VIRUSES
While viruses possess some properties of living systems, including having a genome (Villarreal 2004), they probably are better described as nonliving infectious entities than as living microorganisms (van Regenmortel and Mahy 2004).
Viruses generally lack ribosomes and any independent metabolic activities; they cannot generate ATP, and they cannot reproduce in the absence of a host. All viruses are obligate intracellular parasites and generally have no evidence of living qualities outside of an infected host. However, the finding that some viruses can be infected by other viruses and become “sick” in the process complicates the separation of viruses from other living organisms (Ogata and Claverie 2008).Free viral particles generally consist of a nucleic acid core surrounded by a protein coat (capsid); the complete structure is termed a nucleocapsid, and the individual protein units of the capsid are termed capsomeres (Voyles 2002). Nucleocapsids typically vary from 20 to 300 nanometers in size. Many animal viruses also have an envelope surrounding the capsid. When present, this envelope is derived from the cellular membrane of the host in which the virus replicated; the envelope also has additional viral proteins or glycoproteins used to bind to host cell receptors (Voyles 2002).
Viral Identification
Individual viruses contain either DNA or RNA, but not both; single- and double-stranded forms of both RNA and DNA viruses occur. Classification of viruses is determined by their capsid size and shape (helical, icosahedral, complex), kind of viral nucleic acid (RNA or DNA), nucleic acid strandedness and direction of the nucleic acid twist, number of base pairs and base sequences of their RNA or DNA, and the presence or absence of a lipoprotein envelope. Other taxonomically important features include the mode of replication, as well as the general chemical composition (antigenic properties) of the capsid (Melnick 1999, Murphy et al. 1999).
Viral isolation often first requires isolating and cultivating the viruses in living, susceptible cells such as laboratory animals, embryonated hens' eggs, or tissue culture cells. Viral identification is based on identifying the viral size and morphology through electron microscopy techniques; characterizing the viruses based on gel electrophoresis; DNA profiling by restriction endonuclease analysis, DNA hybridization, or polymerase chain reactions (PCR) (Gough 1997); and using specific antigen-detecting tests (App. 2). Many of the methods used to diagnose viruses are similar in principle to those applied to bacterial infections, particularly regarding the use of serology and recently developed molecular techniques; the primary differences between diagnosing bacterial and viral infections lies in the ability to isolate and cultivate bacteria in prepared sterile growth media, whereas viruses require susceptible living cells in which to replicate (Gough 1997).
Viruses currently are separated into about 73 families and groups (Melnick 1999). Current taxonomic schemes for viruses have been maintained by the International Committee on Taxonomy of Viruses (ICTV) (Buchen- Osmond 2003). Although the species concept for viruses is controversial and complicated (van Regenmortel and Mahy 2004), the ICTV currently recognizes about 3,000 viral species; however, at least 30,000 different viral strains and isolates have been identified by virologists (Buchen-Osmond 2003). A considerable number of viruses have been identified as important to the study of wildlife diseases (App. 1: Table 8).