PLEUROPNEUMONIA. 
825 
illustration of the difficulty which was felt in deciding the 
question of contagion. Hugard says that he possesses no 
positive facts or satisfactory experiments by which to deter- 
mine the contagious character of pleuropneumonia. The 
general course of the disease furnishes, he seems to conclude, 
evidence sufficiently convincing : wherever pleuropneumonia 
has appeared there it has been communicated from the sick 
to the healthy. The disease never attacks the members of a 
herd simultaneously, but commences with one or two cattle, 
generally those which have been recently bought. In a short 
time the number of cases increases, and then diminishes, but 
the disease does not cease until the majority of the animals 
have been attacked. Farmers who breed and rear their own 
cattle and do not replenish their stock from fairs and markets, 
it has been noticed, escaped the disease even while their 
neighbours who pursue a different course are suffering from 
its ravages. Many examples of the introduction of the dis- 
ease by newly-purchased stock are related by Gerlach Werth, 
and Hertwig, and our experience in this country for the last 
thirty years is fruitful in similar instances. 
Giving full weight to all the evidence on both sides of the 
0^0 
question, it is impossible to doubt the fact of the frequent 
communication of pleuropneumonia from sick to healthy 
animals, but it is evident that the degree of activity of the 
contagium varies under different circumstances, and it is also 
certain that a considerable number of animals resist the in- 
fluence of the virus even when they are persistently exposed 
to it. In the experiments of the French Commission, twenty 
animals out of a hundred were proof against the infection, 
and only fifty took the disease in a severe form. A much 
larger proportion, however, will often be atttacked, and in 
some instances the percentage of susceptible animals in a 
herd appears to be much less than this, the disease ceasing 
after two or three animals have succumbed. The different 
degrees of susceptibility to the infection, the sudden cessation 
of the disease in some cases without any assignable reason, 
the failure of inoculation to induce the disease, and the fre- 
quent escape of animals after being exposed to contact with 
diseased cattle, are facts which account for the difference of 
opinion which prevails among both scientific and practical 
men in reference to the nature and mode of propagation of 
pleuropneumonia. 
Admitting, as we are bound, from the evidence before us, 
that pleuropneumonia is communicable from sick to healthy 
cattle, it is nevertheless a fact that it arises in some districts 
independently of contact with diseased animals, and it is also 
