Biological Therapy 



117 



the placenta, particularly from the chorionic layer, and from 

 the stomach, liver and lymphatic glands of the aborted fetus. 



Bacteria other than B. abortus have in some instances 

 been isolated from the uterus or vaginal discharge of affected 

 cattle. Among the other organisms which have been cred- 

 ited with ability to cause abortion may be mentioned B. coli, 

 B. pyogenes, B. paratyphosus and vibrio fetus. Notwith- 

 standing the presence of such micro-organisms and the ab- 

 sence of B. abortus in a few cases it should be considered 

 that whenever abortions assume epizootic proportions we 

 are dealing with true infectious abortion and that the Bang 

 bacillus is the etiological factor. Other bacteria with the 

 possible exception of vibrio fetus are undoubtedly secondary 

 invaders and are responsible not for the abortion itself but 

 for the metritis and sterility which follow. 



SOURCE OF INFECTION 



Many of the older beliefs regarding the mode of trans- 

 mission of this disease have of late been disproven by care- 

 fully controlled experiments and investigations. The bull 

 as a possible spreader is no longer feared since Schroeder 

 has shown that infection does not occur through the female 

 genitalia. 



It is now believed that infection always occurs through 

 the digestive system after pregnant animals ingest food or 

 water contaminated with discharges from affected cows. 

 The bacteria in this infectious material pass through the 

 blood and lymphatic systems to the epithelial layer of the 

 chorion and become established in that tissue which, during 

 the period of pregnancy, constitutes an extremely favorable 

 medium for the bacterial growth. After parturition the non- 

 gravid uterus is no longer a favorable habitat for these or- 

 ganisms and they then establish a permanent abode in the 

 udder. From here they migrate to the uterus only during 

 subsequent pregnancies. The expelled placental membranes 

 and subsequent discharge containing large numbers of B. 

 abortus, contaminate foodstuffs, bedding, etc., which is then 

 a menace to susceptible individuals. 



CARRIERS 



When the abortion bacillus establishes itself in the 

 udder the affected animals may remain carriers for years. 



