416 CONTAGIOUS PLEURO-PNEUMONIA IN CATTLE 
reach 50 per'cent. In general it may be said that about 40 per cent. 
of the exposed animals will contract the disease and about one-half 
of these will prove fatal. Pregnant cows usually abort. 
The duration of the disease in acute cases is usually from 7 to 20 
days. In chronic ones the time of death or recovery is uncertain. 
Morbid anatomy. There is a progressive interstitial pneumonia 
with secondary hepatization of the lungs and exudative pleuritis. 
Usually only one lung, the left, is affected. The anatomical changes 
vary according to the duration of the disease. 
The otherwise healthy lung shows, in the initial stage, small, cir- 
cumscribed, inflammatory centers from the size of a hazelnut to that 
of a walnut. The interlobular tissue in it is hyperemic, permeated 
by single hemorrhages and infiltrated with serum. The reddened 
lobules of the lungs are surrounded by bright margins, which are 1 to 2 
mm. broad and which are filled with a serous or lymphatic fluid. 
When the deposits are superficial, the plure become opaque and 
covered with slight clots. 
At the height of the disease there is a lobular pneumonia with 
pleuritis which is usually spread over the greater part of one lobe of 
the lung. The exudate may be soft, membranous, fibrinous, lumpy 
and easily detached. The lung is considerably enlarged, of firm 
consistency, very heavy (weighing up to one hundred pounds), sinks 
in water and does not crackle when cut. Its section appears marbled, 
in consequence of the interstitial connective tissue having become 
thickened into broad lines which vary in color from orange to dirty 
white and which surround the dark colored lobules of the lung. The 
larger lobules have a thickness of from 0.2 to 5 cm.; and the smaller 
ones of from 0.25 to 0.50 cm. The color of the enclosed lobules of the 
lungs depends on the duration of the process and varies from brown- 
red to dirty yellow. The recently infected lobules have a blood-red, 
reddish-brown or dark brown color (stage of red hepatization). The 
color of the older ones varies from orange to yellow (yellow hepatiza- 
tion) and that of a still older date is gray (gray hepatization). The 
central foci, because they are the oldest, are usually in a stage of 
yellow or gray hepatization. Some of the enclosed lobules of the 
lungs are normal or only compressed, while others are merely hypere- 
mic. If we closely examine the bright interstitial lines, we find that 
they consist at first of an edematous infiltration, which later on 
becomes plastofibrinous, gelatinous, indurated and finally tends to 
the formation of adventitious connective tissue. The lymph-spaces 
