370 Strangles. 



There is another possibility which has been emphasized 

 by Schiitz, that among other streptococci which are very widely 

 distributed in nature the streptococci of strangles may live 

 outside of the animal bodies as saprophytes, and under favor- 

 able conditions may attack the colts, especially when the 

 animals are weakened by outside influences. In this manner 

 those cases can be explained in which the disease appears in 

 localities after years of absence, without any demonstrable in- 

 troduction. 



Solipeds are exclusively susceptible to the disease, and they 

 become infected at the age of i/o to 5 years, mostly however 

 while they are colts. In rare cases the disease may occur at 

 the age of 1 to 2 months and also in animals over five years 

 of age. The greater susceptibility of young animals is as- 

 sociated with the lesser resistance of their mucous membranes. 

 The greater resistance of older animals is however probably 

 the result of their having already passed through an attack 

 of the disease. According to common experience it usually 

 attacks animals only once during their lives, and the immunity 

 resulting from one attack may be proven experimentally 

 (Toggia). In exceptional cases the acquired immunity does 

 not last through the entire life, as some horses may become 

 affected repeatedly, but in such cases the attacks are usually 

 separated by intervals of several years, while cases in which 

 an animal becomes repeatedly infected in one year (Holterback) 

 are exceptionally rare. 



According to Viborg and Hertwig out of 100 horses affected with strangles 

 70 were less than 5 years old, 20 more than 5 years, and 10 over 15 years. Accord- 

 ing to Humbert's observations covering four years in the vicinity of Snippes 

 (France), of all horses over two years of age 2195 were affected once, 543 twice, 

 121 three times, and 1641 animals did not contract the disease. 



Outside influences which reduce the natural resistance of 

 the animals, especially colds, favor the infection, by causing 

 a catarrh of the mucous membrane of the air passages, as a 

 result of which the epithelia become loosened, and thereby the 

 bacteria colonize more readily in the profusely accumulated 

 secretion, from which place they may penetrate the tissue of 

 the mucous membrane. 



Usually weakened and poorly nourished colts are particu- 

 larly susceptible to the disease. In this regard, stabling in 

 poorly ventilated excessively warm stables, insufficient feeding, 

 sudden changes of weather, fatigue during transportation, also 

 pre-existing illness have a modifying effect on the individual 

 susceptibility. The cases in older horses usually have a direct 

 connection with such factors. On the other hand the resistance 

 is increased by work, acclimation to the changeable outside in- 

 fluences and hardening. Other accessory conditions, such as 

 temperament, teething, etc., have no influence on the suscep- 

 tibility. 



Peterson observed an instance in which all the colts of a stud became affected 

 with strangles after standing for half an hour in a pond, and later being exposed on 



