More than 200 viruses have been identified as capable of producing disease. Acute viruses, such as the common cold, have a rapid onset and quick recovery. Chronic viral infections, such as acquired immunodeficiency syndrome (AIDS), have recurrent episodes of exacerbations (increases in severity of symptoms of the disease) and remissions (periods of partial or complete disappearance of the signs and symptoms). Display 14-1 describes the viruses discussed in this chapter.

Although viral infections are common, for many years only a limited number of drugs were available for their treatment. Over the past several years, the number of antiviral drugs has increased significantly. Several of the antiviral drugs will be discussed in greater detail than others. These include acyclovir (Zovirax), amantadine (Symmetrel), didanosine (Videx), ribavirin (Virazole), zanamivir (Relenza), and zidovudine (AZT, Retrovir).

Viruses represent a large group of infective agents that are composed of a core of nucleic acids, either RNA or DNA, surrounded by a layer of protein. They are not really living organisms according to general understanding, since they lack the cell membrane that is associated with living cells. Viruses can reproduce only inside a living cell, and they cause many diseases. Viruses are not normally affected by antibiotics but a small number of viruses can either be destroyed or have their growth stopped by drugs.

Viruses can reproduce only within a living cell. A virus consists of either DNA or RNA surrounded by a protein shell. The virus is capable of reproducing only when it uses the body’s cellular material (Fig. 14-1). Most antiviral drugs act by inhibiting viral DNA or RNAreplication in the virus, causing viral death.

General Characteristics of Viruses

1. Viral structure:Typical viral components are shown in Figure.These components are a nucleic acid core and a surrounding protein coat called a capsid. In addition some viruses have a surrounding lipid bilayer membrane called an envelope.

 

A. Nucleic acid

• Viral genomes are either DNA or RNA (not both)

•  Nucleic acid may be single- or double-stranded

B. Capsid

• protein coat

• Protection of Nucleic Acid

• Provides Specificity for Attachment

• Capsomeres are subunits of the capsid

C. Envelope

• Outer covering of some viruses

• Envelope is derived from the host cell plasma membrane when the virus buds out

• Some enveloped viruses have spikes, which are viral glycoproteins that project from the envelope

• Naked (non-enveloped) viruses are protected by their capsid alone

2. Size of viruses:

Determined by electron microscopy

Ranges from 20 to 14000 nm in length

 

3. Shape of viruses:

• Four basic morphologies

• Icosahedral - efficient means to conserve and enclose space; form capsomers (planar faces formed by association of proteins)

• Helical - capsid is shaped like a hollow protein tube

• Enveloped - outer covering derived from the host cell’s nuclear or plasma membrane and often possessing spikes or peplomer projections involved in attachment and entry into a host cell sometimes via their enzymatic activity

• Complex symmetry - viruses that fit neither of the above categories or which may employ portions in combination, e.g., bacteriophage

4. Host Range: The specific types of cells a virus can infect in its host species represent the host range of the virus.

• Animal virus

• Plant virus

• Bacterial virus (bacteriophage)

• Host range is determined by attachment sites (receptors)