The foot abscess is a common cause of lameness and a circumstance that many horse owners will encounter at some point in time. In this article, veterinary podiatrist Luke Wells-Smith describes how they happen, the clinical signs, different types of foot infections and areas they can affect, as well as detailing the current treatment and management strategies.
What is a foot abscess?
A foot abscess is the accumulation of inflammatory fluid and cells within the structures of the foot. There are two types of abscess - sterile and septic.
A sterile abscess typically forms after a traumatic incidence, such as a stone bruise that causes damage to the underlying soft tissue. Damage to this tissue causes swelling, which is trapped under the rigid hoof capsule (hoof capsule is the hoof wall, sole and frog).
The more the soft tissue swells under the hoof capsule, the more pressure and pain that builds up. Eventually, inflammatory cells are recruited to repair the damaged tissue, forming an abscess.
A septic abscess occurs whenever micro-organisms, such as fungi or bacteria, are involved and the tissue becomes infected.
Micro-organisms can cause an infection and abscess in two ways: direct infection of the sensitive tissue from a penetrating injury or a break in the integrity of the hoof capsule or blood borne, with the former being by far the most common cause of abscessation.
A nail driven close to the sensitive tissue surrounding the pedal bone is an example of a penetrating injury, seeding micro-organisms into the foot.
There are many structures within the hoof capsule that can be affected by infection and abscessation - bone, synovial and soft tissue structures.
The two major bones within the hoof capsule are the pedal and navicular bones, with the short pastern bone being within the hoof capsule to a lesser extent. Synovial structures within the hoof capsule include the coffin joint and the navicular bursa. The digital flexor tendon sheath is another synovial structure which is in close proximity to the foot and can sometimes be affected. The collateral cartilages of the pedal bone can also be infected, this is known as ‘quittor’.
The foot is comprised of many different soft tissue structures. These include the deep digital flexor tendon, suspensory apparatus of the navicular bone, collateral ligaments of the coffin joint, the coronary band, the digital cushion and sensitive tissue surrounding the pedal bone.
All of the areas above can be associated with abscessation or infection. However, the majority of the time, the infection occurs between the hoof capsule and the sensitive tissue surrounding the pedal bone, known as the dermis.
The dermis comprises of blood vessels, lymphatics and nerves. The dermis in contact with the hoof wall also contains the pedal bone side of the lamellar attachment. The dermis on the underside of the pedal bone is responsible for the growth and development of the sole, bars and frog, whereas the dermis at the coronary band is responsible for hoof wall growth.
Depending upon where the abscess started will determine the most likely area the abscess will drain.
Typically, the infection travels up the white line (i.e. junction between the sole and hoof wall) or through a crack in the external hoof wall, it will travel along the dermis and drain through the coronary band.
Penetrating injuries or bruising to the sole and/or frog will travel along the dermis on the underside of the pedal bone and drain through the heel bulbs. Severe abscesses may also track underneath the skin causing cellulitis and eventually drain somewhere along the limb.
Signs of a foot abscess or infection
Typically horses with a foot abscess have a mild lameness in the very early stages. This lameness rapidly worsens, usually over 24-48 hours. The time when the majority of veterinarians and farriers are called to a foot abscess is when the horse is toe touching lame and unable to bear weight on the limb. The reason for this is likely the accumulation of fluid and pressure within the hoof capsule, causing a compartmental-like scenario.
On examination, the horse will likely have an easily palpable digital pulse to the foot and may have some accumulation of swelling in the lower limb. The digital pulse can be assessed by placing your fingers across the palmar digital vessels at the level of the fetlock. The palmar digital vessels originate further up the leg and course down the back half of the leg, over both the medial (inside) and lateral (outside) proximal sesamoids. The best way to find the vessels is to run your fingers horizontally across the back half of the fetlock, and under your fingers you should feel three thread-like structures: palmar digital vein, artery and nerve. It is important to only apply light pressure over the vessels, otherwise it makes it more difficult to feel the digital pulse.
After palpating the digital pulse to the affected leg, it is also important to check the digital pulses in the three other limbs to rule out laminitis. In an episode of laminitis, the digital pulses will be typically elevated in two or more limbs. In some cases of chronic laminitis, re-current abscesses form and these can occur in all four limbs. Therefore, it is important to rule in/rule out laminitis in cases where the pulse is elevated in multiple limbs.
If the lameness associated with the abscess has been present for greater than three days and the horse will not stand on the affected limb, it is important to put in place a plan to prevent supporting limb laminitis of the other foot. Again, horses with supporting limb laminitis will also have an easily palpable digital pulse. Therefore, it is important to involve your veterinarian and farrier to help assess the horse.
