Such a relatively large proportion of cases of abortion without Bacillus abortus as the inciting agent is noteworthy. In general Bacillus abortus was associated with first pregnancies. Its presence diminished rapidly in frequency in later pregnancies. Assuming in a general way that purchased cows coming from small herds were free from any immunity and that their first pregnancy in the new herd is equivalent to that of a native heifer and may be counted as the first, we have Bacillus abortus associated with the first pregnancy in 42, with the second in 14, with the third in 5, and with the fourth in 1. Spirilla were distributed as follows: (a) in purchased cows, first pregnancy, 6; second pregnancy, 9; third pregnancy, 5; and fourth pregnancy, 3; (b) in native cows, first pregnancy, 0; third pregnancy, 1; sixth pregnancy, 1; and eighth pregnancy) 1. The relation of infection with spirilla to acquired immunity is not clear and more data from large herds are needed to define both etiological and immunological bearings of the spirilla.

Thus far spirilla have not been encountered in native heifers of the herd giving birth the first time. A tentative explanation to be offered is that the young stock is kept segregated from the older and purchased cows until shortly before calving. The occasional discharge of a fetus among the young stock in pasture tends to keep up the disease due to Bacillus abortus. Later on association with older cows brings about infection with spirilla (Vibrio fetus) and more rarely with other possible agencies of fetal disease. On the other hand, abortions may occur among the pastured stock from time to time and remain unrecognized. Not until both groups of animals are subjected to the same daily scrutiny will it be possible to affirm that abortion associated with spirilla does or does not occur among young stock.