© James R. Rooney, D.V.M

O’Grady has recently provided an excellent review and summation on the current status of so-called white line disease, seedy toe. (1) I quote from a portion of that paper, part of which I have italicized, "White line disease (WLD) is a term used to describe a keratolytic process on the solar surface of the hoof, which is characterized by a separation of the inner zone of the hoof wall (Redden, 1991). The separation occurs in the non-pigmented horn at the junction between the stratum medium and the stratum lamellatum. This separation is invaded by opportunistic bacteria/fungi at the toe, quarter and/or heel leading to infection, which progresses to varying heights and configurations toward the coronet. The disease process occurs secondary to a primary hoof problem such as chronic laminitis, abnormal hoof conformation, hoof imbalance or any other condition that causes a hoof wall separation.”

Specifically, we can say separation of the hoof wall in the toe area. In the earliest stages of this condition there must be damage and/or damage leading to separation in the sole/wall junction, the white line, in order to allow invasion by any of the several fungi and fungi which have been isolated. Chronic laminitis is an obvious cause. “Abnormal hoof conformation, hoof imbalance” can be made more specific. That is, if the toe is too long, there is increased leverage on the attachments between the coffin bone and the hoof wall in the toe region during the rolling of the hoof off the surface at the time of lift-off. (I shall spare the reader the formal mathematical statement of that action but will be happy to provide it to anyone who asks.)

Farriers not infrequently find spots of blood in parings of the bearing surface of the hoof wall, specifically in the area of the toe. In my opinion these spots arise from slight tearing of the laminae near the distal border of the coffin bone. This tearing, in turn, is the result of the increased leverage being exerted on the laminar attachments by a “too long” toe as noted above. Such laminar tearing has been observed and given the name (by me!) road founder (2).

If the tearing is sufficiently severe, there will be pain in the foot which blocks only if the nerves (dorsal branch of the ulnar nerve) on the front aspect of the pastern are infiltrated; the usual nerve blocks of the metacarpal and/or palmar digital nerves will not completely desensitize the toe portion of the foot. If less severe, there may well be no detectable pain at all. Detectable, that is, to the human observer.

The bleeding subsequent to such tearing provides a nidus or weak point, medium, for the growth of the opportunistic fungi, ergo, white line disease.

Why, that being the case, is there such a high frequency of this condition? There is some anecdotal evidence that habitually bare foot horses are less commonly afflicted than habitually shod horses. Also, it has been noted that pulling shoes and turning out may allow many white line disease cases to self cure. I have made attempts to quantify these anecdotal observations but have not received enough replies from questions to farriers and veterinarians to be of use.

I suggest the following hypothesis: the horseshoe prevents the rolling wearing of the toe of the barefoot horse (3). In the absence of such wear the toe simply lengthens continuously from one shoeing to the next. Even though the levering effect may seem small, it can have a cumulative effect over time.

It would obviously be of importance for this hypothesis to determine the frequency of white line disease: in horses shod at shorter or longer intervals, horses shod with half rounds rather than flat shoes, and even some real numbers for habitually shod and barefoot horses. An interesting dissertation project for some horse studies major!

References:

1: O’Grady, S. White line disease update. American Farriers Journal. July/August 2000, page 29

2: Rooney, J. R. and Robertson, J L. 1996. Equine Pathology. Iowa State Univ. Press, Ames, Iowa

3: Rooney, J. R. 1999 Surfaces and Wearing of Hooves. Online Journal of Veterinary Research 4:73-93.

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