350 ACUTE GENERAL INFECTIOUS DISEASES 



a specific virus, which may be filtrable, forms the true cause 

 of the disease. In all probability, however, other bacteria, 

 principally the Streptococcus pyogenes equi, and the Bacillus 

 equi septicus, are secondary invaders, contributing to the 

 underlying disease process and clinical phenomena. 



Natural Infection. — ^The way in which the disease spreads 

 naturally is at present not known. The infection seems to be 

 spread by more or less intimate contact between the sick 

 horse and susceptible ones. The transmission through inter- 

 mediate agents, such as food, water, stable utensils, etc., or 

 through persons or insects, has not been demonstrated experi- 

 mentally. However, practical experience in other diseases 

 and with this disease would not exclude indirect transmission. 

 Sporadic outbreaks in stables could be explained, however, as 

 coming from apparently healthy "germ carriers" or "missed 

 cases" of infectious pneumonia, i. e., where still exist in the 

 lungs or occasionally in other organs unhealed foci of infec- 

 tion. As predisposing factors anything which reduces the 

 resistance of the susceptible animal unquestionably has a 

 bearing on the origin of the disease. Therefore, refrigera- 

 tion, overwork, bad sanitary conditions, become predisposing 

 factors. The disease is rare in very young or aged horses, 

 and occurs usually in animals in the prime of life. 



One attack produces immunity for only a short period. 

 Individual instances are recorded in which a given horse has 

 suffered repeated mild attacks of the disease. An attack 

 of infectious pneumonia in no way influences the susceptibility 

 to influenza infection. 



Necropsy. — On postmortem, where the disease has assumed 

 a typical form, the lesions are those identified with fibrinous 

 or even hemorrhagic pneumonia, with a marked tendency to 

 gangrene. The extent and distribution of the inflammation 

 varies. In some cases the pneumonia is of the lobar, in others 

 the lobular type. The exudate is usually of a hemorrhagic 

 character. Yellow-gray areas of necrosis throughout the 

 lung tissues are commonly noted appearing as encapsuled 

 pus centers or gangrenous foci. 



The pleuritis is serofibrinous; the thorax may contain 

 several gallons of serous exudate. Adhesions between the 



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