Virology and Viral Infections
Viruses are small (10–400 nm in diameter) infectious units consisting of a single-stranded or double-stranded nucleic acid genome and a protein shell, or capsid, with or without an external lipid envelope. Viral nucleic acid consists of either RNA or DNA. RNA viral genome may be either single-stranded or double-stranded. Antiviral medications typically target viral gene transcription. Therefore, the clinical significance of the nucleic acid type lies principally in differences in susceptibility to antiviral medications.
The viral capsid interacts internally with the genome to stabilize it, protects the genome from the external environment, and, in the case of nonenveloped viruses, expresses on its surface the ligand for virus–host cell binding.
In some virus families, a host cell–derived lipid bilayer, or envelope, surrounds the protein capsid. Viral genome–encoded glycoproteins bound to the membrane act as ligands (antigens) for neutralizing antibodies directed against the virus. The lipid envelope is vulnerable to damage by ultraviolet (UV) light, detergents, alcohols, and general-use antiseptics. Because of this vulnerability, enveloped viruses, such as herpes simplex virus and human immunodeficiency virus (HIV), are intrinsically susceptible to the external environment, and their infectivity is short-lived outside the host. Enveloped viruses are difficult to transmit via fomites or medical instruments, and alcohol treatment of medical instrumentation is generally sufficient to prevent iatrogenic infection.
In contrast, nonenveloped viruses, such as adenoviruses, are relatively resistant to environmental insult and, in some cases, can persist for weeks outside the human host. To reduce the spread of such viruses in an office practice, the Centers for Disease Control and Prevention recommends the application of dilute bleach (1 part household bleach [containing 5.25%–6.15% sodium hypochlorite] and 9 parts water) to tonometer tips for at least 10 minutes; however, care must be taken to clean residual bleach from the tonometer tip before use to prevent corneal toxicity. Alcohol sterilization is ineffective in this setting. For further discussion of infectious diseases, see BCSC Section 1, Update on General Medicine.
King AMQ, Lefkowitz E, Adams MJ, Carstens EB. Virus Taxonomy: Ninth Report of the International Committee on Taxonomy of Viruses. Oxford, UK: Elsevier; 2012.
Rutala WA, Weber DJ; Healthcare Infection Control Practices Advisory Committee (HICPAC). Guideline for disinfection and sterilization in healthcare facilities, 2008. Centers for Disease Control and Prevention website. www.cdc.gov/infectioncontrol/guidelines/disinfection/index.html. Updated December 29, 2009. Accessed November 20, 2017.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.