Understanding one of the most common causes of lameness

A hoof abscess is a localised accumulation of purulent fluid (pus) within the horse’s hoof. The pus is produced as a reaction by the horse’s body to infection. A hoof abscess causes sudden-onset, severe lameness. Despite the dramatic onset and severity of the lameness associated with a hoof abscess, it is thankfully relatively quick and easy to treat and in most cases does not result in any long-term problems.

What’s going on in the hoof?

A hoof abscess usually starts with a localised, walled-off infection, which the body will try to fight with its white blood cells and inflammatory mediators. As the infection is walled-off inside the abscess, it is difficult for the immune system to get into the area to clear the infection, which means that even though the body is trying to fight the infection, it will typically build up, causing more inflammation, attracting more white blood cells to the area and essentially increasing the pressure within the hoof. As the hoof wall is rigid it cannot expand to relieve this pressure, and this pressure causes pain.

The immune system ultimately ‘rejects’ the infected tissue and tries to expel it from the hoof. During this process the pus from the abscess will take the path of least resistance to relieve the pressure in the foot. If left untreated, it will usually work its way to the hoof wall, breaking out of the coronary band or the bulbs of the heel (the softer tissues).

Lameness

The severity of the lameness is different in each individual case. Some abscesses will rupture on their own, giving the horse instant relief, meaning the horse may never appear lame, or may be lame only transiently. Once the wall of the infected area has ruptured, the infective material is more accessible and therefore much easier for the immune system to attack. Alternatively, the abscess may create pressure so high that a horse is unwilling to put his foot on the ground, let alone walk. Many horses will opt to put the weight on their toes to minimise the discomfort. This is called ‘pointing’. This severe lameness can develop overnight, so a sound horse the evening before can be crippled with lameness the next morning.

Common causes

Most abscesses are caused by bacteria entering the interior hoof structures, usually at the sole-wall junction (just inside the hoof wall). Listed below are common causes.

Environmental conditions

If the weather is dry for a period of time, the hoof can dry out and shrink slightly, causing tiny cracks and fissures to occur in the sole-wall junction. When the weather becomes wet again, these cracks and fissures are easily filled with all kinds of dirt, giving bacteria easy access to the internal hoof structures.

Penetrating wounds

If a horse steps on something sharp such as a nail, rock or broken glass, it may penetrate the sole. The result may be a very small, almost unnoticeable break in the integrity of the sole, but even a small weakness is enough to allow contamination of the hoof by bacteria. In this case the abscess usually develops two to four days after the penetrating wound originally occurred.  

Incorrectly placed nails from shoeing

If a nail is placed too close to or into the foot’s sensitive inner structures, bacteria can be introduced that go on to form an abscess. Even if the nail was removed because of its incorrect placement, a tract has now been opened, which can allow bacteria into the interior of the hoof. Many farriers recommend flushing the hole made by the nail with an antiseptic solution as soon as it is opened and wrapping the foot for between three to five days to reduce the chances of infection.

Ground conditions

Muddy or rocky ground can soften feet or cause bruises. Many bruises will develop into abscesses either through infection entering through a small external break in the integrity of the hoof, or by bacteria circulating in the horse’s own bloodstream infecting the area. These areas of haemorrhage are ideal breeding grounds for bacteria.

Poor hoof balance

Poor hoof balance can result in extra stress on the sole-wall junction of the foot and can create cracks that make the hoof vulnerable to infection. Areas of the hoof that become bruised due to the poor balance are also areas in which abscesses develop.

Management factors

Dirty stables tend to be wet and contain lots of bacteria that can invade the foot. This is exacerbated if the conditions outside are dry, because, as seen above, fissures and cracks form in dry hooves, which give the bacteria in the stable easy access to the interior of the hoof.  

Horses must not be allowed to stand for long periods in ‘dirty’ beds

Finding an abscess

Apart from the sudden lameness, an owner may also notice swelling of the hoof, heat coming from the hoof, heat generally in the limb, a low-grade fever, draining tracts from the abscesses (pus, often grey or black in colour, seen at the sole/coronary band), increased digital pulse, and evidence of the hoof injury that has led to the introduction of the bacteria. If an abscess is left untreated or is situated deep within the hoof, you can sometimes see bone erosion on x-rays secondary to the destructive nature of the infection.

Vets generally begin by feeling for the digital pulse in that limb. If this is palpable and they have strong suspicions that the cause is an abscess, the first step is usually to remove the shoe (if shod) and clean the foot. Hoof testers are then used focally to find the area in which the abscess is situated. When the foot is examined or trimmed, you may then also see a black spot on the sole or sole-wall junction (the tract through which the abscess is draining) or you may see the original crack or puncture wound. This can lead you to the abscess.

Treatment

Abscess treatment essentially involves opening the abscess – breaking down its wall and letting it drain. If possible the vet will try to drain the abscess through the sole. This works for two reasons:

  1. The signs of the tract are usually on the sole (making it easier to find the abscess).
  2. Gravity will help the abscess to drain.

The drainage hole is made using a hoof knife or loop knife. When the pressure is released you will often see a black, brown or grey exudate begin to drain from the site. The horse will often experience some immediate relief as the pressure has been slightly reduced. If an abscess has gone undetected for a long time, but is still relatively superficial, a large amount of the sole may need to be pared away. This may mean that the foot requires longer-term protection from infection in the form of a pad or plate.

If, however, the point of origin and the abscess cannot be identified, or the infection is too deeply situated within the internal hoof structures, or the abscess is within the heel, frog or bars region, the vet will not cut or make holes to go looking for the abscess. Cutting too much or going too deep can be very dangerous for the rest of the hoof tissue. In the scenario of a deep or ‘hidden’ hoof abscess, the horse will usually be given heavy pain relief and a high dose of anti-inflammatories and the abscess allowed to find its own drainage route, usually through the coronary band or sole. This is encouraged by poulticing the hoof with Epsom salts, which act as a drawing agent. This is a painful and unpleasant process for the horse, and risks the infection causing damage to important structures within the hoof.

Keep it clean

Treatment then requires cleaning of the foot, keeping the foot wrapped and protected to prevent further debris entering, and using drawing agents to encourage the remaining pus to drain. Hoof boots can be used for protection or soaking, and poultices with drawing agents are used to aid drainage. Anti-inflammatory medications may help with pain relief, and some vets give antibiotics in these cases. Softening the hoof capsule via foot soaks to encourage rupture or drainage was once considered an essential part of treatment. This was then followed by applying a poultice to the hoof containing drawing agents. Now some veterinarians are recommending against soaking the foot as it is thought that it may weaken the hoof wall. Instead, some are opting to use just the drawing agents, such as Epsom salts and glycerin ichthamol, applied to a poultice to encourage the drawing out of the remaining pus. Once successful drainage has been established, improvement should be seen rapidly. However, if the lameness continues, it is important to investigate alternative causes.

Preventing abscesses

Generally speaking the prognosis of a hoof abscess is excellent. However, if the hoof abscess has caused deep infection such as infection of the coffin joint, navicular bone, tendon sheath or collateral cartilages, the prognosis is much more guarded. Good hoof care is the only way to reduce the risk of abscesses. This requires routine farrier care to keep feet balanced and avoid bruising, and also regular, thorough cleaning of the feet to remove debris that collects in and around the foot.