Hoof Abscesses

Stephen E. O'Grady, DVM, MRCVS 
Hoof abscesses are probably the most common cause of acute lameness in horses. Foreign matter (such as gravel, dirt, sand, etc.) or infectious agents such as bacteria or fungal elements gains entry into the hoof through a separation in the sole-wall junction (white line). This foreign debris will migrate in the hoof to the sensitive subsolar or submural tissue leading to infection. Another common cause of subsolar abscesses is penetration of the bottom of the foot (sole) by a sharp object. Infection may also gain entry into the foot by way of a hoof wall crack or multiple old nail holes.

Hoof abscesses are less likely to occur with a solid sole-wall junction (white line). Conditions that cause mechanical breaks or weakness in the continuity of the white line are hoof imbalance (long toe-underrun heel syndrome, excessive toe length, heels too high) hoof wall separations (white line disease, seedy toe), aggressive removal of sole and chronic laminitis. Excessive moisture or dryness may also contribute to weakness in the white line. If left untreated, the subsolar abscess will follow the path of least resistance up the hoof wall, rupture and then form a draining tract at the coronet. This often leads to a permanent scar in the hoof wall.

Clinical Signs

Most affected horses show sudden (acute) lameness. The degree of lameness varies from subtle to non-weight bearing. The digital pulse felt at the level of the fetlock is usually bounding and the involved foot will be warmer than the opposite foot. The site of pain can be localized through the use of hoof testers. A small tract or fissure will commonly be observed in the white line where the pain is noted. The wound or point of entry may not always be visible, as some areas of the foot such as the white line and frog are somewhat elastic and wounds in these areas typically close. Sometimes pain will be noted over the entire foot with hoof testers and, in this case, the veterinarian may want to rule out a severe bruise or a possible fracture of P3 (coffin bone)

Treatment

The object of treating a simple subsolar abscess is to open and drain the infection. The opening should be of sufficient size to allow drainage but not so extensive as to create further damage. Establishing drainage is the most important aspect of therapy. Preferably, this is done at the onset of lameness before the infection ruptures at the coronet. The offending tract or fissure is opened on the hoof wall side of the white line using a 2 mm bone curette or other suitable probe. A small opening is sufficient to obtain proper drainage and care must be taken to avoid exposing solar corium, as it will invariably prolapse through the opening and create an ongoing source of pain. The draining tract is kept soft and drainage is enhanced by the application of an Animalintex® poultice for the first 48 hours. This is a self-contained, medicated poultice, which is commercially available through your veterinarian or tack shop. In most cases, this eliminates the need for continued foot soaking. The horse should show marked improvement within 24 hours.

The hoof is kept bandaged with a suitable antiseptic such as Betadine® ointment or 2% iodine until all drainage has ceased and the wound is dry. At this point, a small gauze plug is used to fill the opening of the tract and is held in place with super glue. This keeps the affected area clean and prevents the accumulation of debris within the wound. The shoe is replaced when the horse is sound. Many times the painful tract can be located but drainage cannot be established at the white line. In this case, the infection has migrated under the sole away from the white line.

Under no circumstances should an opening be created in the adjacent sole. This only leads to a persistent, non-healing wound and increased susceptibility to bone infection. Instead, a small channel should be made on the hoof wall side of the white line in a vertical direction following the tract to the point where it courses inward. Drainage can be established here in a horizontal plane. Tetanus immunization status of the horse should be determined. Use of systemic antibiotics is optional and based on the needs of the individual horse.

Prevention

Prevention is achieved through proper hoof care and centers around promoting a strong, solid white line which resists penetration by debris. Excessive toe length increases the bending force exerted on the toe, leading to a widening and weakening of the white line. This, along with toe cracks and hoof wall separations, is the most common cause of foot abscesses.

To prevent abscesses it is important that the foot be trimmed in a manner that accentuates a strong healthy foot. A few basic principles can be used when trimming to create a strong foot and strengthen the white line. First, the bars of the foot are left untouched and the heels are trimmed back toward the widest part of the frog, or as far back as possible. This allows a large amount of weight bearing to occur in the posterior portion of the foot and not the toe area. Sole is only removed adjacent to the white line to identify excess hoof wall to be removed. It is not necessary to concave the sole as this occurs naturally. The toe is then backed up from the dorsal surface (front) of the hoof wall and/ or the breakover is set back accordingly. This assures that there is no excessive toe length. A good rule of thumb to use when trimming the foot is to leave the last few rubs on the bottom of the foot. When applying shoes, fitting the shoes hot may be helpful to seal the sole wall junction. The use of hoof hardeners (Keratix®) and bedding the horse on shavings or sawdust may be useful to harden the feet during extremely wet weather or when the horse is being washed frequently such as during horse shows. During dry weather, a hoof dressing such as a combination of cod liver oil and pine tar (mixed in a ratio of 3:1) painted on the entire foot may help to contain moisture.

Preventing indirect penetration through the white line is therefore dependent on providing adequate protection to the underlying sensitive structures. The hoof capsule has a natural ability to provide such protection and it is imperative that we strive to enhance these strong features through proper trimming. Excessive removal of protective horn is a common practice, as emphasis is often placed on eye appeal instead of functional strength.

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