472 CHRONIC INFECTIOUS DISEASES 



which have collected on the penis during copulation with an 

 infected female. 



Whether the bull is merely a mechanical carrier or the 

 abortus bacillus may proliferate in the seminal vesicles, caus- 

 ing him to become also infected, and therefore a permanent 

 distributor, remains undecided. In a few cases the seminal 

 fluid contained abortion bacilli, but no infection was trans- 

 mitted to cows servo! by these bulls, the service taking place 

 on neutral ground. 



Experimentally abortion has been induced in pregnant 

 animals by introducing pure cultures of the abortus bacillus 

 into the vagina, stomach, and veins. The disease is prac- 

 tically always brought into a herd through an infected female 

 which has either recently aborted or is eliminating the causal 

 germ in her vaginal discharge and milk. The tendency for 

 cattle owners to dispose of cows which have aborted insures 

 a wide dissemination of the disease. In rare instances the 

 premises may become infected through a contaminated bull 

 to which the cows of the herd have been brought for service. 

 In still rarer instances a very young calf from an infected 

 mother may introduce the infection. That persons, dogs, 

 fowls, and birds of the air may too carry infection from 

 infected to non-infected premises is probable. 



Symptoms. — ^The period of incubation following natural or 

 artificial inoculation is varied. It averages from natural 

 exposure about foiu" months (33 to 230 days). Artificial 

 transmission of vaginal discharge from diseased to healthy 

 cows was followed by abortion in 9 to 21 days. The symp- 

 toms which indicate the presence of the disease in a cow 

 herd are : (a) A number of cows are dropping their calves 

 prematurely; (6) cows which have aborted show an abnormal 

 vaginal discharge, and (c) the appearance of symptoms of 

 premature labor especially in heifers. 



The first few cases of abortion in a herd may be entirely 

 overlooked as they are apt to happen during the first weeks 

 (five to seven) of pregnancy when the fetus is very small. 

 A given cow may thus abort, be rebred, conceive and abort 

 again without the owner's attention being attracted to the 

 condition. Finally after aborting twice or more times she 



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