligate intracellular parasite containing genetic material surrounded by protein....

3/5/2024 MEDICAL VIROLOGY - We live and prosper in a literal cloud of viruses - The number of viruses on Earth is staggering A virus is an obligate intracellular parasite containing genetic material surrounded by protein. Viruses infect all living things. We eat and breathe billions of viruses regularly. We carry viral genomes as part of our own genetic material. 1  3/5/2024 How ‘infected’ are we? HSV-1, HSV-2, VZV, CMV EBV, HHV-6, HHV-7, HHV-8 Once infected, it is for life! HSV-1: herpes simplex virus type 1; HSV-2: herpes simplex virus type 2 VZV: Varicella zoster virus; CMV: cytomegalovirus EBV: Epstein-Barr virus HHV-6: Human herpesvirus 6; HHV-7: Human herpesvirus 7; HHV-8: Human herpesvirus 8 HUMAN VIROME 2  3/5/2024 Comparison between Bacteria and Viruses Bacteria Viruses Usually free-living but can be parasitic Obligate intracellular parasites Ribosomes No ribosomes DNA and RNA DNA or RNA but not both 100 – 1000 genes 10 – 100 genes Seen by light microscope Seen by electron microscope What is a virus? Viruses are small non-cellular infectious, obligate intracellular parasite agents or entities, comprising a genetic material (either DNA or RNA) encased in a protein shell and they cannot reproduce on their own and reproduce only in living cells they have invaded. Viruses cause a variety of human diseases and they are not sensitive to the ordinary antibacterial chemotherapeutic agents. 3  3/5/2024 Virology discipline consists primarily of the following major spheres 1. The knowledge of pathogenic viruses relevant to the diagnosis and management of infectious diseases. 2. The establishment and direction of infection control programmes across the continuum of care. 3. Epidemiology of viruses and prevention of viral diseases. 4  3/5/2024 General Characteristics of Viruses Viruses do not have a cellular organisation. They are non-cellular entities. Viruses are about 10 – 100 times smaller than most bacteria, their sizes are about 20 - 300 nm. 5  3/5/2024 General Characteristics of Viruses Viruses are obligate intracellular parasites. Viruses are obligate intracellular parasites of bacteria, protozoa, fungi, algae, plants, animals and humans. 6  3/5/2024 General Characteristics of Viruses The genome of a virus is either DNA or RNA but never both. Viruses have the genetic information for replication and they are dependent on the synthetic machinery of host cells for their replication. They multiply by a complex process and not by binary fission. General Characteristics of Viruses Viruses are unable of independent growth in ordinary artificial media. They are unaffected by antibacterial antibiotics. 7  3/5/2024 General Characteristics of Viruses Viruses do not have the metabolic machinery for the generation of energy or for protein synthesis because they lack the enzymes necessary for protein and nucleic acid synthesis. Viruses have the genetic information for controlling the host’s cells energy generating system. Be careful: viruses do not think! 8  3/5/2024 Medical importance of viruses The medical importance of viruses lies in their ability to cause a very large number of human diseases. Although most viral infections do not result in death, some, such as rabies, AIDS and Ebola, have very high mortality rates and others can lead to long-term debility such as Poliomyelitis. Certain viruses can cause cancer in animals and birds as well as in humans. Viruses that can cause cancer are called oncogenic viruses. Transmission of Viruses Viruses are intracellular parasites and have to find a new host before the original host mounts an effective immune response or dies. 9  3/5/2024 Transmission of Viruses Horizontal virus transmission Person-to-Person transmission Vertical virus transmission Materno-foetal transplacental transmission from infected mother to her baby Viral Zoonoses Transmission of viruses from infected animals to humans Horizontal virus transmission routes - Air-borne (respiratory route) through inhalation - Food-borne by Ingestion e.g. faeco-oral - Sexual contact - Blood-borne through inoculation 10  3/5/2024 VIRUS PARTICLE I. COVERING: capsid envelope (not found in all viruses) Measles virus II. CENTRAL CORE: example of a virion nucleic acid molecules (DNA or RNA) matrix protein enzymes (not found in all viruses) VIRUS PARTICLE All viruses have a protein capsid or shell that surrounds the nucleic acid in the central core. Together the capsid and the nucleic acid are referred to as the nucelocapsid. 