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Fortieth Annual Report of the
Data along this line have been published by Dr. V. A. Moore of the
Cornell Veterinary School. Those obtained from the study of a herd
of cattle are given In Table 111. A herd of 491 animals was tested
and 96 did not react to the test. These non-reacting animals were
removed and placed in quarters where there was no opportunity for
Infection to occur. Retests were made at intervals of six months with
the results as shown. It will be seen that at each test one or more
animals were found to be diseased, so that by the end of four years
but 19 out of the original 96 had not reacted to the test. Possibly In
certain cases the reactions were caused by reinfection from other
animals in which the disease had progressed from the dormant stage
to an open type between the successive tests. It is certain that a large
part of the reactions obtained on the retests were due to the fact
that at the time the previous tests were made the disease was in a
dormant stage. Additional data might be presented along similar lines.
Such facts as these have led the live stock sanitarians to believe that
the healthy herd, not the healthy individual animal, must be taken
as the unit in fighting the spread of tuberculosis. This is emphasized
in the report which was made last year by the commission appointed
by the American Veterinary Association to study the subject of bovine
tuberculosis, when they recommended that in the case of herds in
which over 50% of the animals were found diseased on the first test
the whole herd should be considered as diseased; that In the case of
such herds it Is not worth while to attempt to breed a healthy herd
from this foundation by separating the herd into two parts-the re-
acting and the non-reacting animals-because a considerable portion
of the non-reacting animals have the disease either in the incubation
period or in a dormant form. In a herd of any size in which the
disease has been present for any length of time, such animals are to be
found, and their presence can only be detected by making frequent tests
on the herd.
When the breeders realize the importance of this point in the spread
of bovine tuberculosis, we shall not have a spectacle presented to us
of a dispersion sale of a herd of pure bred animals in which shortly
before the sale the tuberculin test showed 20% of the animals to be in-
fected.
It seems to those best fitted to judge that if one wishes to be ab-
solutely certain of purchasing a healthy animal he must buy his an-
imals from herds that are known to be free from tuberculosis; that If
he buys, even on the tuberculin test, from an infected herd he runs the
chance of purchasing a diseased animal. In the case of herds that are
not extensively diseased, In which but one or two reacting animals have
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