Symptoms and Pathological Anatomy. - The symptoms of canker are seldom noticeable at the commencement of an attack. The disease is slow in its progress; for some time confines its ravages to the sub-horny tissues unseen, and is quite unattended with pain. It is not observed, therefore, until considerable damage has been done, and the disease is far advanced. What is usually first seen is a peculiar softening and raising of the horn of the frog. The infective material has set up a chronic inflammation of the keratogenous membrane, leading to abnormal secretion, and, in place of the horny cells it should normally secrete, is thrown out an abundance of a serous fluid.
This upraised and softened horn once thrown off is not again renewed, and the whole of the sensitive frog and perhaps a portion of the sensitive sole is left uncovered. In place of the normal horn, however, is often found a hypertrophy of the elements of the keratogenous membrane leading to huge fungoid-looking growths with a papillomatous aspect, damp in appearance and offensive in smell, and readily bleeding when injured (see Fig. 131).
The horn immediately surrounding the lesion is loose and non-adherent to the sensitive structures. This indicates, of course, that the disease has spread further beneath the horny covering than is at first sight apparent. Portions of this loose horn removed reveal beneath it a caseous foetid matter, easily removed by scraping (the perverted secretion of the keratogenous membrane). When this is carefully scraped away, the sensitive structures appear to be covered with a thin, smooth membrane, gray in colour and almost transparent, while beneath it may be seen the red appearance of normal sensitive structures.
If the horn surrounding the lesion is not touched with the knife, but little is seen of the extent of the disease, for that removed by natural means is often very small in quantity. To all intents and purposes the disease appears to be confined to the frog. This appearance is misleading, especially if the disease has been in existence for some time, for it may have easily spread to the whole of the sole, and even to the greater portions of the laminae secreting the wall.
It is, in fact, not until the pressure exerted by the normal horn is removed by its breaking away that the vascular structures of the keratogenous membrane begin to swell, and the perverted secretions to enlarge in size. Once the pressure is removed, however, this quickly comes about, and the characteristic fungoid growths rapidly make their appearance.
This tendency to spread is highly indicative of canker. The serous matter exuding from the diseased keratogenous membrane appears, in fact, to be highly infective. Once its flow is commenced, it slowly, but surely, invades the sensitive structures near it, appearing, as Elaine has put it, to 'inoculate' them. What is really the case, of course, is not that the discharge itself is infective, but that it is contaminated with infective material.
The fungoid-looking growths to which we have before referred are, in reality, nothing more than the villi of the sensitive frog and sole greatly hypertrophied and irregular in shape. At times the hypertrophy is as a huge and compact enlargement occupying the position of the frog. Sometimes, however, it occurs as numerous elongated and twisted fibrous bundles, separated from each other by deep clefts, and the clefts filled with the offensive cankerous discharge (see Fig. 134).
Fig. 135. - Lower Aspect Of Cankered Foot, Showing Destruction Of Wall.
At a very advanced stage canker leads to destruction of much of the horny sole and frog; or even parts of the wall may become separated from the tissues beneath, and break away from the foot (see Fig. 135). At other times the disease brings about a deformity of the whole of the foot. Its longitudinal and transverse diameters become enormously increased, and the whole foot apparently flattened from above to below (see Fig. 136). This indicates that not only has the horny sole been entirely destroyed, but that the destructive process has also extended to the greater part of the lower half of the wall, with a consequent hypertrophy of exposed soft structures, and a sinking of the bony column, similar to that which occurs in laminitis, but not so pronounced.
Fig. 136. - Foot With Advanced Canker.
A further aspect of the badly-cankered foot is to be found in an apparently enormous increase in the length of the wall. This we have seen protruding for quite 5 inches beyond the plane of the sole. It simply indicates that, in order to keep the animal at work, the smith has at every shoeing spared the wall, so that the diseased structures might be kept from contact with the ground.
As we have said before, pain and other symptoms of distress are quite absent. Animals affected with canker for a long time maintain their condition, feed well, and are quite capable of performing work under ordinary conditions.
Differential Diagnosis and Prognosis. - Perhaps the only disease with which canker may be confounded is thrush. They should, however, be easily distinguishable. The discharge from thrush is not so profuse, and is thicker and darker in colour, while the loosening of the horn is almost entirely absent. Furthermore, thrush shows no tendency to spread, and, even when left untreated, may remain confined to the frog for months, and even years. Canker, on the other hand, is slowly progressive, and soon shows the characteristic fungoid excresences, which growths are in thrush never seen. A further point of difference is discovered when treatment is commenced. Canker is found to be refractory to a point that is absolutely disheartening, while thrush, with careful attention, is soon got under hand, and a permanent cure effected.
The prognosis must be guarded. By many canker has been said to be incurable. This, however, has been clearly shown to be wrong. When the animal is young, and treatment may reasonably be judged to be economical, then a favourable prognosis may be indulged in, provided the veterinary surgeon intends to put into that treatment a more than ordinary amount of individual care and attendance. Even then, however, he will have to be very largely guided by the condition of his case. He should see that it is not too far advanced, and that a great deformity of the hoof, or actual exploration, does not indicate disease of the greater part of the wall.