Virology Lecture 4: Pathogenesis and Diagnosis of Viral Infections PDF

Introduction to Microbiology: Virology Lecture 4 – Pathogenesis and Laboratory Diagnosis of Viral Infections Darryn Knobel BVSc MSc PhD [email protected] Pathogenesis Pathogenesis Stages of infectious disease in an infected host Infection SUBCLINICAL INFECTIOUS PERIOD Clinical signs Time LATENT...

Introduction to Microbiology: Virology Lecture 4 – Pathogenesis and Laboratory Diagnosis of Viral Infections Darryn Knobel BVSc MSc PhD [email protected] Pathogenesis Pathogenesis Stages of infectious disease in an infected host Infection SUBCLINICAL INFECTIOUS PERIOD Clinical signs Time LATENT PERIOD INCUBATION PERIOD INFECTIOUS PERIOD CLINICAL PERIOD Pathogenesis Stages of infectious disease in an infected host Infection CLINICAL NONINFECTIOUS PERIOD Clinical signs Time LATENT PERIOD INCUBATION PERIOD INFECTIOUS PERIOD CLINICAL PERIOD Pathogenesis Patterns of infection Acute Rhinovirus Rotavirus Influenza virus Latent Herpes simplex virus 1 and 2 Bovine herpesvirus 1 Persistent: asymptomatic Lymphocytic choriomeningitis virus Bovine viral diarrhea virus Persistent: pathogenic Human immunodeficiency virus type 1 Canine distemper virus Pathogenesis The course of a typical acute infection Pathogenesis Pathogenecity and virulence Pathogenesis: The processes following infection of a host that lead to disease Pathogenicity: The potential to produce disease in a host Yes/No – a virus is either pathogenic or it is not (apathogenic) in a host Virulence: A measure of the ability of a pathogenic virus to produce severe disease in a host Quantitative measure Pathogenesis Tissue tropism The spectrum of tissues infected by a virus e.g enterotropic, neurotropic, hepatotropic Ranges from limited to pantropic Some determinants of tissue tropism: susceptibility permissivity accessibility immune defenses Pathogenesis Tissue tropism: highly virulent avian influenza strains Furin In mammals, reproduction of influenza virus is restricted to epithelial cells of the respiratory tract due to local production of tryptase In birds, reproduction of highly virulent influenza virus strains occurs in the intestinal tract as well as the respiratory tract, and there is wide tissue tropism. This is due to mutations in the HA that permits it to be cleaved by ubiquitous proteases such as furins Pathogenesis Mechanisms of host cell damage Loss of cellular function (e.g. immune suppression; neuronal dysfunction) Oncogenesis (development of cancer) By Isabellelyy - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=142143687 Pathogenesis Mechanisms of host cell damage: cytopathic effects Pathogenesis Mechanisms of host cell damage: immunopathology Laboratory Diagnosis of Viral Infections Laboratory Diagnosis of Viral Infections Specimens Laboratory Diagnosis of Viral Infections Diagnostic methods Detection and identification by electron microscopy Virus culture and identification Detection and identification of viral nucleic acids Detection and identification of viral antigens Detection and quantitation of antiviral antibodies Laboratory Diagnosis of Viral Infections Electron microscopy Characteristic morphology of virus families Advantages: Rapid, specific Disadvantages May have low sensitivity (not enough virus, especially in fluids e.g. respiratory fluids – OK for feces and vesicle fluid) Expensive equipment Technically demanding Laboratory Diagnosis of Viral Infections Culture and identification Specimens inoculated into suitable cell cultures and presence of virus detected by various methods 3 main kinds of cell cultures (‘cell lines’): Primary cell cultures: derived from tissues, die after few generations Diploid cell strains: single cell type, grow for up to 100 generations Continuous cell lines: transformed (cancerous) immortal cell lines (e.g. HeLa cell lines, Vero cells, etc.) Laboratory Diagnosis of Viral Infections Culture and identification Detection of cytopathic effects Detection of noncytopathic infected cells: immunoassays or molecular assays Measure of virus virulence: TCID50 = Tissue culture infectious dose: number of viruses required to cause infection in 50% of the cell culture (lower TCID50 = more virulent) Laboratory Diagnosis of Viral Infections Identification of viral nucleic acids As with any laboratory test, positive results need to be interpreted in light of the clinical history, clinical signs, and the pathogenesis of the disease in question. Laboratory Diagnosis of Viral Infections Identification of viral antigens Lateral flow immunochromatographic assay Immunohistochemistry Immunofluorescence Laboratory Diagnosis of Viral Infections Detection of antiviral antibodies (serology) 1:640 1:640 X No rise in titer (= no recent infection) Retrospective diagnosis IgM: Indicates recent illness Single serum specimen OK IgG: ≥4-fold rise in titer (=recent infection)