In this latter connection may be mentioned the harm resulting from the use of certain varieties of moss litter.  This we find pointed out by J. Roalfe Cox, F.R.C.V.S.[A] Tenderness in the foot was first noticed, and, on examination, the horn of the sole and of the frog was found to be peculiarly softened.  It afforded a yielding sensation to the finger, not unlike that which is imparted by indiarubber, and on cutting the altered horn it was almost as easily sliced as cheese-rind.  The outer surface being in this way slightly pared off, the deeper substance of the horn was discoloured by a pinkish stain.  The horn of the frog was in many instances found detaching from the vascular surface, which was very disposed to take on a diseased action, somewhat allied to canker, and became extremely difficult to treat.

[Footnote A:  Veterinary Journal, vol. xvi., p. 243.]

Conditions such as these, although not constituting the disease itself, certainly lay the frog open to infection, especially if afterwards the animal is called upon to work in the mud of the streets of a large town, or to stand in a badly drained and damp stable.

A further cause of thrush is to be found in the condition of the frog, brought about by contraction of the heels (see p. 118).  We have already seen that one of the most prominent factors in the causation of contraction is the removal of the frog from the ground by shoeing, with its consequent diminution in size and deterioration in quality of horn.  This leads to fissures in the horny covering, and favours infection of the sensitive structures beneath.  Thrush is, in fact, nearly always present in the later stages of contracted foot.

By some thrush is believed to be but the commencement of canker.  With this, however, we do not hold.  We believe both to be due to specific causes as yet undiscovered, but that the cause of thrush is not the one operating in canker.  In arriving at this conclusion we are guided by clinical evidence.  The two conditions are quite dissimilar, even in appearance, and, while one is readily amenable to treatment, the other is just as obstinately resistant.

Symptoms.—­The symptoms of thrush are always very evident.  Probably the first thing that draws one’s attention to it is the stench of the puriform discharge.  The foot is then picked up and the characteristic putrescent matter found to be accumulated in the median, and often in the lateral, lacunae.  The organ is wasted and fissured, the horn in the depths of the lacunae softened and easily detachable, and portions of the sensitive frog often laid bare.

With a bad thrush lameness is present, the frog itself is tender to pressure, and often there is considerable heat and tenderness of the heels and the coronet immediately above.  More especially is this noticeable after a journey.

It is, perhaps, more common in the hind-feet than in the fore, and more often met with in heavy draught animals than in nags.  The hind-feet are, of course, more open to infection by reason of their being constantly called upon to stand in the animal discharges in the rear of stable standings, while it is a well-known fact that heavy animals have their stables kept far less clean, and their feet less assiduously cared for, than do animals of a lighter type.