This work was confirmed by Beizerink in 1898, who showed further that tobacco mosaic virus will diffuse through an agar layer suggested that it was a “contagium vivum fluidiurri”.
In 1898, Loeffler and Frosch were the first to show virus of animal origin to be filterable when they demonstrated that the foot and mouth disease of cattle was incited by an entity which passed through bacteria proof filter. The ‘Sirah’ disease of sugarcane caused by a virus was observed in Jawa in 1882, and the methods of its control were also investigated. Mosaic disease of sugarcane, which was then known as ‘yellow stripe’ disease was also discovered in Jawa in 1890. In 1887, the ‘mosaic’ of tomato was observed in England, and the disease was studied in detail in America. Mayer (1886), discovered tobacco ‘mosaic’ in north Europe. He proved that this disease can be transmitted to healthy plants by mechanical means. Iwanowsky (1892) discovered the first virus and thought it to be the small bacterium causing tobacco mosaic.
Beijerinck (1898) discarded the Iwanowsky’s statement and proposed his theory of cantagium vivum fludium. In 1901, Takami of Japan, transmitted the dwarf or stunt disease of paddy by an insect Nephotettix apicalis, and it was known for the first time that viruses could be transmitted by insects. For the first time in 1935, W.M. Stanley isolated the ‘Tobacco Mosaic Virus’ in paracrystalline form. Green (1935) and Laidlaw (1938) independently put forward the theory that viruses are obligate parasites which have developed parasitism to the highest degree.
Bawden and Pirie (1938) isolated a fully crystalline form of the tomato bushy stunt virus. Stanley (1936) and Northrop (1938) suggested that the viruses are autocatalysts and multiply in susceptible cells by activating previously formed inactive precursors. According to Darlington (1944), the viruses are derived from cell proteins. In his view these undifferentiated proteins may come to acquire the properties of infection. The discovery of the relationship between viruses and insects was not made in a day and a period of years elapsed between the time when insects were first suspected to transmitting plant viruses and the actual demonstration of this method of transmission. The first to prove experimentally the relationship between an insect and a plant virus seems to have been a Japanese fanner, Hashimoto, who worked in 1894 with the dwarf disease of rice and the leaf hopper Nephotettix apicalis var. cincticeps. About 1907, three workers in America, Ball, Adams and Shaw suggested that there was some connection between curly top of sugarbeet and the leaf hopper Eutettix tenella.
Some years later, Smith and Boncquet (1915) confirmed this and showed that a single insect from an infected plant placed on a healthy plant for 5 minutes would produce the disease. The original criterion of a virus was an infectious entity that could pass through a filter with a pore size small enough to hold back all known cellular agents of diseases. However, diseases were soon found that had virus like symptoms not associated with any pathogen visible in the light microscope, but which could not be transmitted by mechanical inoculation.
With such disease, the criterion of filterability could not be applied. The infectious nature was established by graft transmission and sometimes by insect vectors. Thus, it came about that certain diseases of the yellows and witches broom type such as aster yellows, came to be considered as due to viruses on quite inadequate grounds. Later on, critical examination by electron microscopy and the use of inhibitory drugs have been strong indications that a number of so called virus diseases are really caused by mycoplasma like agents (Doi et al, 1967). During most of the period between 1900 and 1935 attention was focused on description of diseases.
Ineffective attempts were made to refine filtration methods in order to define the size of viruses more closely. There were almost the only aspects of virus diseases that could be studied with the techniques that were available. The influence of various physical and chemical agents on virus infectivity was investigated but methods for assay of infective material were not refined. Holmes (1929) showed the local lesions produced in some hosts following mechanical inoculation could be used for the rapid quantitative assay of infective virus. This technique enabled properties of viruses to be studied much more readily and paved the way for the isolation and purification of viruses a few years later.
In 1926 the first enzyme urease was isolated, crystallised and identified as a protein (Sumner, 1926). The isolation of others soon followed. In the early 1930’s workers in various countries began attempting to isolate and purify plant viruses using methods similar to those that had been used for enzymes. Following detailed chemical studies suggesting that the infectious agent of TMV might be protein.
Stanley (1935) announced the isolation of this virus in an apparently crystalline form. At first Stanley (1935, 1936) considered that the virus was a globulin containing no phosphorus. Bawden et al (1936) described the isolation from TMV infected plants of liquid crystalline nucleoprotein containing nucleic acid of the pentose type.
The present concept of the virus particle is that it consists of a protein shell within which is contained a nucleic acid molecule which by its own structure and coding property determined the arrangement of the amino acid sequences in the subunits of the protein cell. The nucleic acid is also the infective agent which as it passes into a healthy cell, dictates a change in the protein metabolism which results in the buildup of virus protein usually at the expense of the normal protein. When Louis Pasteur (1822-1895) and Robert Koch (1843-1910) discovered that microbes were the cause of many diseases, they opened up a new field of experiment and research. The word ‘virus’ which, before their discoveries, was applied to any poisonous substance came to be used for all kinds of infection agents. As research progressed further, it was found that certain agents were so fine that they could pass through specially designed filters which did not allow even bacteria and other known microorganisms to pass through. Some scientists, unable to conceive their extreme minuteness, hazarded the opinion that these agents were contagious fluids and did not consist of discrete organisms. Finally, when it was established that they consist of sub-microscopic organisms, which were invisible to the most powerful microscopes of the conventional type, the term virus was restricted to apply only to these ‘filter-passing’ or filterable sub-micro-organisms. It was the invention of electron microscope in recent years that has enabled these tiniest of organisms to be identified and photographed.
The best of the light microscopes can magnify an object 2,000 times its original size. But the electron microscope gives a direct magnification of 10,000 and can enlarge photographically up to 200,000. Some of the diseases caused by viruses have been known for a long time. Small-pox for instance, has been known for ages and has caused innumerable deaths. Even in these days it is responsible for thousands of deaths every year. But the nature and cause of these diseases have been understood only recently. The behaviour of viruses has been a matter of great surprise and speculation; they possess certain characteristics of inorganic substances and they also behave like living organisms. This has led scientists to call them the missing links between dead and living matter.
In 1935, W.M. Stanley, an American biochemist, isolated the tobacco, mosaic virus and was surprised to find that it had all the properties of a crystalline solid.
Its chemical constitution lent itself to analysis and produced regular shaped crystals like many chemical salts. In its isolated condition it was incapable of reproduction which is typical of all living organisms. Thus it was not alive in the accepted sense of the word. But as soon as it was brought into contact with a healthy tobacco leaf, it appeared to spring into life, flourishing and multiplying like any primitive living thing. Chemically, viruses consist of nucleoproteins — a combination of nucleic acid and protein essential constituents of living matter.
The more complex viruses contain some fats in addition. Their extreme simplicity leads one to hope that someday it may be possible to produce them in the laboratory if only for the purpose of producing vaccines. There are a number of diseases caused by virus infection.
Some of these are small-pox, measles, mumps, yellow fever, poliomycilitis (infantile paralysis), usually called polio, chicken pox, rabies, and influenza. It is believed that even common cold is caused by virus infection. In addition there are a number of animal and plant diseases caused by viruses. The control of virus diseases is not adequate. Usually viruses are not affected by antibiotics.
It is easier to control pneumonia than the common cold. Viruses can mutate just like other living organisms. By mutation is meant a sudden variation in some well-marked character. This gives rise to new types of viruses. The sudden outbreaks of some epidemics in unexpectedly virulent forms are attributed to such mutations.
Naturally it takes some time before the new character is studied and brought under control. The deadly influenza epidemic of 1918 has been cited as a possible example.