Pathogenicity. Natural Infection. 175 



the animal, and they then subsequently produce the inflammatory 

 changes attributed to them. The resistance of the horse may be 

 weakened also by other influences such as colds or overexertion, in 

 which condition the action of the bacteria mentioned may be exerted 

 and lead to the development of the inflammatory processes in the 

 lungs. The pleuro-pneumonia which develops in such cases would 

 anatomically and bacteriologically manifest the same character, inas- 

 much as it is caused by these same bacteria, but it would differ from 

 influenza by the absence of all contagious properties. This conception, 

 which is based only on theoretical considerations, tends to explain the 

 peculiarities in the development of the disease, which sometimes appears 

 as an enzo-otic, at other times only in a sporadic form, while the clinical 

 and anatomical manifestations, especially the croupous pneumonia, are 

 practically the same. 



Poels' observation, which refers to a stallion, deserves special mention. This 

 animal for months infected, in the act of coitus, the mares which he covered, and 

 its semen, fresh as well as filtereil, when injected intravenously in healthy animals, 

 produced influenza after an incubation of from 5 to 6 days. The blood of horses 

 infected in this manner also proved virulent, even after being filtered through a 

 Berkefeld filter. 



The natural infection is evidently transmitted nsiially by 

 secretions of the lungs, and by the feces of affected animals, 

 while infectiousness of the exhaled air is very improbable. 



Tlie secretions and excrements are especially infectious 

 during- the development and at the height of the disease. The 

 convalescing animals may also infect, and even apparently re- 

 covered animals may do likewise, in cases when the lungs still 

 contain gangrenous areas after the disappearance of the acute 

 symptoms. The development of the disease among horses, by 

 the introduction of an apparently healthy animal, can be ex- 

 plained by such a mode of infection. The animal introduced 

 had probably been affected with the disease but a short time 

 before. This may also explain the continued existence of the 

 disease in some stables, when it is continuously transmitted to 

 other animals from one which has not completely recovered 

 from the disease. Such apparently recovered animals, whose 

 lungs contain necrotic lesions, which are not encapsulated, may 

 carry the infection for months (Siedamgrotzky). Usually, how- 

 ever, the disease is introduced by animals which are in the in- 

 cubation period or in the first stages of the disease, and in 

 this respect animals bought in horse exchanges are especially 

 dangerous, while among army horses the disease frequently 

 breaks out after the introduction of remounts. 



According to some English authors (Pottie, J. Clark, Rieks), as well as the 

 observations of Jensen, stallions which recover from influenza may transmit the 

 disease to mares in the act of coitus months later and even after one to two years. 

 Grimme reports a case in which apparently 14 out of 22 mares were said to have 

 been infected by a stallion, which, besides a slight reddening of the conjunctivae, 

 manifested no other indications of the disease. (See also the observation of Poels, 

 above. ) 



Indirect infection is transmitted by the most varied objects 

 which may be contaminated. Such are the secretions and ex- 



