Classification Of Animal Viruses Research Articles

Classification of medically important viruses II: RNA viruses

The early attempts to classify viruses were based on diseases they caused or on the site of their recovery. They were grouped according to the assumed trophism or affinity for different systems or organs of the body. Thus, human viruses were classified as neurotropic--i.e., those affecting the nervous system (poliomyelitis, rabies), pneumotropicm i.e., those affecting the respiratory system (influenza, common cold), dermotropic i.e., those producing the skin lesions (smallpox, chickenpox, measles), and viscerotropic---i.e., those affecting the visceral organs (yellow fever, hepatitis). This system did not gain acceptance because different viruses could cause similar symptoms. In 1941, Bawden suggested that the viral nomenclature and classification should be based on the properties of the viruses and not upon the host responses. From early 1950s, viruses began to be classified into groups on the basis of their physicochemical and structural features. Currently, viruses are classified into two major groups on the basis of the type of nucleic acid they possess: the riboviruses (RNA-containing) and deoxyriboviruses (DNA-containing). Further classification is based on the other properties of the viruses such as the strandedness of nucleic acid (single or double), polarity of nucleic acid (positive or negative), capsid symmetry (icosahedral, helical, or complex), site of capsid assembly (nucleus or cytoplasm), envelope (presence or absence), site of envelopment (surface membrane, nuclear membrane, or intracytoplasmic membrane), virion and capsid diameter, number of capsomeres, and molecular weight of nucleic acid in the virion. Another striking feature of the present classificatiou is that the virus family names end in ...viridae, subfamily names end in ...virinae, and genus names end in ...virus. So far, twenty-one families of animal viruses have been described. As the knowledge increases, periodic revisions among different groups are made by an international nomenclature committee. The committee, originally established in 1966 as the International Committee on Nomenclature of Viruses (ICNV), was changed in 1974 to International Committee on Taxonomy of Viruses 0CTV). Information on classification of animal viruses will appear in two successive articles in this newsletter, starting in this issue with Part I, the DNA viruses. The chemical and mo~phologic properties of DNA-containing animal virus families are shown in Table 1.

Nature and Classification of Viruses Affecting Small Animals

The classification of animal viruses is predicated upon the various parameters discussed herein and the molecular biology of viruses. Although many of the parameters used for classification purposes have little obvious relation to the disease the veterinarian observes, it is necessary to utilize this information collectively to understand viral classification and its relationship to disease pathogenesis, treatment, diagnosis, and prevention because all these are predicted on the classification of animal viruses (Table 2).

Biochemical and biophysical characteristics of vesicular exanthema virus and the viral ribonucleic acid.

The properties of three strains of VEV have been examined in an attempt to define its place in the classification of animal viruses. Each strain grew to high titre in a pig kidney cell line, IB-RS-2, was resistant to lipid solvents and pH 5 and could be purified by precipitation with ammonium sulphate, differential ultracentrifugation, detergent treatment and sedimentation in a sucrose gradient. The purified viruses had sedimentation coefficients in sucrose gradients of 160–170S and densities in caesium chloride ranging from 1.370 to 1.377 g/ml. In the electron microscope the 35–40 mμ particles had an appearance very similar to that described for the feline picornaviruses. Infectious RNA was obtained from each of the strains by extraction with 1% sodium deoxycholate and phenol. The infectivity of the RNA was destroyed by trace amounts of ribonuclease, indicating that the RNA is single-stranded. Labelled RNA prepared from32P-virus sedimented in a sucrose gradient at 35–38S and was disrupted into slowly sedimenting molecules by incubating with 0.01 μg ribonuclease per ml prior to centrifugation. The percentage distribution of32P in the four nucleotides was adenylic acid 29.0: cytidylic acid 25.1; guanylic acid 20.6; uridylic acid 25.3.

The Emerging Classification of Animal Viruses: A Review

Order is the backbone of science, but a scientific discipline must progress to a reasonably advanced state of knowledge before it can be put in order. In this respect, virology has lagged behind most other biological disciplines. About 20 years ago, not more than fourscore animal viruses had been recognized. The main reason for this paucity of information compared to what is known today, was the lack of convenient methods for propagating and isolating viruses in the laboratory. For example, for several decades relatively few animal viruses could be studied, and these only in their natural hosts. The virus of hog cholera could be studied only in susceptible pigs, and the virus of foot-and-mouth disease only in susceptible cattle. Such methods were primitive by today's standards, but in the hands of gifted investigators they yielded valuable information despite their obviously limited scope. The continuing search for better methods of propagating and isolating virus was rewarded in the 1930's (16,40,) when embryonated eggs were found to support the growth of many viral agents of human or veterinary importance. Although the list of the contributions of the chicken embryo to virus research and prophylaxis is impressive and is still growing, many problems could not be solved by this technique, and had to await the emergence of the cell-culture era (15). Meanwhile, for lack of a rational taxonomic system, investigators labeled any new viruses they isolated in various ways: by geographic location, as in the cases of Newcastle disease (13) and St. Louis encephalitis viruses (31); by a major symptom associated with infection, as in the cases of

Contributions to characterization and classification of animal viruses.

SummarySystematic organization of the known viruses is important from the taxonomic viewpoint and in the practical sense in providing inferential knowledge based on known qualities of related agents. Determination of nucleic acid type, sensitivity to organic solvents, pH 3.0 lability, size, and structure provide excellent basic criteria for grouping viruses. Further subgrouping can be based on factors such as heat stability, sensitivity to drugs, cytopathology, and type of inclusions. Acid lability tests and nucleic acid typing were carried out for a number of viruses and methods for determining essential lipid, drug sensitivity and heat lability were explored. A simplified working scheme for classification and identification of viruses is presented.

Properties of bovine virus diarrhoea virus.

RECENTLY it has been shown1 that a virus isolated from cattle with signs of ‘mucosal disease’2 is antigenically related to the virus of bovine virus diarrhœa3. In northern Sweden, a type of mucosal disease (‘Umea disease’) was found to be accompanied or preceded by a respiratory disease. In paired serum samples from diseased animals increasing titres of antibodies were demonstrated against both bovine virus diarrhœa virus and para-influenza 3 virus4, but only para-influenza 3 virus was isolated5. The incidence of infections with bovine virus diarrhœa virus among Swedish cattle seems to be about 20 per cent as judged from the results of a serological survey in 1959–604. In the present work the properties on which Cooper6 bases his classification of animal viruses have been investigated for bovine virus diarrhœa virus.

Zone electrophoresis of Rous sarcoma virus.

THE value of zone electrophoresis in the study of animal viruses has been demonstrated by previous workers1,2. By this method new criteria for the classification of animal viruses can be obtained. In addition, the study of electrophoretic inhomogeneity of viruses is of special interest. Thus it could be shown for polivirus type I that the virus activity is shared by at least two components with different electrophoretic mobilities2.