CHAPTEE XLL 



INFECTIOUS DISEASES DUE TO ULTRAMICROSCOPIC VIRUSES 



PLEUROPNEUMONIA IN CATTLE CATTLE PLAGUE 



HOOF-AND-MOUTH DISEASE. 



IT would be illogical to assume the non-existence of bacteria so 

 small that they cannot be recognized, even with the best microscopic 

 objectives and oculars. In fact, at least one pathogenic micro- 

 organism of this type is already known, though even under the 

 best optical apparatuses it appears only as a small, highly refractive 

 point. Yet from its behavior there is every reason to believe that 

 it is indeed a bacterium. The organism referred to is the exceedingly 

 minute microorganism discovered by Nocard and Roux as the cause 

 of pleuropneumonia in cattle. In albuminous natural exudates it 

 will not pass the Chamberland or Berkefeld filters, but it will pass 

 these if suspended in watery solutions, such as bouillon, etc. It can 

 be cultivated on certain artificial media and forms something like 

 very delicate, hardly visible colonies. A further step in reasoning 

 leads to a living virus which may or may not pass the filters in 

 albuminous exudates, and which cannot be seen even as a highly 

 refractive point. Another step leads to a living virus so small 

 that it easily passes the pores of porcelain and clay filters even in 

 natural albuminous fluids. With this the form of "contagium 

 vivum fluidum" is reached which is designated as an ultramicroscopic, 

 invisible, filterable, living virus. Some of the diseases due to such 

 viruses which are either still on the boundary line of visibility or 

 which are absolutely invisible and perhaps never can be seen are 

 briefly considered in this chapter. 1 



CONTAGIOUS PLEUROPNEUMONIA IN CATTLE. 



Occurrence and Historical. Lung plague in cattle, pleuropneumonia 

 contagiosa bovum, "Lungenseuche der Kinder" (German), peri- 

 pneumonie contagieuse (French), is an acute or subacute infectious 

 disease of cattle characterized by exudative inflammatory changes 

 in the lungs combined with sero-fibrinous pleuritis. The disease 



1 Helmholtz, the celebrated German physicist, has shown that in consequence of the inherent 

 properties of light, it will be impossible, no matter how much the microscope is improved, to 

 see objects smaller than a certain minimum which he calculated exactly. There is of course, 

 no reason whatever for assuming that there are no live objects in nature beyond this micro- 

 scopic limit. 



