Predisposing Causes. - Starting with the assumption that the disease is due to local infection, we may relate as predisposing causes anything having a prejudicial effect upon the horn, disintegrating it, and so laying the tissues beneath open to attack. The most prominent in this connection is certainly a continued dampness of the material on which the animal has to stand. Particularly is this the case when the material is also excessively foul and dirty, contaminated with the animal discharges, and presumably swarming with the lower forms of animal and plant life. We shall therefore find bad cases of canker in stables where the "sets" are irregular, or where no paving at all is attempted, where the drainage is defective, and where darkness and want of proper ventilation favours organismal growth. The fact that with modern drainage and a general hygienic improvement in stabling, canker has to a large extent died out, supports this contention.
Again, as with thrush, anything removing the counter-pressure of the frog with the ground and throwing that organ out of play, may be looked upon as a predisposing cause. The atrophy of the frog thus occurring, the deterioration in the quality of its horn and the fissures in its surface lay it specially open to infection. That one of the principal factors in the treatment of canker is a restoration of ground-pressure to the frog and the sole is sufficient proof of this.
Further, it is well to note that, although playing no part in the actual causation, certain constitutional conditions may in some measure predispose the foot to attack. Clinical observation teaches us that animals of a lymphatic nature, with thick skins and an abundance of hair, with flat feet and thick, fleshy frogs, are far more liable to attack than are animals with reverse points.
Exciting Causes. Those who give this subject careful consideration cannot fail to arrive at the conclusion that canker is most certainly due to local infection with a specific poison, and that poison a germicidal one from the ground. The symptoms arising may be due to the action of a single germ, or to two or more germs acting in conjunction. As to whether the parasitic invasion is single or multiple we cannot feel certain, but that it is parasitic we feel absolutely assured.
It is simply the light that bacteriological advance has made during the last two decades that enables us to make the statement with such feelings of assurance. We arrive at our conclusions by reasoning from analogy. Here we have a disease always exhibiting the same symptoms, more or less peculiar to one class of animal, always with a similar characteristic appearance and smell, always obstinately refractory to treatment, showing always a tendency to spread to the other feet of the same animal, and often to the feet of other animals near enough to become infected, and always cured - when cured it is - by a treatment which may be summed up in two words as 'rigid antisepsis.' Other diseases, with points in common with this, have been directly proved to be due to a specific cause. Common regard for logic compels us to admit the same for canker.
Fig. 134. - A Foot, The Subject Of Canker, Showing Destruction Of The Horny Frog, And A Fungoid-Looking Hypertrophy Of The Tissues Beneath.