Differential Diagnosis. - Navicular disease may be mistaken for ordinary contracted foot. It will be remembered, however, that in the early stages of navicular disease contraction is absent, and that it is only when the disease in the bursa is of long standing that contraction comes on. With ordinary contracted foot, too, careful paring and suitable shoeing soon sees a diminution in the degree of lameness, and a return to the normal in shape (see Treatment of Contracted Foot, p. 125). With navicular disease, however, such shoeing as is beneficial in the treatment of contracted foot (notably the various methods of giving to the frog counter-pressure with ground) soon brings on an aggravation of the lameness.
It is, perhaps, even more likely to be confounded with contraction when we have with the contraction a state of atrophy and thrush of the frog. With a frog in this condition pressure will give rise to pain, and navicular disease be erroneously judged to be present. In such a case we must rely wholly upon either extreme flexion or extreme extension of the joint to guide us, when, if contraction only is the offending condition, no symptom of pain will be shown.
Navicular disease may also be confused with rheumatic affections, with sprain of the posterior ligaments of the first interphalangeal articulation, and with sesamoid lameness. Mistakes are sometimes made, too, especially with a hasty observer, in confounding it with shoulder lameness.
In rheumatism the constant changing of the seat of pain, the sometimes elevated temperature, and the appearance of symptoms of heat, tenderness, and swelling in the affected area should guide one to a right conclusion.
In sprain of the posterior ligaments of the coronet and in sesamoid lameness, nothing but a careful examination and manipulation of the parts will ward off error, for in each of these cases there is 'pointing' and resting of the limb, and considerable disinclination to put weight firmly upon it. If at the same time manipulation gives distinct evidence of pain, all doubt may be set at rest.
Roughly speaking, sesamoid lameness is a condition of the gliding surface of the sesamoids, and the face of the tendon playing over them, similar to that found in navicular disease. All symptoms of pointing, the constant maintaining of the limb in a state of flexion, and a feeling manner of progression are again all present. It is plain from this that in all cases where an animal with a gait at all suggestive of navicular disease is brought for our examination, the manipulation of the limb should be thorough. The character of the lameness is almost sure to deceive us; and it is not until we are able to obtain local symptoms pointing to the one or the other of the conditions we have enumerated that a decisive opinion may be given. In sesamoid lameness the local symptoms are those of heat and pain in the fetlock on palpation, and a swelling of the affected parts, such swelling being at first slight, yielding, and barely distinguishable, and afterwards larger, bony and hard, and more marked. Later still there is distinct evidence of 'knuckling' over at the fetlock and inability to fully flex it.
In cases of shoulder lameness the gait alone should be sufficient to render liability of error small, for with nearly every case there is a manifest inability to 'get the limb forward', and this is best seen at a side view when the animal is trotting past the observer. When trotting towards one, there is a further and unmistakable symptom common to most shoulder lamenesses that serves to distinguish it at once, and that is the peculiar 'sweeping' outwards with the affected limb.
Lastly, with either of the conditions we have just mentioned, it is the exception to get contracted foot follow on. With navicular disease it sooner or later makes its appearance.
Prognosis. - The prognosis of navicular disease (once diagnosed with certainty) must almost of necessity be unfavourable. The facts that the disease has made serious progress before it is really noticeable, that the situation of the parts prohibits operative interference, and that the disease is one of a chronic and slowly progressive type, all point to an unfavourable termination.