Hoof testers can be helpful in localising which area of the foot the abscess is located. Hoof testers apply pressure to the sole and hoof wall and, when placed over an area of inflammation/abscessation, will cause a pain response. However, not all horses that react to hoof testers have an abscess. Other diagnoses include laminitis, pedal bone fracture, thin soles and navicular syndrome. Similarly, some horses with a foot abscess won’t react to hoof testers. This typically occurs in Summer when the feet are dry and hard, therefore less reactive to the hoof testers.
Palpation of the coronary band is also important. Before the abscess matures and drains, many times the area where it is likely to drain will become painful to the touch. The coronary band may also become swollen and red.
Depending upon which areas of the foot are affected by the abscess will also determine the clinical signs. As mentioned previously, infection under the dermis results in either drainage from the coronary band or the heel bulbs. Infection of the collateral cartilages of the pedal bone (quittor) typically cause a draining tract from above the coronary band.
Infection of the coffin joint and navicular bursa can cause increased coffin joint fluid and pain on flexion of the lower limb. A foul swelling, white-yellow discharge that persists for many days can be an indicator that the pedal bone is infected.
Abscessation between the dermis and the hoof capsule is a relatively straightforward diagnosis and typically resolves quickly once drainage has been established. When other areas of the foot are infected, it is best to consult with both your veterinarian and farrier for further diagnostics. These include x-ray, ultrasound, joint fluid analysis and even CT/MRI.
Foot abscess management
A foot abscess between the dermis and hoof capsule is relatively easy to manage. Once the area where the foot abscess has formed is localised, a small hoof knife or horseshoe nail can be used to explore the white line. It is important to make only a small hole. If the hole is too large, the sensitive dermis may prolapse or dirt and debris may become trapped in the area, causing further pain and possibly infection.
Once the abscess is located and drained, the foot needs to be wrapped in a warm, wet poultice and secured in place with an Elastoplast wrap. Duct tape can be applied to the bottom of the foot to reduce the wear of the bandage. The poultice needs to be changed daily until the drainage and discharge reduces. Drainage typically stops with four days.
After the drainage is complete, a plan is made depending upon where the abscess has drained from (white line, coronary band or heel bulbs). If the hoof capsule is intact and the abscess did not affect a significant amount of the dermis, the horse can continue in work either barefoot or in a shoe. Typically, if the abscess was drained from the white line, this area would need to be packed with either cotton wool soaked in iodine or a medicated hoof wax (i.e. Keratex Hoof Putty) to prevent the area from becoming impacted with debris.
If the abscess affects are larger area of the hoof capsule, hoof wall or sole/frog, resections may be required to facilitate the healing process. If a resection is required, some form of stabilisation of the hoof capsule is required, such as a shoe or a hoof capsule cast.
If the drainage of the abscess continues for greater than seven days or the lameness is not improving, further diagnostics are required to determine what other structures may be involved. If the pedal bone is infected this usually requires surgery to remove any areas that are affected. After the surgery, a hospital plate is applied so that the surgery site is protected, and can be cleaned and medicated on a regular basis.
If other structures are infected, such as the deep digital flexor tendon, suspensory apparatus of the navicular bone, coffin joint and navicular bursa, intensive management is required in a veterinary hospital and, typically, these cases have a guarded prognosis for soundness. If these cases are identified and treated early, the horse will have a better prognosis for long-term soundness.
In some cases of severe soft tissue damage, sterile medical maggots can be used to help debride the area. The sterile medical maggots only debride necrotic and dead tissue, and leave the healthy tissue.
As mentioned, micro-organisms can be involved in a foot abscess. These micro-organisms typically are environmental in origin (i.e. from the soil). One bacteria that commonly lives in soil is Clostridium tetani, which is responsible for the disease tetanus. As foot abscesses are relatively common, it is important to keep your horse’s tetanus vaccination status up to date.
If your horse is not vaccinated for tetanus and develops a foot abscess, contact your local veterinarian for advice as soon as possible. Tetanus is very difficult to treat and has a poor prognosis for survival in many cases. However, vaccination against tetanus is the best and relatively inexpensive prevention for development of the disease.
Take home messages
The majority of foot abscesses are relatively easy to treat and have a good prognosis for soundness. It is important to understand that other areas within the foot can be affected, and that a team approach between your farrier and veterinarian is imperative. If the foot abscess does not resolve within seven days or the lameness continues to deteriorate despite treatment, other areas of the foot need to be investigated and other treatments may be required. It is also important to keep your horse vaccinated for tetanus as a foot abscess is one of the ways your horse can develop tetanus.