Hoof abscesses are probably the most common cause of acute lameness in horses encountered by veterinarians and farriers.
A hoof abscess can be defined as a localized accumulation of purulent exudates located between the germinal and keratinized layers of the epithelium, most commonly subsolar or submural.
Much debate still abounds between the veterinary and farrier professions as to who should treat a hoof abscess and the best method in which to resolve the abscess.
Remember the origin of the organisms that are responsible for a hoof abscess gain entry through the hoof capsule (epidermis) into the inner subsolar/submural tissue (dermis) where the organisms initiate an abscess. Foreign matter (such as gravel, dirt, sand and manure, coupled with infectious agents such as bacteria or fungal elements) generally gains entry into the hoof in one of three ways:
- Through a break or fissure in the sole-wall junction (white line).
- A misplaced nail or a puncture wound somewhere in the solar surface of the foot.
- By way of a full thickness hoof wall crack or multiple old nail holes.
Formation Of An Abscess
It may be easier for us to understand how to treat an abscess if we briefly look at the mechanism by which an abscess forms.
Foreign debris will gain entry and accumulate in a small separation or fissure
located in the sole-wall junction anywhere around the perimeter of the foot, including the inner surface of the bars adjacent to the sole (Figure 1).
As the animal bears weight, foreign matter will migrate through the fissure until it reaches the subsolar or submural tissue (dermis). Once inside the hoof capsule, the defense mechanism within the dermal tissue recognizes the matter as foreign and sets off a reaction. The bacterium contained within the debris invades the dermal tissue and leads to inflammation, the bacteria continue to grow and cause neutrophils (white cells) to migrate into the area.
Enzymes released from the bacteria and from the invading white cells lead to liquefaction tissue necrosis and the development of the gray/black exudate. The inflamed area is quickly walled off with a thin layer of fibrous tissue to form an abscess. The inflammation and the pressure from the accumulation of the exudate exerted on the surrounding tissue leads to the clinical signs associated with a hoof abscess.
Dermal tissue can be inoculated by bacteria from a misplaced nail in two ways. The nail can be driven directly into the laminar corium. When the nail enters dermal tissue, the horse will show discomfort as the nail is driven into the foot and there will be hemorrhage present where the nail exits the outer hoof wall.
Blood observed at the exit of the offending nail will alert the farrier of the misplaced nail and the blood also acts as a "physiologic rinse" to dilute or eliminate bacterial contamination. Removal of the nail and application of an appropriate antiseptic will usually prevent infection.
Another scenario that occurs frequently is that while the farrier is driving a nail, the horse shows pain indicating the nail is invading sensitive tissue. The farrier will generally remove the nail, place it in another spot or direction and again drive it into the foot. When this occurs, the farrier should remove the shoe and examine the spot where the nail entered the foot. When a nail enters dermal tissue (even if removed), it can seed the area with organisms and lead to an abscess.
If the nail has entered the foot inside the sole-wall junction, the owner should be alerted as to potential problems and the horse could be placed on a broad-spectrum antibiotic for 3 to 5 days as a prophylactic measure.
Finally, we have the condition described as a "close nail" where the nail is placed so that it lies against the border of the dermal corium just inside the hoof wall. Pressure against the corium, the movement of the horse combined with the organisms introduced with the nail, will lead to an abscess as described above. There is a lag period of 7 to 14 days or even longer before clinical symptoms or discomfort is observed following the placement of a "close nail."
Another common cause of perceived subsolar abscesses is penetration of the bottom of the foot (sole) by a sharp object. This is not actually an abscess but rather a diffuse infection caused by the solar corium being seeded with organisms from the penetrating object.
Pain is immediate and usually followed by infection within 3 days. A full-thickness puncture wound in the sole always requires veterinary input.
Most affected horses show sudden (acute) lameness. The degree of lameness varies from being subtle in the early stages 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. With careful observation — unless the abscess is in the middle of the toe — the intensity of the digital pulse will be much stronger on the side of the foot where the abscess is located.
If the abscess is long standing, there may be soft tissue swelling in the pastern or above the fetlock on the side of the limb corresponding to the side of the foot where the abscess is located.
The site of pain can be localized to a small focal area through the careful use of hoof testers. Sometimes with acute lameness, the pain will be noted over the entire foot with hoof testers and, in this case, veterinary assistance is used to rule out laminitis, a severe bruise or even a possible fracture of the distal phalanx (P3).
The most important aspect of treating a subsolar/submural abscess is to establish drainage. The opening should be of sufficient size to allow drainage, but not so extensive as to create further damage.
When pain is localized with hoof testers, a small tract or fissure will commonly be found in the sole wall junction (white line). The wound or point of entry may not always be visible, as some areas of the foot such as the white line are somewhat elastic and wounds in this area tend to close. In this case, a suitable poultice should be applied to the foot daily in an attempt to soften the affected area and eventually a tract will become obvious.
The offending tract or fissure is followed within the white line using a thin small loop knife, a 2 mm bone curette or other suitable probe (Figure 2). The tract is slowly followed until a gray/black exudate (pus) is released and the probe will enter the "belly" of the abscess. At this point, the tract is open into the cavity of the abscess.
A small opening is all that is necessary to obtain proper drainage (Figure 3). This can be determined by placing thumb pressure on the solar side of the tract and observing more drainage being expressed or a bubble at the opening when pressure is applied. Care should be taken to avoid exposing any corium, as it will invariably prolapse through the opening, prevent closure of the tract and create an ongoing source of pain. Under no circumstances should an abscess be approached through the sole.
The draining tract can be kept soft and drainage promoted in many ways. The application of an Animalintex poultice that has been soaked in hot water is applied for the first 24 to 48 hours has been useful in the author's hands. This is a self-contained, medicated poultice, which is commercially available through your veterinarian or tack shop.
The author prefers the sheet version of this poultice rather than the poultice pad distributed by this company, as the whole foot, including the coronet, should be enveloped in the poultice.
Using Soak Bandages
Another method to encourage drainage is to apply a soak bandage. Here layers of practical cotton are crisscrossed to form a heavy bandage that envelops the foot. MgSo4 (Epson salts) is placed in the inner foot surface of the bandage and the bandage is attached to the foot.
The bandage is now saturated with hot water and saturated periodically over the next 24 to 48 hours. Using either of these methods eliminates the need for continued foot soaking.
Ichthammol ointment is a coal tar derivative with mild antiseptic properties that has been described for treating skin disease in both humans and animals. The use of an Ichthammol bandage for treating hoof abscesses, both before and after drainage, has become another traditional treatment among veterinarians and horse owners with reportedly good results. The author has not used this form of treatment, therefore would be unable to render an opinion as to its efficacy.
The tetanus immunization status of the horse should always be determined.
The horse should show marked improvement within 24 hours. Following the poultice or foot soak bandage, the hoof is kept bandaged with an appropriate antiseptic such as Betadine solution/ointment or 2 percent iodine until all drainage has ceased and the wound is dry. At this point, the opening is filled with Keratex Hoof Putty that keeps the affected area clean and prevents the accumulation of debris within the wound. The shoe is replaced when the horse is sound.