11  3/5/2024 What is a VIRION? A virion is a complete virus that occurs extracellularly and is capable of infecting new cells. Example of a virion An illustrated diagram of the human immunodeficiency virus (HIV) 12  3/5/2024 Example of a virion An illustrated diagram of the EBOLA virus Classification of Viruses Viruses are classified into two main divisions based on the type of nucleic acid they possess: 1. Riboviruses (RNA viruses) contain RNA. 2. Deoxyriboviruses (DNA viruses) contain DNA. 13  3/5/2024 DNA viruses RNA viruses 14  3/5/2024 Viruses are classified differently according to Classification of the following properties Viruses 1. VIRION MORPHOLOGY, including particle size, shape, type of symmetry of the capsid and presence or absence of envelope. 2. VIRUS GENOME PROPERTIES, including type of nucleic acid (DNA or RNA), size of genome , strandedness (single or double), whether linear or circular and nucleotide sequence. 3. GENOME ORGANISATION AND REPLICATION, including gene order, number and position of open reading frames, strategy of replication and cellular sites. Viruses are classified differently according to the following properties 4. VIRUS PROTEIN PROPERTIES, including number, size and functional activities of structural and non-structural proteins, amino acid sequence. 5. ANTIGENIC PROPERTIES. 6. PHYSICOCHEMICAL PROPERTIES OF THE VIRION, including molecular mass, pH stability, thermal stability and susceptibility to physical and chemical agents. 7. BIOLOGIC PROPERTIES, including natural host range, mode of transmission, vector relationships, pathogenicity, tissue tropisms and pathology. 8. IMMUNOLOGIC PROPERTIES. 9. SYMPTOMATOLOGY. 15  3/5/2024 Universal System of Virus Taxonomy Viruses are quite diverse that require classification to aid in their study and identification. The present universal system for virus taxonomy includes family, genera and species. Virus families are designated by terms ending in – viridae. The International Committee on Taxonomy of Viruses (ICTV) had organised more than 60 genera and about 25 families of viruses infecting humans and animals. (ICTV): http://ictvonline.org/ STRUCTURE of VIRUSES Viruses come in many shapes and sizes. The general plan of virus organisation is the utmost in simplicity and compactness. Viruses contain only those parts needed to invade and control a host cell: an external coating and a core containing one or more nucleic acid strands of either DNA or RNA. 16  3/5/2024 STRUCTURE of VIRUSES Knowledge of virus structure furthers our understanding of the mechanisms of certain processes such as the interaction of virus particles with cell surface receptors and neutralising antibodies. Knowledge about virus structure may lead also to the rational design of antiviral drugs capable of blocking viral attachment, uncoating or assembly in susceptible cells. 17  3/5/2024 SHAPES of VIRUSES SHAPES of VIRUSES Viral architecture can be grouped into three types based on the arrangement of morphologic subunits. A virus can have one of three shapes. These are: 1. Helical Symmetry (e.g. orthomyxoviruses) 2. Icosahedral or Cubic Symmetry (e.g. adenoviruses) 3. Complex Structures (e.g. poxviruses). 18  3/5/2024 Helical Symmetry The virus particle is elongated or pleomorphic (not spherical) and the nucleic acid is spiral. Helical Symmetry A helical virus is road or thread shaped. The nucleic acid and capsid protein are closely associated in the ribonucleoprotein, which takes the form of a helix. 19  3/5/2024 Helical Symmetry Helical Symmetry 20  3/5/2024 Icosahedral or Cubic Symmetry (rotational symmetry) An icosahedral virus is spherically shaped. Most viruses that have icosahedral symmetry do not have an icosahedral shape rather; the physical appearance of the particle is spherical. Complex viruses (atypical viruses): Complex Symmetry Complex structural patterns are found in bacteriophages and the smallpox virus. For example, certain bacteriophages are composed of icosahedral heads attached to filamentous shafts. 21  3/5/2024 Poxvirus Bacteriophages 22