CHAPTER 41 

 THE ANIMAL VIRUSES: GENERAL PROPERTIES 



Tentative Definition of Pathogenic Forms 



]\Iinute bodies m hich are usually invisible by ordinary microscopic methods of examina- 

 tion, which have a diameter of less than 0-2 /<, which can often be filtered through candles 

 and membranes impermeable to ordinary bacteria, which have not yet been cultivated 

 in cell-free media but which multiply freely in the presence of susceptible cells in vitro 

 or in vivo, which generally have a high resistance to glycerol, which frequently invade 

 one particular species of host and tend to affect one particular tissue, which give rise to 

 characteristic inclusion bodies in the tissues, and which cause a latent or overt infection 

 often followed by a lasting immunity. 



There is a large group of diseases affecting man, animals, insects, and plants, 

 which have most or all of the characteristics of infectious diseases, yet in which no 

 visible microscopical organism has been satisfactorily demonstrated. That these 

 diseases are infectious is shown by the fact that it is usually possible to reproduce 

 them in normal hosts by inoculation, not only of the ground-up diseased tissue, 

 but also of cell-free extracts or filtrates of the tissue. Since these cell -free extracts 

 are most commonly obtained by filtration, the practice has grown up of referring 

 to the specific infecting agents contained in them as " filtrable viruses." This 

 term is sanctioned by usage ; but it is necessary to point out that as not all these 

 viruses have yet been shown to be filtrable, and as the exact standardization of 

 filtration has not yet proved possible, the term " filtrable " is necessarily a loose 

 one. With increasing knowledge of the properties of viruses, the present tendency 

 is to omit all qualifying adjectives, and to speak of them simply as viruses. Alter- 

 native names, which have from time to time found favour with some workers, 

 but which have now been generally discarded, are " ultramicroscopic viruses," 

 " protista," " chlamydozoa," and " strongyloplasms." 



Methods of Study 



Filtration. — The ability of a particulate body to pass through a filter is not 

 a simple function of the relation of the. size of the body to the size of the pores ; 

 that is to say, a filter is not a mere mechanical sieve. Several factors other than 

 the size of the filter pore determine whether a given body will pass through it 

 or not. Thus, according to Rivers (1928), " the electrical charge on the virus, 

 the electrical charge on the filter, the adsorption of the virus by aggregates of 

 protein or by cell detritus, the amount of protein or other substances in the virus 

 emulsion, the temperature at which the filtration is conducted, the amount of 

 negative or positive pressure employed, the duration of filtration, and other factors 



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