Modes of Infection. 27 



taken out of the stomach of a horse dying of this disease, 

 crushed in a mortar, filtered, and injected into healthy 

 animals, but the result was negative. Furthermore, in 

 1912 bot-flies were exterminated from all the horses 

 before pasturing and it was noticed that the removal of 

 the insect did not influence upon the spread of the 

 disease in that particular summer. 



C. Mosquitoes {Culecidae)— They can safely be ex- 

 cluded from our present problem; for they are found 

 only in the stable and not in the pasture. 



D. Sand-flies [Simulium)— This insect is very abun- 

 dant in the pasture. The power of sucking blood is very 

 strong ; horses are, therefore, greatly annoyed by the 

 sand-fly. This insect appears very early and stays until 

 late autumn. Therefore there is a time when this insect 

 monopolises the pasture, all other insects disappearing. 

 Choosing such a time of the year healthy horses and the 

 patients were kept near each other in the same pasture, 

 preventing, however, the intermingling of these two 

 groups of animals. None of the healthy horses contracted 

 the disease. In spite of the fact that the sand-flies at- 

 tacked both the healthy and the sick animals for nearly 

 a month, the disease was not transmitted. Hence we 

 have reached the conclusion that the sand-flies have 

 nothing to do with the process of infection. 



E. Stable-flies {Stomoxis calcitrans) — This is also 

 a blood-sucking insect occurring as abundant and living 

 as long as the sand-flies. It abounds in the stables of 

 the plains. We, therefore, tried an experiment as early 

 as 1908, letting the fly, which had sucked much blood 

 from a patient, bite healthy horses; but the result was 

 negative. During the season of the stable-fly, the pati- 

 ents and the healthy horses were kept in two enclosures 

 near each other. The result here was also negative. 



