1917, to March 1, 1922, on 3,911,546 cattle by State, county, and Federal
officers engaged in cooperative tuberculosis eradication work showed
153,046 reactions, or 3.9 per cent.

All cattle in the District of Columbia, numbering 1,701, were tested with
tuberculin in 1909-10, and 18.87 per cent reacted. In 1909-11 herds in
Maryland and Virginia supplying milk to the District of Columbia were
tested, with 19.03 and 15.38 per cent of reactions, respectively, among
4,501 cattle.

All cattle in the District of Columbia were tuberculin tested in 1920-21,
numbering 1,313, and 5 animals reacted, or 0.4 per cent, demonstrating that
tuberculosis may be eradicated from all the herds in a circumscribed area.

The beef cattle of the United States show a much smaller proportion of the
disease than dairy cattle, though the percentage of cattle found
tuberculous in the Government meat-inspection service has increased
considerably in recent years. This increase is due partly, but not wholly,
to more stringent inspection. Of 7,781,030 adult cattle slaughtered under
Federal inspection during the fiscal year ended June 30, 1911, 76,448 were
found tuberculous, a percentage of 0.98.

From the statistics above referred to, and other data, it appears that in
the more densely populated areas of Europe and America from 5 to 50 per
cent of the dairy cattle are more or less affected with tuberculosis, while
the proportion of beef cattle affected is distinctly less, ranging from
0.14 to 30 per cent. This difference is due to a number of causes. Beef
cattle average younger when slaughtered. They are not so frequently
stabled, and are for that reason less liable to infection, and as the males
constitute a large proportion of this class of animals the effect of milk
secretion in lowering the vital forces is not so apparent. In the United
States it has been estimated that about 10 per cent of the dairy cattle are
tuberculous, while only about 2 per cent of the beef cattle are so
infected.

_Cause and nature of the disease._--The cause of tuberculosis is the
tubercle bacillus, which gains entrance to the body, lodges somewhere in
the tissues, and begins to grow and multiply at that point. As this
bacillus vegetates and increases in numbers it excretes substances which
act as irritants and poisons and which lead to the formation of a small
nodule, called a tubercle, at the point of irritation. As the bacilli are
disseminated through the animal body