836 THE FILTRABLE VIRUSES 



whether by compression or aspiration should be as small as possible (in the former 

 case, say, the pressure produced by an india-rubber syringe, and in the latter that 

 equivalent to 50-500 mm. of mercury) : the experiment should be carried out 

 at the ordinary temperature of the atmosphere and if possible not above 20 C. : 

 the emulsion should be diluted so as to avoid blocking the pores of the filter with 

 the albuminous matter present : and finally, since even those ultra- microscopic 

 viruses which are most easily filtered are partially retained in the filters, several 

 animals should be inoculated each with a large volume of the filtrate. 



SECTION I. PLEURO-PNEUMONIA OF CATTLE. 



[Pleuro-pneumonia contagiosa.] 



All attempts to demonstrate the micro-organism of contagious pleuro- 

 pneumonia in cattle failed until Nocard and Roux in 1898 devised a new 

 method of investigation (vide post) which resulted in the discovery of the 

 organism. " The discovery of the cause of pleuro-pneumonia," wrote these 

 observers, " is interesting not only because peculiar difficulties have been 

 overcome but because it affords hope that a similar success may attend the 

 study of those other diseases of which the micro-organism is up till the 

 present unknown." 



Pleuro-pneumonia of cattle may run either an acute or a chronic course. In the 

 acute form of the disease the respiratory symptoms are the most marked : the 

 respiratory movements are increased in frequency and are shallow, friction 

 sounds can be heard as well as rhonchi denoting bronchial disease, and there is a 

 frequent cough and running from the nose ; the animal ceases to chew its cud and 

 loses its appetite : the disease may resolve, become chronic, or may terminate 

 fatally. In the chronic form of the disease which may either begin as such or 

 follow an attack of the acute form a considerable area of the lungs is congested 

 and hepatized : as a rule, the disease is incurable. 



The essential lesion in pleuro-pneumonia is the distension of the meshes of the 

 inter- lobular connective tissue with a large amount of clear amber- coloured fluid. 



1. Experimental inoculation. Vaccination. 



Willens has shown that the exudate in pleuro-pneumonia will reproduce 

 the disease on inoculation into bovine animals. Goats, sheep, pigs, dogs, 

 guinea-pigs, rabbits and birds are immune. 



The inoculation of a drop of the fresh serous exudate from a case of pleuro- 

 pneumonia into the sub-cutaneous tissues of a cow is followed, after an 

 incubation period of from 8-25 days, by a disease the severity of which will 

 depend upon the site of inoculation. 



(i) When the material is inoculated beneath the skin of the trunk or neck, 

 the temperature of the inoculated animal becomes very much raised and is 

 accompanied by an enormous but painless inflammatory swelling which 

 may extend throughout the cellular tissue of the trunk. Most frequently 

 the disease is fatal, but if the animal recover it will be immune both to inocu- 

 lation and to the spontaneous disease. If the animal die, the meshes of the 

 connective tissue are found, post mortem, to be distended with a clear serous 

 fluid which is present in such large amount that occasionally several litres 

 can be collected. The cedema never affects the lungs or internal organs : 

 the animal dies of an intoxication. 



(ii) If, on the other hand, the inoculation be made into the dense cellular 

 tissue at the tip of the tail, the disease is usually benign ; the swelling at the 

 site of inoculation is very little marked and not extensive, and in the great 

 majority of cases the illness which follows is slight and the animal quickly 

 recovers ; it is then found to be immune. 



