968 



THE ANIMAL VIRUSES 



of viruses to ultra-violet light in the presence of cells, particularly dividing cells 

 than in their absence also suggest that viruses occupy an intracellular position 

 and multiply most readily in growing cells. How far, in fact, the viruses are 

 cytotropic and how far they are cytotrophic is a matter for dispute — if in fact any 

 clear distinction can be drawn between these two properties. Goodpasture (1930) 

 believes that actual growth occurs only in the living cells of the body, while Leding- 

 ham (1932) is not prepared to go to this length. Probably they take advantage 

 of ferment action in the body cells, and receive their nutritive material in a partly 

 digested state. If this is so, then viruses, including the bacteriophage, must be 

 among the most dependent parasites of which we have knowledge in the unicellular 

 world. 



Inclusion Bodies. — Histological examination of the lesions occurring in filtrable 

 virus diseases often reveals the presence within the cytoplasm or the nucleus, or 

 sometimes both, of peculiar bodies whose nature is at present unknown, and which 

 are usually referred to as " inclusion bodies." The appearance of these bodies 

 varies in different diseases, and 

 often in the same disease in dif- 

 ferent animals (Figs. 234, 235). 

 The bodies may be rounded, oval, 

 pyriform, or irregular in shape ; 



Ectromelia virus on left (Fig. 2.34), inclusion body from foot of mouse ; on right (Fig. 235), 

 inclusion body after maceration, showing the Uberated elementary bodies. Photographed 

 in visible light (x 1250). (After Barnard.) 



their substance may be hyaline or granular ; in structure they may be homo- 

 geneous, or they may contain one or more, often several, elementary corpuscles; 

 in their staining reactions they may be basophilic or acidophilic, and within the 

 same inclusion body the granules or elementary corpuscles may stain differently 

 from the ground substance ; lipoid substances staining with osmic acid are some- 

 times found. In many diseases afiecting the skin, such as fowl-pox, human variola, 

 and the common wart, the formation of inclusion bodies is restricted to the epidermis, 

 but in others, such as zoster, varicella, and venereal herpes, they are found both 

 in the epidermis and in the corium. Moreover, according to Lipschiitz (1925), 

 only certain layers of the epidermis may be affected ; thus in the common wart, 

 inclusion bodies are found in the prickle- and horn-cell layers but not in the basal 

 cell layer. Inclusion bodies can be produced experimentally only by the inoculation 

 of living viruses ; they are not formed after inoculation of dead viruses, even 

 though the latter have immunizing properties, e.g. vaccinia and herpes. After 

 inoculation of the virus, the inclusion bodies appear at different times in different 

 infections. Thus in common warts, the nuclear inclusion bodies are demonstrable 



