Animal viruses are now classified into 16 separate families. Classification criteria include morphology, physiochemical properties,
and genetic and antigenic properties. There is clinically relevant information in the classification schema of animal viruses in relationship to pathogenesis of disease
as well as prevention and control.
The morphologic characteristics are clinically relevant to an understanding of the relative complexity of the individual virus
and its associated disease. The larger the virion (virus particle), the more genetic information contained in the genome.
The more genetic information, the more complex the virion structure: the more complex the virion, the more complex the disease.
Whether a virus has a spherical shape or a helical shape has little direct bearing on disease, although it is related to the
molecular and genetic makeup and complexity of the virus and the complexity and pathogenesis of the disease produced.
Viruses commonly affecting cattle (part 1)
The basic viral components are protein, and nucleic acid; some viruses also contain lipid in their makeup. Protein constitutes
the major structural component of all virions and functions primarily as a protective coat that surrounds the nucleic acid
of each virus particle (virion); this external protein is called the capsid and is composed of repeating protein subunits
(capsomeres). The capsid and its enclosed nucleic acid constitute the nucleocapsid. Viruses may contain internal proteins,
which include (1) a membrane protein that lines the capsid to provide nucleocapsid stability; (2) nucleoproteins, such as
histones, that are intimately associated with the nucleic acids and provide flexibility and stability for the nucleic acid;
and (3) enzymatic proteins that function in the replication of nucleic acid, i.e., reverse transcriptase.
Viruses commonly affecting cattle (part 2)
The nucleic acid possessed by a virus is limited to either DNA or RNA. Virion DNA may be either single- or double-stranded
while virion RNA may be either single- or double-stranded, linear, segmented, positive (able to function as messenger), or
negative (anti-messenger). The type, size, and weight of the nucleic acid are proportional to the size and complexity of
the individual virion, replication mechanisms, and, ultimately, the disease course.
One family of DNA viruses and almost all RNA viruses have an outer envelope that is composed of lipid (derived primarily from
host cell membranes) with specific viral proteins (inserted into the cell membranes during virus replication). The envelope
is necessary for the infectious integrity of the virus. The envelope proteins participate in the viral replication, disease
pathogenesis and disease control and are exemplified by the hemaglutinins and the neuraminidase of the influenza viruses;
these proteins also function as the cellular receptors to initiate viral infection. Enveloped viruses are generally more
antigenically complex and less immunogenic. Significantly, enveloped viruses routinely elicit an immune-mediated component
to the disease. Enveloped viruses are more unstable facilitating hygienic control since destruction of the envelope destroys
the infectivity of the virus.
Genetic properties and, ultimately, antigenic properties are utilized for characterization and classification purposes and
for determining the relationships within a given family. These properties also play a significant role in the production
of vaccines that can provide cross-protection between various viruses. The genetic makeup of viruses also has become important
in the separation of closely related viruses, or the separation of field virus strains from vaccine virus strains as well
as facilitating diagnoses, i.e., PCR.
DNA viruses replicate in the nucleus, except poxviruses, of the infected cell with the production of inclusion bodies; this
site of replication corresponds to the normal site of DNA replication within eukaryotic cells. DNA viruses routinely have
longer incubation times and longer disease duration. DNA viruses, except the herpeviruses, are non-enveloped. Lastly all
except the Parvoviridae possess oncogenic members.
RNA viruses replicate in the cytoplasm of the infected cells often with the production of intracytoplasmic inclusion bodies;
the Orthomyxoviridae and the Paramyxoviridae also have nuclear phases in their replication process with the production if
both intranuclear and intracytoplasmic inclusion bodies. The oncogenic RNA viruses are in the Retroviridae family. Only
the Picornaviridae, Caliciviridae, and the Reoviridae are nonenveloped. The RNA viruses possess their genome in various combinations
of the following configurations: (1) double-stranded or single-stranded, (2) positive or messenger type, (3) negative or antimessenger-type,
and (4) segmented or non-segmented. RNA virus diseases generally have a shorter incubation time and a shorter duration. RNA
viruses have much higher mutation rates during replication.