In both man and beast, age appears to have a predisposing effect, but
apart from debility or trauma, this may be fully explained on the ground
of often repeated infection, an indoor life, and, in dairy cows, the
excessive drain upon the system. In thoroughbred herds of cattle, in
which the cows are preserved as long as they will breed, and in dairies
where they are kept as long as the milk yield is satisfactory, the
oldest show by far the largest percentage tuberculous. In secluded
individual dairies we have found 100 per cent. affected, Ostertag gives
the average for Berlin at 75 per cent., and Bang for Denmark at 69 per
cent.