Infections of the Ovum, Embryo and Fetus 497 



nor admission that the proper feeding of animals does not 

 play a vital part in reproduction. While abortion occurs 

 only in the presence of intra-uterine infection, infection of 

 both uterus and fetus is common without causing, or visibly 

 threatening to cause abortion. The bacteria commonly pres- 

 ent in the gravid uterus and in the contained fetus cause no 

 visible injury in many cases. If intra-uterine bacteria uni- 

 formly prevented birth, the bovine species would soon be- 

 come extinct. Under good hygienic conditions, if the bac- 

 terial content of the uterus is low, a degree of resistance 

 may be acquired sufficient to hold in leash the bacteria pres- 

 ent. In the presence of menstrual blood or of an embryo, 

 the virulence of the bacteria may increase and injury follow. 

 If the resistance of the animal is lowered by faulty feeding 

 or otherwise, infection within the utero-chorionic space be- 

 comes more active. The amount of food may be deficient or 

 excessive. It may be damaged by fungi. The ration may 

 be badly balanced, or, as in the experiments of McCullom 

 and others, though the food may contain all necessary ele- 

 ments in the quantities technically required, may fall short 

 in variety; then reproduction is faulty or absent. Repro- 

 duction is dependent upon a nutritive surplus beyond other 

 body requirements. If the nutritive supply is barely suf- 

 ficient to maintain the general functions of the body, ova 

 and spermatozoa are not formed. If the nutritive supply is 

 somewhat greater, the reproductive cells form, but tend to 

 perish later, either before or after fertilization. In this de- 

 struction bacteria play an important part. 



Other factors influence reproduction. Pregnant cows or 

 heifers moved from one establishment to another are far 

 more liable to abort, to calve prematurely, or to suffer from 

 metritis, probably with retained membranes, and the calf is 

 more apt to suffer from dysentery than if the pregnant fe- 

 males had been left at home with otherwise equally good 

 care. The interferences with reproduction are not the re- 

 sult of inferior care nor of exposure to a present infection, 

 but of the lowering of their power of resistance to a present 

 infection. Twin pregnancy is far more liable to interrup- 



32 



