Many diseases in humans and animals are caused by agents which cannot be seen through the ordinary microscope. However, they can be visualized with the help of electron microscope, and their infectious nature proved by inoculating into susceptible hosts or into a suitable cell-culture system-a suspension of the diseased tissues or body fluids that have been rendered bacteria-free. Such disease-producing agents were termed in the early days of microbiology as ‘filterable viruses’ because of their ability to pass through conventional filters which retain bacteria. The first indication of the existence of viruses was obtained as early as in 1892, in the course of investigations on the mosaic disease of the tobacco plants. It was discovered in 1898 that the foot-and-mouth disease of cattle was caused by a virus. Since then a large number of viruses from plants, animals and man have been recognized and extensive studies made.
NATURE OF VIRUSES
A virus is an entity, the genetic determinant of which consists of elements of nucleic acid, either RNA or DNA, replicating in living cells depending solely on the metabolic activities of the living susceptible host cells. Viruses vary in their sizes ranging from 20 nm (200 angstroms) to 350 nm (3,500 angstroms) and in shapes ranging from spherical to bullet, od, brick or filamentous forms. Some of the viruses are exceedingly fragile and are easily destroyed by common physical and chemical agents such as heat, sunlight, alkalies and disinfectants. Viruses resist cold. They can be stored at ultra-low temperatures ranging from-70° to-196°C for several years without any loss in their infectivity or other properties. They can also be lyophilized or their stability improved further by the addition of cryopreservatives such as 5-10% dimethyl sulphoxide or 50% glycerol. Extensive work has been done in recent years in concentrating, purifying and even crystallizing viruses by special processes for further analysis of their chemical, physical and biological properties.
A virus particle, also called ‘Virion’, consists of a nucleic acid core with its strands measuring about 20 to 25 angstroms in diameter, a protein coat known as ‘Capsid’ with or without an envelope derived from the nuclear or cytoplasmic membrane of the host cell. The nucleic acid which is either DNA or RNA is single or double stranded, and is formed by 3 or 4 genes in smaller viruses to several hundreds in larger viruses. The proportion of nucleic acid in a virion varies from 1 to 50%. The capsid enclosing the nucleic acid genome is made up of a number of subunits called Capsomeres’ which are connected together by chemical bonds. There may be one or two capsids. The capsids are icosahedral or helical depending upon the structural symmetry of the virions. The envelope when present contains lipid layers and proteins which are synthesized as specified by viral genes contributing to the antigenic specificity. The structural chemistry of the virions is studied by electron microscopy, X-ray crystallography or neutron diffraction. Based on the above physico-chemical characteristics and the kind of host and of vectors involved, viruses have been classified into 17 virus families.
Recent advances in nucleic acid and protein biochemistry have provided a detailed insight into the molecular aspects of virus gene expression. Some of the techniques developed such as gene sequencing, visualization through electron microscope and molecular hybridization of nucleic acids have revolutionized the concept of viral pathogenesis and provided means for development of accurate and modern diagnostic tests, diagnostic probes, and cheaper and more effective vaccines. The DNA recombinant and hybridoma technologies are finding more and more use in modern applied virology.
In virus infection there is a close relationship between the infecting agent and the susceptible cell, wherein multiplication or self replication of a virus takes place. The virus particles gain entry into the cells through injury. If the entry has to be effected through the wall of the intact cells, a complex physico-chemical process is involved specifically mediated by viral protein and membrane protein complementarity. On entering the cell the particles break down into a relatively large number of units, each of which has the capacity to reproduce. The host cell subsequently appears to break down and release the virus particles in the circulation where they are able to attach to susceptible cells in the neighbourhood or other parts of the body. The lesions observed in virus infections are the result of such damage to an individual cell or to a group of susceptible cells. In some of the virus diseases, e.g. rabies and pox, the infection results in well-defined pathological changes in the affected tissues. These changes often described as ‘inclusion bodies’ are of considerable diagnostic aid for laboratory workers. There are also situations where the viruses entering a susceptible cell do not lyse the cell or present any demonstrable evidence of cellular damage as in asymptomatic infections, cancers and slow virus infections. There are many factors which determine the pathogenesis of a virus infection such as the characteristic of the virus and the susceptible cells, the number of infecting virus particles, their entry into the susceptible cells, the speed of virus multiplication and spread, the effect of virus on cellular functions and the inflammatory and defensive responses of the host. One of the important responses of vertebrate cells to virus infection is the production of an antiviral substance called ‘Interferon’ which is chemically a class of glycoprotein coded by the cell.
Viruses cannot grow on artificial media such as agar, broth and blood agar, but can be kept viable by propagation in living tissues. Attempts have been made to grow in embryonated hen or duck eggs the viruses of certain diseases such as Ranikhet (Newcastle) disease, rinderpest and rabies. Similarly viruses of rinderpest and Ranikhet diseases are grown on tissue cultures consisting of a suspension of the actively growing specific host cells or fragments of fresh tissue preserved under optimum conditions of survival. Before the advent of cell-culture systems for virus isolation and assay, laboratory animals such as mice, rabbits, guinea-pigs and hamsters were largely in vogue. Primates and rabbits are still being used for the study of human viruses like kuru and hepatitis, and for raising antisera for immunological purposes.