INTRODUCTION
Although they are not alive, viruses are the quintessential parasites in several respects; all viruses are obligate intracellular parasites incapable of replication and assembly without using host cellular processes.
Viruses differ from even the simplest of living cells by their very small size; they range from a diameter of 18-nm for tiny parvoviruses, to about 300 nm for the larger iridoviruses, to approximately 750 nm for the recently discovered mimiviruses. Although the largest viruses overlap the size of the smallest bacteria, most viruses tend to be about i/i00th the size of most bacteria. Viruses have simple structures; they lack internal membranes and have only a single type of nucleic acid—either RNA (ribonucleic acid) or DNA (deoxyribonucleic acid), but not both. Viruses also lack both metabolism and ribosomes necessary for translation (MacLach- lan and Dubovi 2011a). Simple viruses are little more than infectious sequences of nucleic acid wrapped in a protein coat; this combination is called a nucleocapsid. The protein coat, or capsid, is made of a small number of repeated structural units called capsomeres. Each fully assembled viral particle is referred to as a virion. In addition to this basic structure, some viruses are wrapped in an external membrane called an envelope. Envelopes are derived from host membranes in those viruses that bud from host cells; viruses that exit host cells by rupturing or otherwise killing the cells are non-enveloped. The type of membrane incorporated into the envelope depends on where in the host cell the viral replication occurs; thus the viral membrane comprises host plasma membrane for viruses replicating in the host cytoplasm, nuclear membrane for those that replicate in the nucleus, Golgi body membrane for those that replicate in Golgi bodies, and endoplasmic reticular membrane for those that replicate in the endoplasmic reticulum. Once outside the host cell, all viruses are inert, non-metabolizing, nonliving particles (MacLachlan and Dubovi 2011a).There is amazing diversity within and among virus families. Several characteristics are used to classify viruses, including genomic nucleic acid type (RNA or DNA). Important characteristics of genome size and structure include the number of base pairs, whether the genome is a circular loop or linear segment(s), single strandedness or double strandedness, and gene sequences. Other characteristics include the location of replication within the host cell, whether the capsid is covered by a membranous envelope, and the basic size and shape of the infectious particles; most viruses are helical or icosahedral. Unlike living organisms, viruses are not given specific, binomial Latin names. However, viruses are classified into families, subfamilies, and genera based on the characteristics mentioned above (App. 1: Table 8).
Since virus particles are inert outside of host cells, the general “life cycle” of a virus must include cellular invasion, self-replication, and emergence of infectious particles. Specifically, viruses perform the following processes in order to replicate. First, a virus must attach to and gain entry into a host cell; most viruses are specialists, so it must be a very rare event that this succeeds. Next, the RNA or DNA, depending on the virus type, must be freed from within the viral capsid, and the nucleic acid typically makes its way into the nucleus of the host cell, where transcription occurs and the viral genome is replicated. Next, translation of viral mRNA creates the protein coats and new virions are assembled; there are many paths by which this occurs in different viruses, and several locations, including the nucleus, cytoplasm, and Golgi apparatus. Finally, viral particles must be released from the cells to continue the cycle (MacLachlan and Dubovi 2011a).
All viruses use host cellular processes to generate new viral genetic material and the proteins that make up the capsid, although the ways in which viruses utilize cellular machinery vary dramatically among families.
Some viruses include double-stranded (ds) DNA as genetic material; these include poxviruses, herpesviruses, and warts viruses, among others). Others use ds RNA; these include the viruses causing bluetongue and epizootic hemorrhagic disease of deer as well as Colorado tick fever virus. The genetic material in some other families of viruses includes only a single strand (ss) of DNA, including the parvoviruses and the virus that causes avian beak and feather disease. The ss RNA viruses include those that cause rabies, inf luenzas, hantaviruses, along with many others. Single strands of nucleic acid can exist with either positive or negative polarity. Positive strands of RNA are translated directly into protein, whereas negative strands of RNA need to first serve as a template to create mRNA; ss DNA viruses use host polymerases to create ds DNA, from which mRNA is produced, to be translated into capsid proteins and then regenerate the ss DNA for new viral particles. Retroviruses are unique in having genomes with two positive RNA strands that are transcribed by viral RNA- dependent DNA polymerase first into an RNA- DNA hybrid and secondarily into ds DNA that is inserted into the host genome. The RNA viruses typically replicate in the cytoplasm, requiring that they use a variety of strategies to utilize the hosts' enzymes for transcription. The various pathways used in virus replication provide a very interesting topic of study, but this subject is beyond our intentions. Interested readers are referred to texts on veterinary microbiology and virology for more information (Murphy et al.1999, Hirsh et al. 2004, MacLachlan and Dubovi 2011a, Quinn et al. 2011).
Discussions of viral diseases can be organized by several different criteria, including viral taxonomy, host species most commonly affected, host tissue and organ pathology, and patterns of transmission and persistence in their hosts. As with the bacteria, we organize the viruses by life history strategies, including those with latent infections, those based on ongoing clinical infections among affected host populations, viruses transmitted by arthropod vectors and, very briefly, we note evidence for possible helminth-vertebrate systems.
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