VIRUS (Ultravirus, Infravirus, Filterable Viruses). - They are particular agents of diseases of man, animals and plants, characterized by ultramicroscopic dimensions of the order of magnitude of the millimicron (one millionth of a mm. = mμ) and by the fact that they can reproduce only on living cells and not on bacteriological culture media.
Viruses, although all sharing the characteristic of extreme smallness, differ among themselves by their different sizes, in that some, the largest, reach 250-275 mμ, that is, the limits of microscopic visibility, while the smallest known so far measure barely about ten mμ, like large protein molecules. The largest virus known to date is that of psittacosis, a disease peculiar to parrots as well as to other birds, transmissible also to man, although it is not considered by all authors to be a true virus. There follow in order the viruses of smallpox (about 250 mμ), of rabies (150-200 mμ), of influenza (about 100 mμ), of yellow fever (20-25 mμ), of infantile paralysis (10-12 mμ). A virus of extremely small dimensions, perhaps the smallest of all, is that of foot-and-mouth disease (a disease peculiar to cattle and other animals that only exceptionally affects man), measuring even less than 10 mμ.
Because of their minimal dimensions, viruses are not visible under the optical microscope; only the larger ones are, after suitable staining or by resorting to the dark-field device. These microscopically visible forms of viruses go by the name of elementary corpuscles and are considered, rather than pure viral elements, as aggregates of them or viruses surrounded by other material. Viruses prove visible with the new type of microscope, the electron supermicroscope, in which the optical apparatus is constituted, instead of by ordinary lenses, by particular electromagnetic devices functioning as lenses, and the image of the objects, by means of a beam of electrons, is formed on fluorescent screens or on photographic plates. With this instrument, which allows magnifications of over 100,000 diameters, it has been possible to determine the dimensions of viruses and also to learn their shape: some, generally the larger ones, appear polyhedral in shape, sometimes cubic, others spherical or ellipsoidal, and still others present themselves as minute rods or long filaments more or less flexible.
Again because of their small dimensions, viruses all have the property of passing through special porous materials whose pores are so small as to allow passage to them and not to bacteria and other microorganisms. To demonstrate this property of viruses, which has earned them the name of filterable viruses, special filters are used, which because of their shape are called filter candles, being constituted by a hollow cylinder of unglazed porous porcelain (Chamberland candles) or of fossil meal based on diatom skeletons (Berkefeld candles). Filtration is carried out by finely triturating the material containing the virus with physiological solution (9 per 1000 sodium chloride solution) and filtering it by means of suitable negative pressure; through the porous mass there is the passage of the liquid part and of the smallest particles such as viruses, while the larger particles, such as bacteria and other microorganisms, are retained by the candle. To ascertain the presence of the virus in the filtered liquid, an aliquot of it is inoculated into experimental animals. Thus, to demonstrate the presence of the rabies virus in a dog that has died of this disease, the animal's cranial box is opened, that part of nervous substance that joins the brain to the spinal cord and that goes by the name of medulla oblongata is taken, and it is carefully triturated in a mortar with physiological solution, finally proceeding to filtration. The filtrate thus obtained presents itself as a clear liquid that on microscopic observation shows no microorganisms of any sort; however, if it is inoculated into a healthy dog or rabbit, it causes rabies in them after a few days. With the technique described, it is possible to transmit the rabies virus from a sick animal to healthy animals an infinite number of times, which proves that this is an agent capable of reproducing indefinitely and not a toxic substance that would inevitably be exhausted after a few passages. By means of filtration carried out with a series of filters constituted by collodion membranes with pores of known and progressively graded sizes, it is also possible to proceed to measurements of the dimensions of viruses, which have been found in agreement with those obtained with the electron supermicroscope.
The artificial cultivation of viruses is carried out on living cells, because they, by reason of their obligatory parasitism, are incapable of independent life and can therefore multiply only inside or at the expense of other living elements. In practice, viruses are cultivated on tissue cultures, that is, on proliferating living cells or, better still, in fertilized chicken eggs; in these the viruses are inoculated into the yolk sac or into the amniotic cavity or onto the chorioallantoic membrane.
The various viruses prove differently resistant with respect to physical and chemical agents; some, such as, for example, that of smallpox, are very resistant; others, such as that of influenza, very little. The majority of viruses is very sensitive to high temperatures while it resists very much those that are low, so much so that freezing represents one of the best methods of preservation.
On the nature of viruses there is no agreement among the various authors. Many lean toward a vital nature, considering them very small microorganisms having a form of life reduced to a minimum, which depends to a large extent on the cell in which it finds itself. Others instead regard viruses, especially those of minimal dimensions, as non-vital molecular entities, capable nevertheless of multiplying indefinitely in living cells through the action of the latter, and they therefore call them virus-proteins.
Viruses are agents of many diseases of man, such as smallpox, chickenpox, measles, lethargic encephalitis, infantile paralysis, rabies, which is a disease shared with animals, epidemic parotitis, the various forms of herpes, yellow fever, influenza, the common cold and still others. Among the diseases of animals caused by viruses, mention is made first of all of rabies, which can strike, besides the dog, also the wolf, the cat, cattle, equines and all other mammals except aquatic ones, following the bite of rabid animals.
Other animal viruses are those that cause foot-and-mouth disease and rinderpest, swine cholera, infectious anemia and encephalomyelitis in horses, fowl plague, psittacosis and the so-called polyhedral diseases of insects, among which very important is the one that strikes silkworms and goes by the name of "jaundice" because it makes them yellow in color. Some of these diseases, especially when they appear in epidemic form with great spread, cause economic damage of great magnitude. The best-known plant viruses are: that of tobacco mosaic, which causes in this plant a characteristic disease affecting the leaves, the various viruses of potatoes, the virus of yellow mosaic of the cucumber and that of the tomato.
A particular group of viruses is constituted by those that live at the expense of bacteria, multiplying inside them until they lyse them, for which reason they have been called bacteriophages. These are very widespread in nature, in soil, in water, in feces and above all in materials rich in bacteria. Studied with the electron supermicroscope, bacteriophages appear as small spheres or rods provided with a slender appendage; their dimensions range from a few tens of mμ to about a hundred mμ.
Similar in many characteristics to viruses are particular microorganisms called rickettsiae in honor of the researcher Ricketts, who met his death from a laboratory infection in the course of experiments on them. They appear as small rods or spheroidal bodies with dimensions ranging from one third to 1-2 μ, often characterized by an evident polymorphism. The smaller rickettsiae are, like viruses, filterable; the larger ones are instead retained by the filters themselves. Their cultivation cannot be done in the common bacteriological media but one must resort, as for viruses, to tissue culture or to fertilized eggs. Rickettsiae produce in man several morbid forms, among which the foremost for the severity of symptoms and for epidemic spread is typhus fever or exanthematous typhus, which presents with high fever and a characteristic hemorrhagic exanthema and is transmitted to man by the bite of lice that have in turn become infected by sucking the blood of the sick. Other rickettsial diseases are Rocky Mountain spotted fever, a purely North American disease, Mediterranean eruptive fever, the so-called Q fever, recently observed in Italy, and still others. Rickettsial diseases, unlike those caused by viruses, prove to be fairly curable with antibiotics.
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Aldo Cimmino