616 
Journal of Agricultural Research 
Vol. XXVIII, No. 7 
The manner in which the pregnancies of these 12 animals with an abortion his¬ 
tory terminated is regarded as being of particular interest, since it not only indi¬ 
cates that animals may derive a marked degree of immunity as a result of having 
sustained an attack of the disease, but suggests as well that it may be essential 
that centers of Bacterium abortus infection within the animal shall have ceased to 
exist before such immunity may readily be demonstrated. 
While the fetus, placenta, and uterine exudate expelled by cow No. 413 during 
1917 were heavily saturated with abortion bacteria, and from the numerous milk 
examinations made it could reasonably be concluded that her udder carried the 
infection continuously until aborting the second time, during June, 1919, little 
benefit in the way of tolerance to the organism appears to have been acquired. 
When the manner in which the pregnancy of this animal as well as of the 3 other 
udder-infected cows used in the exposure experiment terminated is compared 
with the 8 that gave evidence of having overcome the infection in their bodies, 
it seems reasonable to infer that the pregnant periods of carriers may well be 
regarded as speculative so long as the infection persists, and that its total elimina¬ 
tion may be necessary before a dependable and true immunity is established. 
This feature is presumably responsible to a great degree for the differing views held 
as to whether immunity is derived from an attack of the disease. 
DISCUSSION OF IMMUNIZATION RESULTS 
Considerable encouragement was derived from the results obtained with living 
abortion organisms under the experimental conditions described. It was gratify¬ 
ing to observe that although marked serological reactions were produced in sus¬ 
ceptible heifers by the injections, they became much reduced in intensity or dis¬ 
appeared in a comparatively brief period. This feature was regarded as being of 
especial interest, inasmuch as what has heretofore appeared to be a logical argu¬ 
ment against the use of living organisms has been that heifers so treated may 
thereafter be rendered spreaders of the disease in herds into which they may be 
subsequently introduced. The entire absence of sterility troubles and freedom from 
calving complications in this group of vaccinated subjects constituted further 
grounds for optimism relative to the procedure, and the simplicity of the immuniz¬ 
ing method made it appear especially attractive. 
It was only when experimental animals with functioning udders were vaccinated 
that a certain amount of danger of producing carriers of the disease by the treat¬ 
ment, and of possible defeat of the object desired, was definitely determined. A 
limited amount of experimental work has been directed toward determining with 
what frequency the organisms localize in the udders of vaccinated nonpregnant 
cows. Despite the fact that the percentage of such cases was relatively high in 
the few mature animals employed in the original experimental work, additional 
data pertaining to this feature have failed to indicate that as much cause for 
alarm exists as was suggested by the earlier experiments. 
Twenty cows from a herd regarded as being free from infectious abortion were 
introduced into a herd in which severe abortion losses were being experienced and 
where 75 per cent of the animals gave positive reactions to the agglutination test 
for the disease. These 20 animals gave negative results to the agglutination test 
for infectious abortion at the time of purchase. On November 2, 1921, which 
was shortly after the introduction of the group, they were injected subcutaneously 
with 20 cc. of abortion vaccine. On March 6, 1922, samples of blood and milk 
were taken from those treated and were subjected to the agglutination test. In 
those cases where 0.005 cc. or lesser amounts of the blood serum caused any 
agglutination, or where agglutination of the milk occurred even in 0.04 cc. or lesser 
amounts, guinea-pig inoculations were made with samples of the milk sediment. 
While 9 groups of guinea pigs consisting of 3 each were injected with sediment 
