CHAPTER XXII. 



EPIZOOTIC DISEASES OF UNDETERMINED ETIOLOGY 

 (FILTERABLE VIRUSES). 



General statement. Although microbiology has revealed 

 the definite cause of many of the infectious diseases of man 

 and the lower animals, it has failed thus far to demonstrate 

 the cause of a number of important maladies that can be 

 inoculated from affected individuals to healthy ones or that 

 can be prevented by rigid quarantine of the infected. Diseases 

 of this class exist among lower animals and in man. The more 

 important of the epizootic diseases of this class are "foot and 

 mouth" disease, contagious pleuro-pneumonia, rinderpest, cow, 

 horse and sheep pox, horse sickness, epithelioma contagiosum 

 of fowls, chicken pox (of fowls), dog distemper, influenza, in- 

 fectious cerebro-spinal meningitis of horses and epizootic hog 

 cholera. 1 The literature on these different affections shows that 

 for each disease one or more observations have been made which 

 have caused the announcement of the discovery of the specific 

 cause. For the diseases mentioned, however, reported dis- 

 coveries relative to the causal factors have not been satisfac- 

 torily confirmed and consequently they remain in the class of 

 unknown etiology. In the human subject such diseases as 

 scarlet fever, measles, variola (small pox), chicken pox, yellow 

 fever, typhus fever and Rocky Mountain spotted fever are still 

 included in this class. 



The cause of rabies, which attacks all species, is believed 

 by some pathologists to be undetermined, although there are 

 investigators who believe the bodies discovered by Negri are 

 protozoa and the cause of the malady. For this reason the 

 Negri bodies are included temporarily among the pathogenic 

 protozoa. 



1 M'Fadyean reviews the literature on the ultravisible viruses 

 in a series of articles in the Jour, of Comp. Path, and Therap., Vol. 

 XXI (1908) pp. 58, 168 and 232. 



