Definition. - Under this unscientific, yet expressive term, is indicated a chronic diseased condition of the keratogenous membrane, commencing always at the frog, and slowly extending to the sole and wall, characterized by a loss of normal function of the horn secreting cells, and the discharge of a serous exudate in the place of normal horn.
Causes. - The exact cause of canker has still to be discovered. Therefore, before expressing an opinion as to what the probable cause may be, we may state here that such opinion can only be based upon clinical observation. Such being the case, we are almost duty bound to give the views of older authors before those of more modern writers.
From the mass of material ready to hand we may select the following as serving our purpose.
The earliest opinion appears to have been that canker, as the name indicates, was of a cancerous or cancroid nature. This was also believed by Hurtrel D'Arboval, who looked upon canker as carcinoma of the recticular structure of the foot. The same theory we find enunciated in the Veterinary Journal so late as 1890. Although the word 'cancer' or 'carcinoma' is not there used, the author employs the terms 'Papilloma' and 'Epithelioma' with the evident intention of expressing his belief in the malignant nature of the disease.
Another early opinion was that the disease was a spreading ulcer, gradually extending and changing the tissues which it invaded.
A further early theory, and one which if not still believed in, has died a hard death, is the constitutional theory. This was believed in by nearly all the older writers, and is mentioned so late as 1872 by the late Professor Williams. In his 'Principles and Practice of Veterinary Surgery,' he says: 'Canker is a constitutional disease due to a cachexia or habit of body, grossness of constitution, and lymphatic temperament.' This, we believe, is credited to-day by some, and yet, quite 100 years before the date of the 1872 edition of Williams's work - in 1756, to be exact - we find a veterinary writer when talking of grease (a disease, by-the-by, very closely allied to canker) exclaiming against this habit of referring everything which we do not rightly understand to some ill-humour of the body. The wisdom his words contain justifies us in giving them mention here. 'It is a very foolish and absurd Notion,' he says, 'to imagine a Horse full of Humours when he happens to be troubled with the Grease. But such Shallow Reasoning will always abound while Peoples' Judgments are always superficial. Therefore, to convince such unthinking Folks, let them take a thick Stick and beat a Horse soundly upon his Legs so that they bruise them in several Places, after which they will swell, I dare say, and yet be in no danger of Greasing. Now, pray, what were these offending Humours doing before the Bruises given by the Stick?'
At the present day it is safe to assert that neither the ulcerative, the cancerous, nor the constitutional theory is believed in widely, and, among the mass of contrary opinions as to the cause of this disease, we may find that even quite early many of the older writers had discarded them.
Quoting from Zundel, we may say that Dupuy in 1827 considered canker as a hypertrophy of the fibres of the hoof, admitting at the same time that these fibres were softened by an altered secretion; while Mercier in 1841 stated that canker was nothing more than a chronic inflammation of the reticular tissue of the foot, characterized by diseased secretions of this apparatus.
Saving that they make no mention of a likely specific cause, these last two statements express all that we believe to-day. As early as 1851, however, the existence of a specific cause was hinted at by Blaine in his 'Veterinary Art.' We find him here describing canker as a fungoid excrescence, exuding a thin and offensive discharge, which inoculates the soft parts within its reach, particularly the sensitive frog and sole, and destroys their connections with the horny covering.
The use of the word 'fungoid,' and particularly that of 'inoculate,' is suggestive enough, and is evidence sufficient that either Blaine or his editor recognised, simply through clinical observation, the working of a special cause.
Four years later, Bouley is found holding the opinion that canker was closely allied to tetter, thus recognising for it a local specific cause. The same observer also pointed out that the secretion of the keratogenous membrane instead of being suspended was greatly increased, taking care to explain, as did Dupuy, that the products of the secretion were perverted and had lost their normal ability to become transformed into compact horn.
In 1864 this slowly growing recognition of a specific cause received further impetus from the statements of Megnier. This observer claimed to have discovered in the cankerous secretions the existence of a vegetable parasite (namely, a cryptogam, as in favus), which he termed the keraphyton, or parasitic plant of the horn.
Modern research, though failing to substitute anything more definite, has not confirmed this. The exact and exciting cause of canker is therefore still an open question, and a matter for research. We may, however, sum the matter up by briefly discussing the causes, so far as clinical observation teaches us. This we shall do under two headings - namely, Predisposing and Exciting.