4: Viruses and prions

Chapter 4 Viruses and prions

Viruses are one of the smallest forms of microorganism and infect most other forms of life: animals, plants and bacteria. They can also cause severe acute oral and orofacial disease, produce oral signs of systemic infection, and be transmitted to patients and dental staff. The main features that characterize viruses are:

Structure

Viruses consist of a nucleic acid core containing the viral genome, surrounded by a protein shell called a capsid (Figs 4.1 and 4.2). The entire structure is referred to as the nucleocapsid. This may be ‘naked’, or it may be ‘enveloped’ within a lipoprotein sheath derived from the host cell membrane. In many viruses (e.g. orthomyxoviruses, paramyxoviruses), the ensheathment begins by a budding process at the plasma membrane of the host cell, while others, such as herpesviruses, ensheath at the membrane of the nucleus or endoplasmic reticulum.

The protein shell or capsid consists of repeating units of one or more protein molecules; these protein units may go on to form structural units, which may be visualized by electron microscopy as morphological units called capsomeres (Fig. 4.1). Genetic economy dictates that the variety of viral proteins be kept to a minimum as viral genomes lack sufficient genetic information to code for a large array of different proteins. In enveloped viruses, the protein units, which comprise the envelopes and are visualized electron microscopically, are called peplomers (loosely referred to as ‘spikes’).

Taxonomy

Vertebrate viruses are classified into families, genera and species. The attributes used in classification are their symmetry, the presence or absence of an envelope, nucleic acid composition (DNA or RNA), the number of nucleic acid strands and their polarity. Classification of some of the recognized families of RNA and DNA viruses is given in Table 4.1. (Note: to memorize which viruses contain DNA, remember the acronym ‘PHAD’: P is for papova and pox, H for herpes and AD for adenoviruses. Most of the remainder are RNA viruses, including the self-evident picornaviruses.)

Table 4.1 Classification of some of the viruses causing human disease

Morphology Virus
DNA  
Enveloped, double-stranded nucleic acid Herpesviruses
  Herpes simplex virus
  Varicella-zoster virus
  Epstein–Barr virus
  Cytomegalovirus
  Human herpesvirus 6
  Poxviruses
  Vaccinia
  Orf
Enveloped, single-stranded Parvoviruses
Non-enveloped, double-stranded Adenoviruses
  Papovaviruses
  Polyomaviruses
  Papillomaviruses
  Hepadnaviruses
  Hepatitis B virus
RNA  
Enveloped, single-stranded Orthomyxoviruses
  Influenzavirus
  Paramyxoviruses
  Parainfluenza
  Respiratory syncytial
  Mumps
  Measles
  Togaviruses
  Rubella
  Retroviruses
  Human immunodeficiency viruses HTLV-I, -III
  Rhabdoviruses
  Rabies
Non-enveloped, double-stranded Reoviruses
Rotavirus
Non-enveloped, single-stranded Picornaviruses
Rhinovirus
  Enterovirus
  Coxsackievirus
  Echovirus
  Poliovirus

HTLV-I, human T cell leukaemia virus type I.

The following is a concise description of the families of mammalian viruses.

DNA viruses

Herpesviruses

Herpesviruses are the predominant viral cause of oral infections in humans; often the infections are recurrent, and latent.

Structure

These enveloped, icosahedral viruses are 180–200 nm in diameter and contain a linear dsDNA molecule. The Herpesviridae family has over 100 members spread widely among vertebrates, and invertebrates and new species are continuously being added. Herpesviruses are unstable at room temperature and are rapidly inactivated by lipid solvents such as alcohol and other common disinfectants owing to the disruption of the outer lipid envelope.

During reproduction, maturation of the progeny begins in the nucleus of the host cell, which buds through the nuclear membrane and acquires the viral envelope. Typical and highly pathognomonic intranuclear inclusions are therefore found in cells that have undergone active virus replication. As many herpesviruses can fuse with the cells they infect, polykaryocytes or giant cells readily appear in tissue lesions. Such cells, e.g. Tzanck cells or nuclear inclusions (Lipschiitz bodies), are hallmarks of herpetic infections.

Different herpesviruses cause a variety of infectious diseases, some localized and some generalized, often with a vesicular rash. Herpesviruses establish latent infection, which can be readily reactivated by immunosuppression (Table 4.2).

Table 4.2 Latent viruses relevant to dentistry

Virus Site of latency
Herpes simplex virus Trigeminal ganglion
Varicella-zoster virus Sensory ganglia
Epstein–Barr virus Epithelial cells
  B lymphocytes
Cytomegalovirus Salivary gland cells
Papillomaviruses Epi/>

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Jan 4, 2015 | Posted by in General Dentistry | Comments Off on 4: Viruses and prions
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