White Line Disease

by
James Rooney, D.V.M.

White line disease, seedy toe, classical laminitis, and road founder type laminitis are all related.

DFb-Fl=0

Road Founder

Of these only road founder, perhaps, needs to be defined. It was well known in older days, occurring particularly in the heavy carriage horses used on the roadways and turnpikes of England and Europe. These horses were characteristically trimmed with long toes and shod - the long toes providing a flashy, high-stepping gait.

Road founder is tearing of the laminae near the distal border of the coffin bone as indicated in the figure. If severe enough the horse would, of course, be lame with the clinical signs those of any case of laminitis. Both fore feet could and often were affected at virtually the same time, thus closely resembling the classical over-eating type of laminitis.

Less severe cases might show little or no lameness. The presence of the condition, then, is indicated by blood spots in the white line area of the toe when the farrier trims the foot. I note, here, that Henry Heymering was the first to direct my attention to these bloody spots. I had known about them but not thought about their cause. It is now clear that they are hemorrhages migrating down into the white line area because of tearing of the laminae just above that area as shown schematically in the figure.

Torn tissue means hemorrhage and dead tissue, and those two factors predispose the area to infection with whatever bacteria or fungi happen to be prevalent in the horse's environment; that is, white line disease/seedy toe.

Pathogenesis

The tearing of the laminae occurs because of the increased resistance of the long-toed hoof to breaking over at the end of the support phase of the stride - as the foot is leaving the ground. That is simple enough, I think, but if the reader wishes it can be shown easily as:

DFb - Fl = 0

Don't worry about that unless you really want to! It simply indicates that the turning force exerted by the deep flexor tendon is equal to the opposite turning force exerted by the body weight on that leg. These turning forces are called moments. If Fl becomes too large which is to say that l increases, DF must pull harder and that can causes the tearing of the laminae. The laminae, clearly, are the "connectors" between those two forces.

Classical founder (laminitis)

Our interest here is only with this condition as it predisposes to white line disease/seedy toe. As is well known the laminae are destroyed or, at least, severely damaged in this condition. Rather little F and DF are quite sufficient to tear what remains. Again, then, we have hemorrhage and death of tissue which predisposes the foundered foot to infection: white line disease/seedy toe. So, in this case, we have a severe example of the same type of process (though of different ultimate cause) as with road founder.

Shoe and Barefoot

From what I have been able to glean from conversations and the literature white line disease is more frequent in shod than barefoot horses. That is because the toe continues to grow and is not worn down when the hoof is shod. The longer the shoe is in place before being replaced or reset, the longer the toe becomes increasing the risk of laminar tearing, road founder.

Even barefoot horses and ponies, however, can develop road founder and white line disease if the surface on which they move about does not wear down the bearing edge of the hoof wall sufficiently and/or the feet are not trimmed regularly. Shetland ponies with iron hard hooves on lush bluegrass pastures are excellent examples. (And who wants to trim a field of Shetland ponies once a month?)

Wet and Dry

Wet conditions are said by some to predispose to white line disease. Certainly, wet standing will contribute to the softening of the already damaged white line area. It seems improbable that wet conditions, per se, would be a cause. Such conditions are more often associated with thrush and scratches.

I hope this makes my case clearly - even if you disagree. I do want to note that Steve O'Grady had an excellent article on this subject in one of the farriery journals recently. I regret I have lost the reference. His observations and findings fit very nicely with mine, so, of course, we are both correct!

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