We have already seen that there probably exists a reservoir of yellow fever virus in certain jungle animals, and just recently has come news of the discovery of the virus of equine encephalomyelitis in the bird known as a “prairie chicken.” There are some of the methods of preventing viruses from reaching their hosts. Now want what can be done to render the host immune or resistant to virus infection?
Pasteur had to start from the beginning without a ready-made vaccine and reason it out for himself. He finally obtained a weakened (attenuated) virus by subjecting the spinal cord of a rabid animal to a series of drying processes. This he studied carefully under a microscope using high power microscopy. The weakened virus serves the same purpose of immunization as does vaccinia virus against smallpox. It is not known for certain exactly how the rabies virus is altered by this treatment, or if it is altered at all. It may be that the drying process merely reduces the concentration of the virus below the limit at which it will produce the disease while still retaining the power to stimulate production of antibodies. The exact mechanism, however, is rather obscure. Unlike most forms of vaccination, this is given after infection has taken place, i.e. after the mad dog has bitten its victim.
The work of Jenner with smallpox and of Pasteur with rabies which were made possible with the use of the microscope using high power microscopy showed that viruses could be changed, and that such changed viruses were able to confer immunity to these dreadful diseases. It seems as if passing a virus through animals which are not the natural hosts, or even through an unusual part of an animal which is, may produce a change in the virus. By using animals like the ferret and the mouse, changes have been induced in several such important viruses as those of yellow fever, equine encephalomyelitis and African horse sickness.
It can be concluded therefore that after series of scientific experiments conducted with the aid of the microscope using high power microscopy that different viruses can be grown or cultivated in small pieces of tissue which are themselves growing in a nutritive broth. There is one im¬portant discovery about the tissue culture of viruses is that in some cases a change takes place in the virus. Let us take two striking examples of this. If the virus of yellow fever be passed through a number of mice in serial inoculations and then cultivated in tissue culture, the virus becomes modified and loses much of its virulence for man. A combination of these two processes has enabled more than a million people to be vaccinated against yellow fever. Our second example is influenza virus. In the case of the influenza the virus was cultivated on the membrane of a developing egg. After a considerable time as what has been observed under the microscope using high power microscopy, the virus became adapted to the chick embryo and seemed to lose its virulence for ferrets, mice and men. This opens up the possibility of immunizing human beings with a strain of influenza virus which has lost its virulence but still retains its power of forcing the production of antibodies. It is best to point out that much of this work is still rather hypothetical and many difficulties still remain to be overcome, and one of these difficulties perhaps, is the possibility of the virus mutating or chang¬ing back again from a non-virulent state to a virulent one. As an appropriate conclusion the following remark of Dr. Wilson Smith was quoted as “I would like to emphasize that a healthy man has a natural resistance against most infectious agents, be they viruses or bacteria. Pathologists and research workers tend to forget that the first and possibly the most important of all preventive measures is the maintenance of good general health by means of fresh air, exercise and adequate nutrition.”
What about immunity against the virus diseases of plants? Is there a possibility for vaccination? Can we not vaccinate potatoes and tomatoes against the many viruses from which they suffer? These are some of the questions that we may want to ask. However, the answer to these questions is a big no. It is a common knowledge that plants do not have the capacity to produce antibodies, despite the fact that they do develop a rather unstable resistance to certain virus diseases. This aspect needs careful study and investigation with the use of the microscope.
Aside from warding off the insect vectors of plant viruses what more can be done to save the crops? Quite a lot has been done already by the plant breeder in this direction and probably more will be achieved. It is the aim of the plant breeder to produce a plant which will combine resistance to a particular virus with the necessary qualities which make the crop a commercial success. Farmers around the world spend millions of dollars to for this purpose. Not long ago, a virus disease (cotton leaf-crinkle), suddenly appeared in the cotton crops of the Sudan and caused tremendous havoc. It continued to do this for some years, until a variety of cotton was produced which was resistant to the disease. Similarly with curly-top of sugar beet in the U.S.A., the annual losses due to this disease were tremendous and in fact stopped the pro¬duction of sugar beet altogether in certain areas. Now a curly-top-resistant sugar beet has been evolved. This goes far to reduce the losses due to this virus. It does not matter very much from the practical point of view what form this resistance takes. In some cases the plant itself may be resistant to the virus. In other cases the plant may be unpalatable to the insect vector of the virus. For instance the leaves are too hairy or too tough for the insect to penetrate. Whatever is the actual reason for the immunity the practical result is much the same, for the crop avoids infection. This has been carefully studied under a microscope.
Plant breeders all over try a variety of ways and means to help in the fight against the effects plant viruses in the crops. They pay large some of money to support scientific studies involving the use of the microscope. Tobacco mosaic is a very infectious disease and causes severe losses every year to the tobacco grower. The virus rapidly permeates the plant and produces the familiar mottling and stunting characteristic of this disease. Now there is a plant closely related to the tobacco plant. This plant has the scientific name of Nicotiaraa glutinnsa. This new plant is resistant to the tobacco mosaic virus in quite a different way from the tobacco plant. Instead of developing an “over all” disease-systemic it is called-local spots or lesions are formed, and these only in the leaves actually inoculated with the virus. In other words, the virus is unable to move out of the leaf into which it was introduced, and the disease produced is therefore practically negligible. Now the aim of the plant breeder is to produce a tobacco plant in which this power to restrict the spread of the virus is combined with those properties of the tobacco plant which make it commercially useful. Considerable pro¬gress towards this goal has already been made in America and a tobacco plant has been produced which has the power of localizing the virus in the leaf. It now remains to be seen how far this plant can be improved from the point of view of its smoking and other properties. Further scientific investigation with the aid of the microscope is necessary to realize this dream.