infection.

Tuberculosis as a bovine disease has increased rapidly during recent
decades throughout many portions of the world. This has been most marked
in dairy regions. Its extremely insidious nature does not permit of an
early recognition by physical means, and it was not until the
introduction of the tuberculin test[85] in 1892, as a diagnostic aid
that accurate knowledge of its distribution was possible. The quite
general introduction of this test in many regions has revealed an
alarmingly large percentage of animals as affected. In Denmark in 1894
over forty per cent were diagnosed as tubercular. In some parts of
Germany almost as bad a condition has been revealed. Slaughter-house
statistics also show that the disease has increased rapidly since 1890.
In this country the disease on the average is much less than in Europe
and is also very irregularly distributed. In herds where it gained a
foothold some years ago, often the majority of animals are frequently
infected; many herds, in fact the great majority, are wholly free from
all taint. The disease has undoubtedly been most frequently introduced
through the purchase of apparently healthy but incipiently affected
animals. Consequently in the older dairy regions where stock has been
improved the most by breeding, more of the disease exists than among the
western and southern cattle.

[Illustration FIG. 21: Front view of a tuberculous udder, showing extent
of swelling in single quarter.]

~Infectiousness of milk of reacting animals.~ Where the disease appears in
the udder the milk almost invariably contains the tubercle organism.
Under such conditions the appearance of the milk is not materially
altered at first, but as the disease progresses the percentage of fat
generally diminishes, and at times in the more advanced stages where the
physical condition of the udder is changed (Fig. 21), the milk may
become "watery"; but the percentage of animals showing such udder
lesions is not large, usually not more than a few per cent. (4 per cent.
according to Ostertag.)

On the other hand, in the earlier phases of the disease, where its
presence has been recognized solely by the aid of the tuberculin test,
before there are any recognizable physical symptoms in any part of the
animal, the milk is generally unaffected. Between these extremes,
however, is found a large proportion of cases, concerning which so
definite data are not available. The results of investigators